Date:  2021-11-22>
Project:  Programming Language C++
Reference:  ISO/IEC IS 14882:2020
Reply to:  William M. Miller
 Edison Design Group, Inc.
 wmm@edg.com


C++ Standard Core Language Active Issues, Revision 106


This document contains the C++ core language issues on which the Committee (INCITS PL22.16 + WG21) has not yet acted, that is, issues with status "Ready," "Tentatively Ready," "Review," "Drafting," and "Open." (See Issue Status below.)

This document is part of a group of related documents that together describe the issues that have been raised regarding the C++ Standard. The other documents in the group are:

Section references in this document reflect the section numbering of document WG21 N4885.

The purpose of these documents is to record the disposition of issues that have come before the Core Language Working Group of the ANSI (INCITS PL22.16) and ISO (WG21) C++ Standard Committee.

Some issues represent potential defects in the ISO/IEC IS 14882:2020 document and corrected defects in the earlier 2017, 2014, 2011, 2003, and 1998 documents; others refer to text in the working draft for the next revision of the C++ language and not to any Standard text. Issues are not necessarily formal ISO Defect Reports (DRs). While some issues will eventually be elevated to DR status, others will be disposed of in other ways.

The most current public version of this document can be found at http://www.open-std.org/jtc1/sc22/wg21. Requests for further information about these documents should include the document number, reference ISO/IEC 14882:2017, and be submitted to the InterNational Committee for Information Technology Standards (INCITS), 1250 Eye Street NW, Suite 200, Washington, DC 20005, USA.

Information regarding C++ standardization can be found at http://isocpp.org/std.


Revision History

Issue status

Issues progress through various statuses as the Core Language Working Group and, ultimately, the full PL22.16 and WG21 committees deliberate and act. For ease of reference, issues are grouped in these documents by their status. Issues have one of the following statuses:

Open: The issue is new or the working group has not yet formed an opinion on the issue. If a Suggested Resolution is given, it reflects the opinion of the issue's submitter, not necessarily that of the working group or the Committee as a whole.

Drafting: Informal consensus has been reached in the working group and is described in rough terms in a Tentative Resolution, although precise wording for the change is not yet available.

Review: Exact wording of a Proposed Resolution is now available for an issue on which the working group previously reached informal consensus.

Ready: The working group has reached consensus that a change in the working draft is required, the Proposed Resolution is correct, and the issue is ready to forward to the full Committee for ratification.

Tentatively Ready: Like "ready" except that the resolution was produced and approved by a subset of the working group membership between meetings. Persons not participating in these between-meeting activities are encouraged to review such resolutions carefully and to alert the working group with any problems that may be found.

DR: The full Committee has approved the item as a proposed defect report. The Proposed Resolution in an issue with this status reflects the best judgment of the Committee at this time regarding the action that will be taken to remedy the defect; however, the current wording of the Standard remains in effect until such time as a Technical Corrigendum or a revision of the Standard is issued by ISO.

accepted: Like a DR except that the issue concerns the wording of the current Working Paper rather than that of the current International Standard.

TC1: A DR issue included in Technical Corrigendum 1. TC1 is a revision of the Standard issued in 2003.

CD1: A DR issue not resolved in TC1 but included in Committee Draft 1. CD1 was advanced for balloting at the September, 2008 WG21 meeting.

CD2: A DR issue not resolved in CD1 but included in the Final Committee Draft advanced for balloting at the March, 2010 WG21 meeting.

C++11: A DR issue not resolved in CD2 but included in ISO/IEC 14882:2011.

CD3: A DR/DRWP or Accepted/WP issue not resolved in C++11 but included in the Committee Draft advanceed for balloting at the April, 2013 WG21 meeting.

C++14: A DR/DRWP or Accepted/WP issue not resolved in CD3 but included in ISO/IEC 14882:2014.

CD4: A DR/DRWP or Accepted/WP issue not resolved in C++14 but included in the Committee Draft advanced for balloting at the June, 2016 WG21 meeting.

C++17: a DR/DRWP or Accepted/WP issue not resolved in CD4 but included in ISO/IEC 14882:2017.

CD5: A DR/DRWP or Accepted/WP issue not resolved in C++17 but included in the Committee Draft advanced for balloting at the July, 2019 WG21 meeting.

C++20: a DR/DRWP or Accepted/WP issue not resolved in CD5 but included in ISO/IEC 14882:2020.

DRWP: A DR issue whose resolution is reflected in the current Working Paper. The Working Paper is a draft for a future version of the Standard.

WP: An accepted issue whose resolution is reflected in the current Working Paper.

Dup: The issue is identical to or a subset of another issue, identified in a Rationale statement.

NAD: The working group has reached consensus that the issue is not a defect in the Standard. A Rationale statement describes the working group's reasoning.

Extension: The working group has reached consensus that the issue is not a defect in the Standard but is a request for an extension to the language. The working group expresses no opinion on the merits of an issue with this status; however, the issue will be maintained on the list for possible future consideration as an extension proposal.

Concepts: The issue relates to the “Concepts” proposal that was removed from the working paper at the Frankfurt (July, 2009) meeting and hence is no longer under consideration.

Concurrency: The issue deals with concurrency and is to be handled by the Concurrency and Parallelism Study Group (SG1) within WG21.


Issues with "Ready" Status


2502. Unintended declaration conflicts in nested statement scopes

Section: 6.4.3  [basic.scope.block]     Status: ready     Submitter: Jens Maurer     Date: 2021-08-26

The changes of P1878R6 inadvertently made constructs like

  if (int a = 1)
    if (int a = 1)
      ...

ill-formed.

Proposed resolution (September, 2021):

Change 6.4.3 [basic.scope.block] bullet 2.2 as follows:

If a declaration whose target scope is the block scope S of a

potentially conflicts with a declaration whose target scope is the parent scope of S, the program is ill-formed.

(See editorial issue 4843.)




2499. Inconsistency in definition of pointer-interconvertibility

Section: 6.8.3  [basic.compound]     Status: ready     Submitter: Jason Merrill     Date: 2021-07-29

The changes for issue 2254 included the following:

Change 6.8.3 [basic.compound] bullet 4.3 as follows:

Two objects a and b are pointer-interconvertible if:

This should also have removed the phrase,

or, if the object has no non-static data members,

since the change to 11.4 [class.mem] paragraph 25 specifies that all bases of a standard-layout class have the same address, regardless of whether the derived class has non-static data members.

Proposed resolution (November, 2021):

Change 6.8.3 [basic.compound] bullet 4.3 as follows:

Two objects a and b are pointer-interconvertible if:






Issues with "Tentatively Ready" Status




Issues with "Review" Status


2197. Overload resolution and deleted special member functions

Section: _N4750_.15.8  [class.copy]     Status: review     Submitter: Maxim Kartashev     Date: 2015-11-11

[Detailed description pending.]

Notes from the November, 2016 meeting:

This issue is to be handled editorially and is in "review" status to check that the change has been applied.




1924. Definition of “literal” and kinds of literals

Section: 5.13  [lex.literal]     Status: review     Submitter: Saeed Amrollah Boyouki     Date: 2014-05-12

The term “literal” is used without definition except the implicit connection with the syntactic nonterminal literal. The relationships of English terms to syntactic nonterminals (such as “integer literal” and integer-literal) should be examined throughout 5.13 [lex.literal] and its subsections.

Notes from the November, 2016 meeting:

This issue will be handled editorially. It is being placed in "review" status until that point.




2345. Jumping across initializers in init-statements and conditions

Section: 8.5.2  [stmt.if]     Status: review     Submitter: John Spicer     Date: 2017-04-25

According to 8.5.2 [stmt.if] paragraph 1,

If the condition (8.5 [stmt.select]) yields true the first substatement is executed. If the else part of the selection statement is present and the condition yields false, the second substatement is executed. If the first substatement is reached via a label, the condition is not evaluated and the second substatement is not executed.

Although 8.8 [stmt.dcl] paragraph 3 forbids bypassing a declaration with initialization, a condition is not syntactically a declaration, and the permission to jump into a then clause and the statement that the condition “is not evaluated” could be read to indicate that a jump across a condition with initialization is permitted. Presumably the prohibition in 8.8 [stmt.dcl] would apply to an init-statement, since it can be a declaration syntactically, but one would expect the same restrictions to apply to both.

Notes from the April, 2018 teleconference:

This issue will be handled editorially (see editorial issue 1949) and will be left in "review" status until CWG verifies that the necessary changes have been made.




2495. Glvalue result of a function call

Section: 8.7.4  [stmt.return]     Status: review     Submitter: Jim X     Date: 2021-07-04

According to 8.7.4 [stmt.return] paragraph 1,

A return statement with any other operand shall be used only in a function whose return type is not cv void; the return statement initializes the glvalue result or prvalue result object of the (explicit or implicit) function call by copy-initialization (9.4 [dcl.init]) from the operand.

It is not clear what a “glvalue result” is or what it means to initialize it.

Suggested resolution:

A return statement with any other operand shall be used only in a function whose return type is not cv void;. If the function call is a prvalue, the return statement initializes the glvalue result or prvalue result object of the (explicit or implicit) function call by copy-initialization (9.4 [dcl.init]) from the operand. Otherwise, the return statement is equivalent to the following hypothetical declaration

If the operand of the return statement, X, is a comma expression without parentheses, e is (X), otherwise e is X. T is the return type of the function call; the invented variable t is the result of the function call.

Notes from the August, 2021 teleconference:

A simpler approach would be simply to use a phrase like “returned object or reference” in place of the current wording referring to glvalues and prvalues. This change was regarded as editorial. The issue will remain in "review" status until CWG can look over the wording change.




2410. Implicit calls of immediate functions

Section: 9.2.6  [dcl.constexpr]     Status: review     Submitter: John Spicer     Date: 2019-03-27

The intent for immediate functions is that they can only be called at compile time. That rule is enforced by the wording of 7.5.4 [expr.prim.id] paragraph 3:

An id-expression that denotes an immediate function (9.2.6 [dcl.constexpr]) shall appear as a subexpression of an immediate invocation or in an immediate function context (7.7 [expr.const]).

However, this restriction does not apply to implicit function calls such as constructor and operator invocations. Presumably some additional wording is needed for such cases.

Additional note, July, 2019:

This issue would appear to be NAD because of the following wording from 7.7 [expr.const] paragraph 10:

An expression or conversion is an immediate invocation if it is an explicit or implicit invocation of an immediate function and is not in an immediate function context. An immediate invocation shall be a constant expression.



2228. Ambiguity resolution for cast to function type

Section: 9.3.3  [dcl.ambig.res]     Status: review     Submitter: Richard Smith     Date: 2016-02-02

[Detailed description pending.]

Proposed resolution (January, 2019):




2252. Enumeration list-initialization from the same type

Section: 9.4.5  [dcl.init.list]     Status: review     Submitter: Richard Smith     Date: 2016-03-22

According to 9.4.5 [dcl.init.list] bullet 3.8,

Otherwise, if T is an enumeration with a fixed underlying type (9.7.1 [dcl.enum]), the initializer-list has a single element v, and the initialization is direct-list-initialization, the object is initialized with the value T(v) (7.6.1.4 [expr.type.conv]); if a narrowing conversion is required to convert v to the underlying type of T , the program is ill-formed.

This could be read as requiring that there be a conversion from v to the underlying type of T, leaving the status of an example like the following unclear:

  enum class E {}; 
  struct X { operator E(); }; 
  E{X()}; // ok? 

Notes from the March, 2018 meeting:

CWG disagreed that the existing wording requires such a conversion, only that if such a conversion is possble, it must not narrow. A formulation along the lines of “if that initialization involves a narrowing conversion to the underlying type of T...” was suggested to clarify the intent. This will be handled editorially, and the issue will be left in "review" status until the change has been verified.




2221. Copying volatile objects

Section: 9.5.2  [dcl.fct.def.default]     Status: review     Submitter: 2016-01-09     Date: Vinny Romano

[Detailed description pending.]

Notes from the November, 2016 meeting:

This issue is to be resolved editorially and is placed in "review' status until the corresponding change appers in a working draft.




2451. promise.unhandled_exception() and final suspend point

Section: 9.5.4  [dcl.fct.def.coroutine]     Status: review     Submitter: Lewis Baker     Date: 2020-02-14

According to 9.5.4 [dcl.fct.def.coroutine] paragraph 14,

If the evaluation of the expression promise.unhandled_exception() exits via an exception, the coroutine is considered suspended at the final suspend point.

However, the “final suspend point” is defined as being “the await-expression containing the call to final_suspend” (bullet 5.2), and it is not desired to evaluate the final_suspend expression in this case.

Suggested resolution:

  1. Change 9.5.4 [dcl.fct.def.coroutine] paragraph 5 as follows:

  2. ...where

  3. Change bullet 3.2 of 7.6.2.4 [expr.await] as follows:

  4. Evaluation of an await-expression involves the following auxiliary types, expressions, and objects:

  5. If needed, change 9.5.4 [dcl.fct.def.coroutine] paragraph 14 as follows:

  6. If the evaluation of the expression promise.unhandled_exception() exits via an exception, the coroutine is considered suspended at the final suspend point and the exception propagates to the caller or resumer.

Notes from the August, 2020 teleconference:

CWG expressed some concern about the lack of a precise definition of “suspend point”. Gor Nishanov suggests the following change, in 7.6.2.4 [expr.await] bullet 5.1:




2194. Impossible case in list initialization

Section: 12.2.2.8  [over.match.list]     Status: review     Submitter: Robert Haberlach     Date: 2015-11-04

According to 12.2.2.8 [over.match.list] paragraph 1 says,

If the initializer list has no elements and T has a default constructor, the first phase is omitted.

However, this case cannot occur. If T is a non-aggregate class type with a default constructor and the initializer is an empty initializer list, the object will be value-constructed, per 9.4.5 [dcl.init.list] bullet 3.4. Overload resolution is only necessary if default-initialization (or a check of its semantic constraints) is implied, with the relevant section concerning candidates for overload resolution being 12.2.2.4 [over.match.ctor].

See also issue 1518.

Proposed resolution (January, 2017):

Change 12.2.2.8 [over.match.list] paragraph 1 as follows:

When objects of non-aggregate class type T are list-initialized such that 9.4.5 [dcl.init.list] specifies that overload resolution is performed according to the rules in this section, overload resolution selects the constructor in two phases:

If the initializer list has no elements and T has a default constructor, the first phase is omitted. In copy-list-initialization, if an explicit constructor is chosen...

Additional notes, February, 2017:

The statement of the issue is incorrect. In an example like

  struct A { A(); A(initializer_list<int>); };
  void f(A a);
  int main() { f({}); }

the rule in question is not used for the initialization of the parameter. However, it is used to determine whether a valid implicit conversion sequence exists for a. It is unclear whether an additional change to resolve this discrepancy is needed or not.




1789. Array reference vs array decay in overload resolution

Section: 12.2.4.3  [over.ics.rank]     Status: review     Submitter: Faisal Vali     Date: 2013-10-01

The current rules make an example like

  template<class T, size_t N> void foo(T (&)[N]);
  template<class T> void foo(T *t);

  int arr[3]{1, 2, 3};
  foo(arr);

ambiguous, even though the first is an identity match and the second requires an lvalue transformation. Is this desirable?

Proposed resolution (June, 2021):

Add the following as a new bullet following 12.2.4.3 [over.ics.rank] bullet 3.2.6:

Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:




2482. bit_cast and indeterminate values

Section: 26.5.3  [bit.cast]     Status: review     Submitter: Richard Smith     Date: 2019-06-20

As currently specified, bit_cast from an indeterminate value produces an unspecified value rather than an indeterminate value. That means this can't be implemented by a simple load on some implementations, and instead will require some kind of removing-the-taint-of-an-uninitialized-value operation to be performed. (A similar concern applies to reading from padding bits.)

The intent is as follows:

Some examples:

  struct A { char c; /* char padding : 8; */ short s; };
  struct B { char x[4]; };

  B one() {
    A a = {1, 2};
    return std::bit_cast<B>(a);
  }

In one(), the second byte of the object representation of a is bad. That means that the second byte of the produced B object is bad, so x[1] in the produced B object is an indeterminate value. The above function, if declared constexpr, would be usable in constant expressions so long as you don't look at one().x[1].

  A two() {
    B b;
    b.x[0] = 'a';
    b.x[2] = 1;
    b.x[3] = 2;
    return std::bit_cast<A>(b);
  }

In two() , the second byte of the object representation of b is bad. But a bit_cast to A doesn't care because it never looks at that byte. The above function returns an A with a fully-defined value. If declared constexpr, it would produce a normal, fully-initialized value.

  int three() {
    int n;
    return std::bit_cast<int>(n);
  }

In three(), the entirety of n is bad. A bit_cast from it produces an int whose value is indeterminate. And because we have an expression of non-byte-like type that produced an indeterminate value, the behavior is undefined.

  B four() {
    int n;
    return std::bit_cast<B>(n);
  }

In four(), just like three(), the entirety of n is bad, so the scalar subobjects of B are bad too. But because they're of byte-like type, that's OK: we can copy them about and produce them from prvalue expressions.

Proposed resolution (May, 2021):

Change 26.5.3 [bit.cast] paragraph 2 as follows:

Returns: An object of type To. Implicitly creates objects nested within the result (6.7.2 [intro.object]). Each bit of the value representation of the result is equal to the corresponding bit in the object representation of from. Padding bits of the result are unspecified. For the result and each object created within it, if there is no value of the object's type corresponding to the value representation produced, the behavior is undefined. If there are multiple such values, which value is produced is unspecified. A bit in the value representation of the result is indeterminate if it does not correspond to a bit in the value representation of from or corresponds to a bit of an object that is not within its lifetime or has an indeterminate value (6.7.4 [basic.indet]). For each bit in the value representation of the result that is indeterminate, the smallest object containing that bit has an indeterminate value; the behavior is undefined unless that object is of unsigned ordinary character type or std::byte type. The result does not otherwise contain any indeterminate values.



2407. Missing entry in Annex C for defaulted comparison operators

Section: Annex C  [diff]     Status: review     Submitter: Tomasz Kaminski     Date: 2019-02-26

The changes from P1185R2 need an entry in Annex C, because they affect the interpretation of existing well-formed code. For example, given:

  struct A {
    operator int() const { return 10; }
  };

  bool operator==(A, int); // #1
  //built-in: bool operator==(int, int); // #2

  A a, b;

The expression 10 == a resolves to #2 in C++17 but now to #1. In addition, a == b is now ambiguous, because #1 has a user-defined conversion on the second argument, while the reversed order has it on the first argument. Similarly for operator!=.

Notes from the March, 2019 teleconference:

The ambiguity in 10 == a arises from the consideration of the reverse ordering of the operands.

CWG found this breakage surprising and asked for EWG's opinion before updating Annex C.

Proposed resolution (April, 2019):

Add the following as a new subclause in C.1 [diff.cpp17]:

C.5.6 Clause 12: Overloading

Affected subclause: 12.2.2.3 [over.match.oper]
Change: Overload resolution may change for equality operators 7.6.10 [expr.eq].
Rationale: Support calling operator== with reversed order of arguments.
Effect on original feature: Valid C++ 2017 code that uses equality operators with conversion functions may be ill-formed or have different semantics in this International Standard.

  struct A {
    operator int() const { return 10; }
  };

  bool operator==(A, int);               // #1
  // built-in: bool operator==(int, int);  // #2
  bool b = 10 == A();                   // uses #1 with reversed order of arguments; previosly used #2





Issues with "Drafting" Status


1092. Cycles in overload resolution during instantiation

Section: _N4750_.15.8  [class.copy]     Status: drafting     Submitter: Jason Merrill     Date: 2010-07-15

Moving to always doing overload resolution for determining exception specifications and implicit deletion creates some unfortunate cycles:

    template<typename T> struct A {
       T t;
    };

    template <typename T> struct B {
       typename T::U u;
    };

    template <typename T> struct C {
       C(const T&);
    };

    template <typename T> struct D {
       C<B<T> > v;
    };

    struct E {
       typedef A<D<E> > U;
    };

    extern A<D<E> > a;
    A<D<E> > a2(a);

If declaring the copy constructor for A<D<E>> is part of instantiating the class, then we need to do overload resolution on D<E>, and thus C<B<E>>. We consider C(const B<E>&), and therefore look to see if there's a conversion from C<B<E>> to B<E>, which instantiates B<E>, which fails because it has a field of type A<D<E>> which is already being instantiated.

Even if we wait until A<D<E>> is considered complete before finalizing the copy constructor declaration, declaring the copy constructor for B<E> will want to look at the copy constructor for A<D<E>>, so we still have the cycle.

I think that to avoid this cycle we need to short-circuit consideration of C(const T&) somehow. But I don't see how we can do that without breaking

    struct F {
       F(F&);
    };

    struct G;
    struct G2 {
       G2(const G&);
    };

    struct G {
       G(G&&);
       G(const G2&);
    };

    struct H: F, G { };

    extern H h;
    H h2(h);

Here, since G's move constructor suppresses the implicit copy constructor, the defaulted H copy constructor calls G(const G2&) instead. If the move constructor did not suppress the implicit copy constructor, I believe the implicit copy constructor would always be viable, and therefore a better match than a constructor taking a reference to another type.

So perhaps the answer is to reconsider that suppression and then disqualify any constructor taking (a reference to) a type other than the constructor's class from consideration when looking up a subobject constructor in an implicitly defined constructor. (Or assignment operator, presumably.)

Another possibility would be that when we're looking for a conversion from C<B<E>> to B<E> we could somehow avoid considering, or even declaring, the B<E> copy constructor. But that seems a bit dodgy.

Additional note (October, 2010):

An explicitly declared move constructor/op= should not suppress the implicitly declared copy constructor/op=; it should cause it to be deleted instead. This should prevent a member function taking a (reference to) an un-reference-related type from being chosen by overload resolution in a defaulted member function.

And we should clarify that member functions taking un-reference-related types are not even considered during overload resolution in a defaulted member function, to avoid requiring their parameter types to be complete.




1499. Missing case for deleted move assignment operator

Section: _N4750_.15.8  [class.copy]     Status: drafting     Submitter: John Spicer     Date: 2012-04-27

Bullet 4 of _N4750_.15.8 [class.copy] paragraph 23 says that a defaulted copy/move assignment operator is defined as deleted if the class has

a non-static data member of class type M (or array thereof) that cannot be copied/moved because overload resolution (12.2 [over.match]), as applied to M's corresponding assignment operator, results in an ambiguity or a function that is deleted or inaccessible from the defaulted assignment operator

The intent of this is that if overload resolution fails to find a corresponding copy/move assignment operator that can validly be called to copy/move a member, the class's assignment operator will be defined as deleted. However, this wording does not cover an example like the following:

  struct A {
    A();
  };

  struct B {
    B();
    const A a;
  };

  typedef B& (B::*pmf)(B&&);

  pmf p =&B::operator=; 

Here, the problem is simply that overload resolution failed to find a callable function, which is not one of the cases listed in the current wording. A similar problem exists for base classes in the fifth bullet.

Additional note (January, 2013):

A similar omission exists in paragraph 11 for copy constructors.




1548. Copy/move construction and conversion functions

Section: _N4750_.15.8  [class.copy]     Status: drafting     Submitter: Nikolay Ivchenkov     Date: 2012-09-02

The current wording of _N4750_.15.8 [class.copy] paragraph 31 refers only to constructors and destructors:

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side effects.

However, in some cases (e.g., auto_ptr) a conversion function is also involved in the copying, and it could presumably also have visible side effects that would be eliminated by copy elision. (Some additional contexts that may also require changes in this regard are mentioned in the resolution of issue 535.)

Additional note (September, 2012):

The default arguments of an elided constructor can also have side effects and should be mentioned, as well; however, the elision should not change the odr-use status of functions and variables appearing in those default arguments.




1594. Lazy declaration of special members vs overload errors

Section: _N4750_.15.8  [class.copy]     Status: drafting     Submitter: Richard Smith     Date: 2012-12-06

The implicit declaration of a special member function sometimes requires overload resolution, in order to select a special member to use for base classes and non-static data members. This can be required to determine whether the member is or would be deleted, and whether the member is trivial, for instance. The standard appears to require such overload resolution be performed at the end of the definition of the class, but in practice, implementations perform it lazily. This optimization appears to be non-conforming, in the case where overload resolution would hit an error. In order to enable this optimization, such errors should be “no diagnostic required.”

Additional note (March, 2013):

See also issue 1360.

Notes from the September, 2013 meeting:

The problem with this approach is that hard errors (not in the immediate context) can occur, affecting portability. There are some cases, such as a virtual assignment operator in the base class, where lazy evaluation cannot be done, so it cannot be mandated.




2203. Defaulted copy/move constructors and UDCs

Section: _N4750_.15.8  [class.copy]     Status: drafting     Submitter: Vinny Romano     Date: 2015-11-20

[Detailed description pending.]




2264. Memberwise copying with indeterminate value

Section: _N4750_.15.8  [class.copy]     Status: drafting     Submitter: Hubert Tong     Date: 2016-05-06

It appears that the following example may have unwanted undefined behavior in C++, although not in C:

  struct A { int x, y; }; 
  A passthrough(A a) { return a; } 
  int main(void) { 
   A a; 
   a.x = 0; 
   return passthrough(a).x; 
  } 

The default memberwise copying operation is not specified to be done in a way that is insensitive to indeterminate values.




1089. Template parameters in member selections

Section: _N4868_.6.5.6  [basic.lookup.classref]     Status: drafting     Submitter: Daveed Vandevoorde     Date: 2010-06-29

In an example like

    template<typename T> void f(T p)->decltype(p.T::x);

The nested-name-specifier T:: looks like it refers to the template parameter. However, if this is instantiated with a type like

    struct T { int x; };
    struct S: T { };

the reference will be ambiguous, since it is looked up in both the context of the expression, finding the template parameter, and in the class, finding the base class injected-class-name, and this could be a deduction failure. As a result, the same declaration with a different parameter name

    template<typename U> void f(U p)->decltype(p.U::x);

is, in fact, not a redeclaration because the two can be distinguished by SFINAE.

It would be better to add a new lookup rule that says that if a name in a template definition resolves to a template parameter, that name is not subject to further lookup at instantiation time.

Additional note (November, 2020):

Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.




1647. Type agreement of non-type template arguments in partial specializations

Section: _N4868_.13.7.6  [temp.class.spec]     Status: drafting     Submitter: John Spicer     Date: 2013-04-04

The Standard appears to be silent on whether the types of non-type template arguments in a partial specialization must be the same as those of the primary template or whether conversions are permitted. For example,

  template<char...> struct char_values {};
  template<int C1, char C3>
  struct char_values<C1, 12, C3> {
    static const unsigned value = 1;
  };
  int check0[char_values<1, 12, 3>::value == 1? 1 : -1]; 

The closest the current wording comes to dealing with this question is _N4868_.13.7.6 [temp.class.spec] paragraph 8 bullet 1:

In this example, one might think of the first template argument in the partial specialization as (char)C1, which would violate the requirement, but that reasoning is tenuous.

It would be reasonable to require the types to match in cases like this. If this kind of usage is allowed it could get messy if the primary template were int... and the partial specialization had a parameter that was char because not all of the possible values from the primary template could be represented in the parameter of the partial specialization. A similar issue exists if the primary template takes signed char and the partial specialization takes unsigned int.

There is implementation variance in the treatment of this example.

(See also issues 1315, 2033, and 2127.)




2127. Partial specialization and nullptr

Section: _N4868_.13.7.6  [temp.class.spec]     Status: drafting     Submitter: Faisal Vali     Date: 2015-05-18

An example like the following would seem to be plausible:

  template<class T, T*> struct X { };
  // We want to partially specialize for all nullptrs...
  template<class T> struct X<T, nullptr> { ... }; // NOT OK

This is disallowed by the rule in bullet 8.2 of _N4868_.13.7.6 [temp.class.spec]:

(See also issues 1315, 1647, and 2033.)




2179. Required diagnostic for partial specialization after first use

Section: _N4868_.13.7.6  [temp.class.spec]     Status: drafting     Submitter: John Spicer     Date: 2015-10-12

According to _N4868_.13.7.6 [temp.class.spec] paragraph 1,

A partial specialization shall be declared before the first use of a class template specialization that would make use of the partial specialization as the result of an implicit or explicit instantiation in every translation unit in which such a use occurs; no diagnostic is required.

There are two problems with this wording. First, the “no diagnostic required” provision is presumably to avoid mandating cross-translation-unit analysis, but there is no reason not to require the diagnostic if the rule is violated within a single translation unit. Also, “would make use” is imprecise; it could be interpreted as applying only when the partial specialization would have been selected by a previous specialization, but it should also apply to cases where the partial specialization would have made a previous specialization ambiguous.

Making these two changes would guarantee that a diagnostic is issued for the following example:

   template <class T1, class T2> class A;
   template <class T> struct A<T, void> { void f(); };
   template <class T> void g(T) { A<char, void>().f(); }   // #1
   template<typename T> struct A<char, T> {};
   A<char, void> f;   // #2

It is unspecified whether the reference to A<char, void> at #1 is the “first use” or not. If so, A<char, void> is bound to the first partial specialization and, under the current wording, an implementation is not required to diagnose the ambiguity resulting from the second partial specialization. If #2 is the “first use,” it is clearly ambiguous and must result in a diagnostic. There is implementation divergence on the handling of this example that would be addressed by the suggested changes.




549. Non-deducible parameters in partial specializations

Section: _N4868_.13.7.6.2  [temp.class.spec.match]     Status: drafting     Submitter: Martin Sebor     Date: 18 November 2005

In the following example, the template parameter in the partial specialization is non-deducible:

    template <class T> struct A { typedef T U; };
    template <class T> struct C { };
    template <class T> struct C<typename A<T>::U> { };

Several compilers issue errors for this case, but there appears to be nothing in the Standard that would make this ill-formed; it simply seems that the partial specialization will never be matched, so the primary template will be used for all specializations. Should it be ill-formed?

(See also issue 1246.)

Notes from the April, 2006 meeting:

It was noted that there are similar issues for constructors and conversion operators with non-deducible parameters, and that they should probably be dealt with similarly.




1755. Out-of-class partial specializations of member templates

Section: _N4868_.13.7.6.4  [temp.class.spec.mfunc]     Status: drafting     Submitter: Richard Smith     Date: 2013-09-19

According to _N4868_.13.7.6.4 [temp.class.spec.mfunc] paragraph 2,

If a member template of a class template is partially specialized, the member template partial specializations are member templates of the enclosing class template; if the enclosing class template is instantiated (13.9.2 [temp.inst], 13.9.3 [temp.explicit]), a declaration for every member template partial specialization is also instantiated as part of creating the members of the class template specialization.

Does this imply that only partial specializations of member templates that are declared before the enclosing class is instantiated are considered? For example, in

  template<typename A> struct X { template<typename B> struct Y; };
  template struct X<int>;
  template<typename A> template<typename B> struct X<A>::Y<B*> { int n; };
  int k = X<int>::Y<int*>().n;

is the last line valid? There is implementation variance on this point. Similarly, for an example like

  template<typename A> struct Outer {
   template<typename B, typename C> struct Inner;
  };
  Outer<int> outer;
  template<typename A> template<typename B>
    struct Outer<A>::Inner<typename A::error, B> {};

at what point, if at all, is the declaration of the partial specialization instantiated? Again, there is implementation variance in the treatment of this example.

Notes from the February, 2014 meeting:

CWG decided that partial specialization declarations should be instantiated only when needed to determine whether the partial specialization matches or not.

Additional note, November, 2014:

See also paper N4090.




369. Are new/delete identifiers or preprocessing-op-or-punc?

Section: 5.4  [lex.pptoken]     Status: drafting     Submitter: Martin v. Loewis     Date: 30 July 2002

5.4 [lex.pptoken] paragraph 2 specifies that there are 5 categories of tokens in phases 3 to 6. With 5.12 [lex.operators] paragraph 1, it is unclear whether new is an identifier or a preprocessing-op-or-punc; likewise for delete. This is relevant to answer the question whether

#define delete foo

is a well-formed control-line, since that requires an identifier after the define token.

(See also issue 189.)




1655. Line endings in raw string literals

Section: 5.4  [lex.pptoken]     Status: drafting     Submitter: Mike Miller     Date: 2013-04-26

According to 5.4 [lex.pptoken] paragraph 3,

If the input stream has been parsed into preprocessing tokens up to a given character:

However, phase 1 is defined as:

Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set (introducing new-line characters for end-of-line indicators) if necessary. The set of physical source file characters accepted is implementation-defined. Trigraph sequences (_N4140_.2.4 [lex.trigraph]) are replaced by corresponding single-character internal representations. Any source file character not in the basic source character set (5.3 [lex.charset]) is replaced by the universal-character-name that designates that character.

The reversion described in 5.4 [lex.pptoken] paragraph 3 specifically does not mention the replacement of physical end-of-line indicators with new-line characters. Is it intended that, for example, a CRLF in the source of a raw string literal is to be represented as a newline character or as the original characters?




1901. punctuator referenced but not defined

Section: 5.6  [lex.token]     Status: drafting     Submitter: Richard Smith     Date: 2014-03-25

The syntactic nonterminal punctuator appears in the grammar for token in 5.6 [lex.token], but it is nowhere defined. It should be merged with operator and given an appropriate list of tokens as a definition for the merged term.

Proposed resolution (October, 2017):

  1. Change 5.5 [lex.digraph] paragraph 2 as follows

  2. In all respects of the language except in an attribute-token (9.12.1 [dcl.attr.grammar]), each alternative token behaves the same, respectively, as its primary token, except for its spelling.18 The set of alternative tokens...
  3. Change the grammar in 5.6 [lex.token] as follows:



  4. Change 5.6 [lex.token] paragraph 1 as follows:

  5. There are five four kinds of tokens: identifiers, keywords, literals,19 operators, and other separators and symbols. Blanks, horizontal and vertical tabs, newlines, formfeeds, and comments (collectively, “white space”), as described below, are ignored except as they serve to separate tokens. [Note: Some white space is required to separate otherwise adjacent identifiers, keywords, numeric literals, and alternative tokens containing alphabetic characters. —end note] Each preprocessing-token resulting from translation phase 6 is converted into the corresponding token as follows:

    [Note: Within an attribute-token (9.12.1 [dcl.attr.grammar]), a token formed from a preprocessing-token that satisfies the syntactic requirements of an identifier is considered to be an identifier with the spelling of the preprocessing-token. —end note]

  6. Delete the final sentence of 5.12 [lex.operators] paragraph 1.

  7. Each preprocessing-op-or-punc is converted to a single token in translation phase 7 (5.2 [lex.phases]).



189. Definition of operator and punctuator

Section: 5.12  [lex.operators]     Status: drafting     Submitter: Mike Miller     Date: 20 Dec 1999

The nonterminals operator and punctuator in 5.6 [lex.token] are not defined. There is a definition of the nonterminal operator in 12.4 [over.oper] paragraph 1, but it is apparent that the two nonterminals are not the same: the latter includes keywords and multi-token operators and does not include the nonoverloadable operators mentioned in paragraph 3.

There is a definition of preprocessing-op-or-punc in 5.12 [lex.operators] , with the notation that

Each preprocessing-op-or-punc is converted to a single token in translation phase 7 (2.1).
However, this list doesn't distinguish between operators and punctuators, it includes digraphs and keywords (can a given token be both a keyword and an operator at the same time?), etc.

Suggested resolution:


  1. Change 12.4 [over.oper] to use the term overloadable-operator.
  2. Change 5.6 [lex.token] to use the term operator-token instead of operator (since there are operators that are keywords and operators that are composed of more than one token).
  3. Change 5.12 [lex.operators] to define the nonterminals operator-token and punctuator.

Additional note (April, 2005):

The resolution for this problem should also address the fact that sizeof and typeid (and potentially others like decltype that may be added in the future) are described in some places as “operators” but are not listed in 12.4 [over.oper] paragraph 3 among the operators that cannot be overloaded.

(See also issue 369.)




1723. Multicharacter user-defined character literals

Section: 5.13.8  [lex.ext]     Status: drafting     Submitter: Mike Miller     Date: 2013-07-31

According to 5.13.3 [lex.ccon] paragraph 1, a multicharacter literal like 'ab' is conditionally-supported and has type int.

According to 5.13.8 [lex.ext] paragraph 6,

If L is a user-defined-character-literal, let ch be the literal without its ud-suffix. S shall contain a literal operator (12.6 [over.literal]) whose only parameter has the type of ch and the literal L is treated as a call of the form

A user-defined-character-literal like 'ab'_foo would thus require a literal operator

However, that is not one of the signatures permitted by 12.6 [over.literal] paragraph 3.

Should multicharacter user-defined-character-literals be conditionally-supported? If so, 12.6 [over.literal] paragraph 3 should be adjusted accordingly. If not, a note in 5.13.8 [lex.ext] paragraph 6 saying explicitly that they are not supported would be helpful.




1735. Out-of-range literals in user-defined-literals

Section: 5.13.8  [lex.ext]     Status: drafting     Submitter: Mike Miller     Date: 2013-08-12

The description of the numeric literals occurring as part of user-defined-integer-literals and user-defined-floating-literals in 5.13.8 [lex.ext] says nothing about whether they are required to satisfy the same constraints as literals that are not part of a user-defined-literal. In particular, because it is the spelling, not the value, of the literal that is used for raw literal operators and literal operator templates, there is no particular reason that they should be restricted to the maximum values and precisions that apply to ordinary literals (and one could imagine that this would be a good notation for allowing literals of extended-precision types).

Is this relaxation of limits intended to be required, or is it a quality-of-implementation issue? Should something be said, either normatively or non-normatively, about this question?




1849. Variable templates and the ODR

Section: 6.3  [basic.def.odr]     Status: drafting     Submitter: Richard Smith     Date: 2014-02-03

The description in 6.3 [basic.def.odr] paragraph 6 of when entities can be multiply-declared in a program does not, but should, discuss variable templates.




1897. ODR vs alternative tokens

Section: 6.3  [basic.def.odr]     Status: drafting     Submitter: Hubert Tong     Date: 2014-03-21

According to 5.5 [lex.digraph] paragraph 2,

In all respects of the language, each alternative token behaves the same, respectively, as its primary token, except for its spelling.

However, the primary and alternative tokens are different tokens, which runs afoul of the ODR requirement in 6.3 [basic.def.odr] paragraph 6 that the definitions consist of the “same sequence of tokens.” This wording should be amended to allow for use of primary and alternative tokens.




2242. ODR violation with constant initialization possibly omitted

Section: 6.3  [basic.def.odr]     Status: drafting     Submitter: Hubert Tong     Date: 2016-03-05

Consider the following example:

  // tu1.cpp
  extern const int a = 1;
  inline auto f() {
    static const int b = a;
    struct A { auto operator()() { return &b; } } a;
    return a;
  }

  // tu2.cpp
  extern const int a;
  inline auto f() {
    static const int b = a;
    struct A { auto operator()() { return &b; } } a;
    return a;
  }
  int main() {
    return *decltype(f())()();
  }

Here, b may or may not have constant initialization. This example should be an ODR violation.

(Split off from issue 2123.)




2494. Multiple definitions of non-odr-used entities

Section: 6.3  [basic.def.odr]     Status: drafting     Submitter: Hubert Tong     Date: 2021-07-02

According to 6.3 [basic.def.odr] paragraph 10,

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement (8.5.2 [stmt.if]); no diagnostic required.

This wording could be interpreted as allowing multiple definitions of non-inline variables and functions if they are not odr-used. That is presumably not the intent.

Notes from the August, 2021 teleconference:

CWG observed that there is a similar problem in paragraph 13. See also issue 1849.




2480. Lookup for enumerators in modules

Section: 6.5.1  [basic.lookup.general]     Status: drafting     Submitter: Richard Smith     Date: 2021-02-12

According to 6.5.1 [basic.lookup.general] paragraphs 2-3,

...A declaration X precedes a program point P in a translation unit L if P follows X, X inhabits a class scope and is reachable from P, or else...

A single search in a scope S for a name N from a program point P finds all declarations that precede P to which any name that is the same as N (6.1 [basic.pre]) is bound in S.

These rules cause problems for finding enumerators when qualified by an exported name of its enumeration type, unlike a member of a class. For example:

  export module A;
  enum class X { x };
  enum Y { y };

  export module B;
  import A;
  export using XB = X;
  export using YB = Y;

  // client code
  import B;
  int main() {
    XB x = XB::x; // should be OK because definition of X is reachable, even
                  // though A is not imported
    YB y = YB::y; // similarly OK
    YB z = ::y;   // error, because y from module A is not visible
  }

It would seem that this problem could be addressed by changing “inhabits a class scope” to “does not inhabit a namespace scope.”




2324. Size of base class subobject

Section: 6.7.2  [intro.object]     Status: drafting     Submitter: GB     Date: 2017-02-27

P0488R0 comment GB 9

According to 6.7.2 [intro.object] paragraph 7,

Unless it is a bit-field (11.4.10 [class.bit]), a most derived object shall have a nonzero size and shall occupy one or more bytes of storage. Base class subobjects may have zero size.

Base class objects of zero size is a misleading term, as sizeof such an object is non-zero. Size should not be a property of an object, rather of a type.




2325. std::launder and reuse of character buffers

Section: 6.7.2  [intro.object]     Status: drafting     Submitter: CA     Date: 2017-02-27

P0488R0 comment CA 12

The status of the following code should be explicitly indicated in the Standard to avoid surprise:

  #include <new>
  int bar() {
    alignas(int) unsigned char space[sizeof(int)];
    int *pi = new (static_cast<void *>(space)) int;
    *pi = 42;
    return [=]() mutable {
      return   *std::launder(reinterpret_cast<int *>(space)); }();
   }

In particular, it appears that the call to std::launder has undefined behaviour because the captured copy of space is not established to provide storage for an object of type int (subclause 6.7.2 [intro.object] paragraph 1). Furthermore, the code has undefined behaviour also because it attempts to access the stored value of the int object through a glvalue of an array type other than one of the ones allowed by subclause 7.2.1 [basic.lval] paragraph 8.




2469. Implicit object creation vs constant expressions

Section: 6.7.2  [intro.object]     Status: drafting     Submitter: Hubert Tong     Date: 2020-12-07

It is not intended that implicit object creation, as described in 6.7.2 [intro.object] paragraph 10, should occur during constant expression evaluation, but there is currently no wording prohibiting it.

Notes from the February, 2021 teleconference:

This issue was occasioned by issue 2464, which is also the subject of LWG issue 3495. CWG reviewed the proposed resolution and agrees with it. The intended approach for this issue is to wait for LWG to resolve that issue, then add a note in the core section pointing out the implications of that requirement for implicit object creation.




1027. Type consistency and reallocation of scalar types

Section: 6.7.3  [basic.life]     Status: drafting     Submitter: Gabriel Dos Reis     Date: 2010-02-03

Is the following well-formed?

    int f() {
        int i = 3;
        new (&i) float(1.2);
        return i;
    }

The wording that is intended to prevent such shenanigans, 6.7.3 [basic.life] paragraphs 7-9, doesn't quite apply here. In particular, paragraph 7 reads,

If, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if:

The problem here is that this wording only applies “after the lifetime of an object has ended and before the storage which the object occupied is reused;” for an object of a scalar type, its lifetime only ends when the storage is reused or released (paragraph 1), so it appears that these restrictions cannot apply to such objects.

(See also issues 1116 and 1338.)

Proposed resolution (August, 2010):

This issue is resolved by the resolution of issue 1116.




1530. Member access in out-of-lifetime objects

Section: 6.7.3  [basic.life]     Status: drafting     Submitter: Howard Hinnant     Date: 2012-07-26

According to 6.7.3 [basic.life] paragraphs 5 and 6, a program has undefined behavior if a pointer or glvalue designating an out-of-lifetime object

is used to access a non-static data member or call a non-static member function of the object

It is not clear what the word “access” means in this context. A reasonable interpretation might be using the pointer or glvalue as the left operand of a class member access expression; alternatively, it might mean to read or write the value of that member, allowing a class member access expression that is used only to form an address or bind a reference.

This needs to be clarified. A relevant consideration is the recent adoption of the resolution of issue 597, which eased the former restriction on simple address manipulations involving out-of-lifetime objects: if base-class offset calculations are now allowed, why not non-static data member offset calculations?

(See also issue 1531 for other uses of the term “access.”)

Additional note (January, 2013):

A related question is the meaning of the phrase “before the constructor begins execution” in 11.9.5 [class.cdtor] paragraph 1 means:

For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior.

For example:

  struct DerivedMember { ... };

  struct Base {
    Base(DerivedMember const&);
  };

  struct Derived : Base {
    DerivedMember x;
    Derived() : Base(x) {}
  };

  Derived a;

Is the reference to Derived::x in the mem-initializer valid?

Additional note (March, 2013):

This clause is phrased in terms of the execution of the constructor. However, it is possible for an aggregate to have a non-trivial default constructor and be initialized without executing a constructor. The wording needs to be updated to allow for non-constructor initialization to avoid appearing to imply undefined behavior for an example like:

  struct X {
    std::string s;
  } x = {};
  std::string t = x.s;  // No constructor called for x: undefined behavior?



1853. Defining “allocated storage”

Section: 6.7.3  [basic.life]     Status: drafting     Submitter: Jeffrey Yasskin     Date: 2014-02-09

The term “allocated storage” is used in several places in the Standard to refer to memory in which an object may be created (dynamic, static, or automatic storage), but it has no formal definition.




1634. Temporary storage duration

Section: 6.7.5  [basic.stc]     Status: drafting     Submitter: Richard Smith     Date: 2013-03-04

According to 6.7.5 [basic.stc] paragraph 2,

Static, thread, and automatic storage durations are associated with objects introduced by declarations (6.2 [basic.def]) and implicitly created by the implementation (6.7.7 [class.temporary]).

The apparent intent of the reference to 6.7.7 [class.temporary] is that a temporary whose lifetime is extended to be that of a reference with one of those storage durations is considered also to have that storage duration. This interpretation is buttressed by use of the phrase “an object with the same storage duration as the temporary” (twice) in 6.7.7 [class.temporary] paragraph 5.

There are two problems, however: first, the specification of lifetime extension of temporaries (also in 6.7.7 [class.temporary] paragraph 5) does not say anything about storage duration. Also, nothing is said in either of these locations about the storage duration of a temporary whose lifetime is not extended.

The latter point is important because 6.7.3 [basic.life] makes a distinction between the lifetime of an object and the acquisition and release of the storage the object occupies, at least for objects with non-trivial initialization and/or a non-trivial destructor. The assumption is made in 6.7.7 [class.temporary] and elsewhere that the storage in which a temporary is created is no longer available for reuse, as specified in 6.7.3 [basic.life], after the lifetime of the temporary has ended, but this assumption is not explicitly stated. One way to make that assumption explicit would be to define a storage duration for temporaries whose lifetime is not extended.

See also issue 2256.




1676. auto return type for allocation and deallocation functions

Section: 6.7.5.5.2  [basic.stc.dynamic.allocation]     Status: drafting     Submitter: Richard Smith     Date: 2013-05-04

Do we need explicit language to forbid auto as the return type of allocation and deallocation functions?

(See also issue 1669.)




2073. Allocating memory for exception objects

Section: 6.7.5.5.2  [basic.stc.dynamic.allocation]     Status: drafting     Submitter: Jonathan Wakely     Date: 2015-01-20

According to 6.7.5.5.2 [basic.stc.dynamic.allocation] paragraph 4,

[Note: In particular, a global allocation function is not called to allocate storage for objects with static storage duration (6.7.5.2 [basic.stc.static]), for objects or references with thread storage duration (6.7.5.3 [basic.stc.thread]), for objects of type std::type_info (7.6.1.8 [expr.typeid]), or for an exception object (14.2 [except.throw]). —end note]

The restriction against allocating exception objects on the heap was intended to ensure that heap exhaustion could be reported by throwing an exception, i.e., that obtaining storage for std::bad_alloc could not fail because the heap was full. However, this implicitly relied on the assumption of a single thread and does not scale to large numbers of threads, so the restriction should be lifted and another mechanism found for guaranteeing the ability to throw std::bad_alloc.

Notes from the February, 2016 meeting:

The prohibition of using an allocation function appears only in a note, although there is a normative reference to the rule in 14.2 [except.throw] paragraph 4. CWG was in favor of retaining the prohibition of using a C++ allocation function for the memory of an exception object, with the implicit understanding that use of malloc would be permitted. The resolution for this issue should delete the note and move the prohibition to normative text in the relevant sections.




2042. Exceptions and deallocation functions

Section: 6.7.5.5.3  [basic.stc.dynamic.deallocation]     Status: drafting     Submitter: Richard Smith     Date: 2014-11-13

According to 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 3,

If a deallocation function terminates by throwing an exception, the behavior is undefined.

This seems to be in conflict with the provisions of 14.5 [except.spec]: if a deallocation function throws an exception that is not allowed by its exception-specification, 14.5 [except.spec] paragraph 10 would appear to give the program defined behavior (calling std::unexpected() or std::terminate()). (Note that 14.5 [except.spec] paragraph 18 explicitly allows an explicit exception-specification for a deallocation function.)




1211. Misaligned lvalues

Section: 6.7.6  [basic.align]     Status: drafting     Submitter: David Svoboda     Date: 2010-10-20

6.7.6 [basic.align] speaks of “alignment requirements,” and 6.7.5.5.2 [basic.stc.dynamic.allocation] requires the result of an allocation function to point to “suitably aligned” storage, but there is no explicit statement of what happens when these requirements are violated (presumably undefined behavior).




1701. Array vs sequence in object representation

Section: 6.8  [basic.types]     Status: drafting     Submitter: Lawrence Crowl     Date: 2013-06-14

According to 6.8 [basic.types] paragraph 4,

The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T).

However, it is not clear that a “sequence” can be indexed, as an array can and as is required for the implementation of memcpy and similar code.

Additional note, November, 2014:

An additional point of concern has been raised as to whether it is appropriate to refer to the constituent bytes of an object as being “objects” themselves, along with the interaction of this specification with copying or not copying parts of the object representation that do not participate in the value representation of the object (“padding” bytes).




1294. Side effects in dynamic/static initialization

Section: 6.9.3.2  [basic.start.static]     Status: drafting     Submitter: Daniel Krügler     Date: 2011-04-08

According to 6.9.3.2 [basic.start.static] paragraph 3,

An implementation is permitted to perform the initialization of a non-local variable with static storage duration as a static initialization even if such initialization is not required to be done statically, provided that

This does not consider side effects of the initialization in this determination, only the values of namespace-scope variables.




1986. odr-use and delayed initialization

Section: 6.9.3.2  [basic.start.static]     Status: drafting     Submitter: Richard Smith     Date: 2014-08-21

The current wording of 6.9.3.2 [basic.start.static] allows deferral of static and thread_local initialization until a variable or function in the containing translation unit is odr-used. This requires implementations to avoid optimizing away the relevant odr-uses. We should consider relaxing the rule to allow for such optimizations.

Proposed resolution (November, 2014):

For a variable V with thread or static storage duration, let X be the set of all variables with the same storage duration as V that are defined in the same translation unit as V. If the observable behavior of the abstract machine (6.7.2 [intro.object]) depends on the value of V through an evaluation E, and E is not sequenced before the end of the initialization of any variable in X, then the end of the initialization of all variables in X is sequenced before E.

There is also a problem (submitted by David Majnemer) if the odr-use occurs in a constexpr context that does not require the variable to be constructed. For example,

  struct A { A(); };
  thread_local A a;

  constexpr bool f() { return &a != nullptr; }

It doesn't seem possible to construct a before its odr-use in f.

There is implementation divergence in the handling of this example.

Notes from the November, 2014 meeting:

CWG determined that the second part of the issue (involving constexpr) is not a defect because the address of an object with thread storage duration is not a constant expression.

Additional note, May, 2015:

CWG failed to indicate where and how to apply the wording in the proposed resolution. In addition, further review has raised concern that “sequenced before” may be the wrong relation to use for the static storage duration case because it implies “in the same thread.”

Notes from the October, 2015 meeting:

The suggested wording is intended to replace some existing wording in 6.9.3.2 [basic.start.static] paragraph 2. CWG affirmed that the correct relationship is “happens before” and not “sequenced before.”




2148. Thread storage duration and order of initialization

Section: 6.9.3.2  [basic.start.static]     Status: drafting     Submitter: Hubert Tong     Date: 2015-06-22

The terms “ordered” and “unordered” initialization are only defined in 6.9.3.2 [basic.start.static] paragraph 2 for entities with static storage duration. They should presumably apply to entities with thread storage duration as well.




2444. Constant expressions in initialization odr-use

Section: 6.9.3.3  [basic.start.dynamic]     Status: drafting     Submitter: Davis Herring     Date: 2019-11-06

According to 6.9.3.3 [basic.start.dynamic] paragraph 3,

A non-initialization odr-use is an odr-use (6.3 [basic.def.odr]) not caused directly or indirectly by the initialization of a non-local static or thread storage duration variable.

Paragraphs 4-6 uses this term to exclude such odr-uses from consideration in determining the point by which a deferred initialization must be performed. A static_assert or a template argument expression can odr-use a variable, but it cannot be said to define any time during execution.

Suggestion: Add constant expression evaluation to the definition. Rename the term to “initializing odr-use” (based on effect rather than cause). Add a note saying that no such odr-use can occur before main begins.

Notes from the February, 2021 teleconference:

CWG agreed with the direction.




170. Pointer-to-member conversions

Section: 7.3.13  [conv.mem]     Status: drafting     Submitter: Mike Stump     Date: 16 Sep 1999

The descriptions of explicit (7.6.1.9 [expr.static.cast] paragraph 9) and implicit (7.3.13 [conv.mem] paragraph 2) pointer-to-member conversions differ in two significant ways:

  1. In a static_cast, a conversion in which the class in the target pointer-to-member type is a base of the class in which the member is declared is permitted and required to work correctly, as long as the resulting pointer-to-member is eventually dereferenced with an object whose dynamic type contains the member. That is, the class of the target pointer-to-member type is not required to contain the member referred to by the value being converted. The specification of implicit pointer-to-member conversion is silent on this question.

    (This situation cannot arise in an implicit pointer-to-member conversion where the source value is something like &X::f, since you can only implicitly convert from pointer-to-base-member to pointer-to-derived-member. However, if the source value is the result of an explicit "up-cast," the target type of the conversion might still not contain the member referred to by the source value.)

  2. The target type in a static_cast is allowed to be more cv-qualified than the source type; in an implicit conversion, however, the cv-qualifications of the two types are required to be identical.

The first difference seems like an oversight. It is not clear whether the latter difference is intentional or not.

(See also issue 794.)




2503. Unclear relationship among name, qualified name, and unqualified name

Section: 7.5.4  [expr.prim.id]     Status: drafting     Submitter: Jens Maurer     Date: 2021-08-04

The phrases “name”, “qualified name” and “unqualified name” are used in various places. It is not clear that all names are either one or the other; there could, in fact, be a third kind of name that is neither.

See also editorial issue 4793.




2473. Parentheses in pseudo-destructor calls

Section: 7.5.4.4  [expr.prim.id.dtor]     Status: drafting     Submitter: Mike Miller     Date: 2020-12-15

According to 7.5.4.4 [expr.prim.id.dtor] paragraph 2,

If the id-expression names a pseudo-destructor, T shall be a scalar type and the id-expression shall appear as the right operand of a class member access (7.6.1.5 [expr.ref]) that forms the postfix-expression of a function call (7.6.1.3 [expr.call]).

This would appear to make the following example ill-formed, because it is the parenthesized expression and not the class member access that is the postfix-expression in the function call:

  typedef int T;
  void f(int* p) {
    (p->~T)();   // Ill-formed?
  }

Presumably this is an oversight.




1973. Which parameter-declaration-clause in a lambda-expression?

Section: 7.5.5  [expr.prim.lambda]     Status: drafting     Submitter: Dinka Ranns     Date: 2014-07-16

According to 7.5.5 [expr.prim.lambda] paragraph 5,

The closure type for a non-generic lambda-expression has a public inline function call operator (12.4.4 [over.call]) whose parameters and return type are described by the lambda-expression's parameter-declaration-clause and trailing-return-type respectively.

This is insufficiently precise because the trailing-return-type might itself contain a parameter-declaration-clause. (The same problem also occurs for generic lambdas later in the same paragraph.)




2086. Reference odr-use vs implicit capture

Section: 7.5.5  [expr.prim.lambda]     Status: drafting     Submitter: Hubert Tong     Date: 2015-02-14

Whether a reference is odr-used or not has less to do with the context where it is named and more to do with its initializer. In particular, 7.5.5 [expr.prim.lambda] bullet 12.2 leads to cases where references that can never be odr-used are implicitly captured:

A lambda-expression with an associated capture-default that does not explicitly capture this or a variable with automatic storage duration (this excludes any id-expression that has been found to refer to an init-capture's associated non-static data member), is said to implicitly capture the entity (i.e., this or a variable) if the compound-statement:

For example, ref should not be captured in the following:

  struct A {
    A() = default;
    A(const A &) = delete;
  } globalA;

  constexpr bool bar(int &, const A &a) { return &a == &globalA; }

  int main() {
    A &ref = globalA;
    [=](auto q) { static_assert(bar(q, ref), ""); }(0);
  }



1521. T{expr} with reference types

Section: 7.6.1.4  [expr.type.conv]     Status: drafting     Submitter: Steve Adamczyk     Date: 2012-07-10

According to 7.6.1.4 [expr.type.conv] paragraph 4,

Similarly, a simple-type-specifier or typename-specifier followed by a braced-init-list creates a temporary object of the specified type direct-list-initialized (9.4.5 [dcl.init.list]) with the specified braced-init-list, and its value is that temporary object as a prvalue.

This wording does not handle the case where T is a reference type: it is not possible to create a temporary object of that type, and presumably the result would be an xvalue, not a prvalue.




2283. Missing complete type requirements

Section: 7.6.1.4  [expr.type.conv]     Status: drafting     Submitter: Richard Smith     Date: 2016-06-27

[Detailed description pending.]




1965. Explicit casts to reference types

Section: 7.6.1.7  [expr.dynamic.cast]     Status: drafting     Submitter: Richard Smith     Date: 2014-07-07

The specification of dynamic_cast in 7.6.1.7 [expr.dynamic.cast] paragraph 2 (and const_cast in 7.6.1.11 [expr.const.cast] is the same) says that the operand of a cast to an lvalue reference type must be an lvalue, so that

  struct A { virtual ~A(); }; A &&make_a();

  A &&a = dynamic_cast<A&&>(make_a());   // ok
  const A &b = dynamic_cast<const A&>(make_a()); // ill-formed

The behavior of static_cast is an odd hybrid:

  struct B : A { }; B &&make_b();
  A &&c = static_cast<A&&>(make_b()); // ok
  const A &d = static_cast<const A&>(make_b()); // ok
  const B &e = static_cast<const B&>(make_a()); // ill-formed

(Binding a const lvalue reference to an rvalue is permitted by 7.6.1.9 [expr.static.cast] paragraph 4 but not by paragraphs 2 and 3.)

There is implementation divergence on the treatment of these examples.

Also, const_cast permits binding an rvalue reference to a class prvalue but not to any other kind of prvalue, which seems like an unnecessary restriction.

Finally, 7.6.1.9 [expr.static.cast] paragraph 3 allows binding an rvalue reference to a class or array prvalue, but not to other kinds of prvalues; those are covered in paragraph 4. This would be less confusing if paragraph 3 only dealt with binding rvalue references to glvalues and left all discussion of prvalues to paragraph 4, which adequately handles the class and array cases as well.

Notes from the May, 2015 meeting:

CWG reaffirmed the status quo for dynamic_cast but felt that const_cast should be changed to permit binding an rvalue reference to types that have associated memory (class and array types).




2243. Incorrect use of implicit conversion sequence

Section: 7.6.1.9  [expr.static.cast]     Status: drafting     Submitter: Hubert Tong     Date: 2016-03-08

The term “implicit conversion sequence” is now used in some non-call contexts (e.g., 7.6.1.9 [expr.static.cast] paragraph 4, 7.6.16 [expr.cond] paragraph 4, 7.6.10 [expr.eq] paragraph 4)) and it is not clear that the current definition is suited for these additional uses. In particular, passing an argument in a function call is always copy-initialization, but some of these contexts require consideration of direct-initialization.

Notes from the December, 2016 teleconference:

The problem is that overload resolution relies on copy initalization and thus does not describe direct initialization. See also issue 1781.




232. Is indirection through a null pointer undefined behavior?

Section: 7.6.2.2  [expr.unary.op]     Status: drafting     Submitter: Mike Miller     Date: 5 Jun 2000

At least a couple of places in the IS state that indirection through a null pointer produces undefined behavior: 6.9.1 [intro.execution] paragraph 4 gives "dereferencing the null pointer" as an example of undefined behavior, and 9.3.4.3 [dcl.ref] paragraph 4 (in a note) uses this supposedly undefined behavior as justification for the nonexistence of "null references."

However, 7.6.2.2 [expr.unary.op] paragraph 1, which describes the unary "*" operator, does not say that the behavior is undefined if the operand is a null pointer, as one might expect. Furthermore, at least one passage gives dereferencing a null pointer well-defined behavior: 7.6.1.8 [expr.typeid] paragraph 2 says

If the lvalue expression is obtained by applying the unary * operator to a pointer and the pointer is a null pointer value (7.3.12 [conv.ptr]), the typeid expression throws the bad_typeid exception (17.7.5 [bad.typeid]).

This is inconsistent and should be cleaned up.

Bill Gibbons:

At one point we agreed that dereferencing a null pointer was not undefined; only using the resulting value had undefined behavior.

For example:

    char *p = 0;
    char *q = &*p;

Similarly, dereferencing a pointer to the end of an array should be allowed as long as the value is not used:

    char a[10];
    char *b = &a[10];   // equivalent to "char *b = &*(a+10);"

Both cases come up often enough in real code that they should be allowed.

Mike Miller:

I can see the value in this, but it doesn't seem to be well reflected in the wording of the Standard. For instance, presumably *p above would have to be an lvalue in order to be the operand of "&", but the definition of "lvalue" in 7.2.1 [basic.lval] paragraph 2 says that "an lvalue refers to an object." What's the object in *p? If we were to allow this, we would need to augment the definition to include the result of dereferencing null and one-past-the-end-of-array.

Tom Plum:

Just to add one more recollection of the intent: I was very happy when (I thought) we decided that it was only the attempt to actually fetch a value that creates undefined behavior. The words which (I thought) were intended to clarify that are the first three sentences of the lvalue-to-rvalue conversion, 7.3.2 [conv.lval]:

An lvalue (7.2.1 [basic.lval]) of a non-function, non-array type T can be converted to an rvalue. If T is an incomplete type, a program that necessitates this conversion is ill-formed. If the object to which the lvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior.

In other words, it is only the act of "fetching", of lvalue-to-rvalue conversion, that triggers the ill-formed or undefined behavior. Simply forming the lvalue expression, and then for example taking its address, does not trigger either of those errors. I described this approach to WG14 and it may have been incorporated into C 1999.

Mike Miller:

If we admit the possibility of null lvalues, as Tom is suggesting here, that significantly undercuts the rationale for prohibiting "null references" -- what is a reference, after all, but a named lvalue? If it's okay to create a null lvalue, as long as I don't invoke the lvalue-to-rvalue conversion on it, why shouldn't I be able to capture that null lvalue as a reference, with the same restrictions on its use?

I am not arguing in favor of null references. I don't want them in the language. What I am saying is that we need to think carefully about adopting the permissive approach of saying that it's all right to create null lvalues, as long as you don't use them in certain ways. If we do that, it will be very natural for people to question why they can't pass such an lvalue to a function, as long as the function doesn't do anything that is not permitted on a null lvalue.

If we want to allow &*(p=0), maybe we should change the definition of "&" to handle dereferenced null specially, just as typeid has special handling, rather than changing the definition of lvalue to include dereferenced nulls, and similarly for the array_end+1 case. It's not as general, but I think it might cause us fewer problems in the long run.

Notes from the October 2003 meeting:

See also issue 315, which deals with the call of a static member function through a null pointer.

We agreed that the approach in the standard seems okay: p = 0; *p; is not inherently an error. An lvalue-to-rvalue conversion would give it undefined behavior.

Proposed resolution (October, 2004):

(Note: the resolution of issue 453 also resolves part of this issue.)

  1. Add the indicated words to 7.2.1 [basic.lval] paragraph 2:

    An lvalue refers to an object or function or is an empty lvalue (7.6.2.2 [expr.unary.op]).
  2. Add the indicated words to 7.6.2.2 [expr.unary.op] paragraph 1:

    The unary * operator performs indirection: the expression to which it is applied shall be a pointer to an object type, or a pointer to a function type and the result is an lvalue referring to the object or function to which the expression points, if any. If the pointer is a null pointer value (7.3.12 [conv.ptr]) or points one past the last element of an array object (7.6.6 [expr.add]), the result is an empty lvalue and does not refer to any object or function. An empty lvalue is not modifiable. If the type of the expression is “pointer to T,” the type of the result is “T.” [Note: a pointer to an incomplete type (other than cv void) can be dereferenced. The lvalue thus obtained can be used in limited ways (to initialize a reference, for example); this lvalue must not be converted to an rvalue, see 7.3.2 [conv.lval].—end note]
  3. Add the indicated words to 7.3.2 [conv.lval] paragraph 1:

    If the object to which the lvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, or if the lvalue is an empty lvalue (7.6.2.2 [expr.unary.op]), a program that necessitates this conversion has undefined behavior.
  4. Change 6.9.1 [intro.execution] as indicated:

    Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer division by zero).

Note (March, 2005):

The 10/2004 resolution interacts with the resolution of issue 73. We added wording to 6.8.3 [basic.compound] paragraph 3 to the effect that a pointer containing the address one past the end of an array is considered to “point to” another object of the same type that might be located there. The 10/2004 resolution now says that it would be undefined behavior to use such a pointer to fetch the value of that object. There is at least the appearance of conflict here; it may be all right, but it at needs to be discussed further.

Notes from the April, 2005 meeting:

The CWG agreed that there is no contradiction between this direction and the resolution of issue 73. However, “not modifiable” is a compile-time concept, while in fact this deals with runtime values and thus should produce undefined behavior instead. Also, there are other contexts in which lvalues can occur, such as the left operand of . or .*, which should also be restricted. Additional drafting is required.

(See also issue 1102.)




901. Deleted operator delete

Section: 7.6.2.8  [expr.new]     Status: drafting     Submitter: John Spicer     Date: 20 May, 2009

It is not clear from 7.6.2.8 [expr.new] whether a deleted operator delete is referenced by a new-expression in which there is no initialization or in which the initialization cannot throw an exception, rendering the program ill-formed. (The question also arises as to whether such a new-expression constitutes a “use” of the deallocation function in the sense of 6.3 [basic.def.odr].)

Notes from the July, 2009 meeting:

The rationale for defining a deallocation function as deleted would presumably be to prevent such objects from being freed. Treating the new-expression as a use of such a deallocation function would mean that such objects could not be created in the first place. There is already an exemption from freeing an object if “a suitable deallocation function [cannot] be found;” a deleted deallocation function should be treated similarly.




1935. Reuse of placement arguments in deallocation

Section: 7.6.2.8  [expr.new]     Status: drafting     Submitter: Hubert Tong     Date: 2014-06-04

The description in 7.6.2.8 [expr.new] paragraph 23 regarding calling a deallocation function following an exception during the initialization of an object resulting from a placement new-expression says,

If a placement deallocation function is called, it is passed the same additional arguments as were passed to the placement allocation function, that is, the same arguments as those specified with the new-placement syntax. If the implementation is allowed to make a copy of any argument as part of the call to the allocation function, it is allowed to make a copy (of the same original value) as part of the call to the deallocation function or to reuse the copy made as part of the call to the allocation function. If the copy is elided in one place, it need not be elided in the other.

This seems curious, as it allows reuse of a parameter object that presumably is destroyed immediately upon the return of the allocation function (but see issue 1880 for a question about the timing of such destructions).

Notes from the November, 2014 meeting:

The resolution for issue 1880 should mostly resolve this issue. The resolution should handle the case in which an object can only be constructed into the parameter object and neither copied nor moved.




2102. Constructor checking in new-expression

Section: 7.6.2.8  [expr.new]     Status: drafting     Submitter: Richard Smith     Date: 2015-03-16

According to 7.6.2.8 [expr.new] paragraph 19,

If the new-expression creates an object or an array of objects of class type, access and ambiguity control are done for the allocation function, the deallocation function (11.11 [class.free]), and the constructor (11.4.5 [class.ctor]).

The mention of “the constructor” here is strange. For the “object of class type” case, access and ambiguity control are done when we perform initialization in paragraph 17, and we might not be calling a constructor anyway (for aggregate initialization). This seems wrong.

For the “array of objects of class type” case, it makes slightly more sense (we need to check the trailing array elements can be default-initialized) but again (a) we aren't necessarily using a constructor, (b) we should say which constructor — and we may need overload resolution to find it, and (c) shouldn't this be part of initialization, so we can distinguish between the cases where we should copy-initialize from {} and the cases where we should default-initialize?




2281. Consistency of aligned operator delete replacement

Section: 7.6.2.8  [expr.new]     Status: drafting     Submitter: Richard Smith     Date: 2016-06-27

We should require that a program that replaces the aligned form of operator delete also replaces the sized+aligned form. We only allow a program to replace the non-sized form without replacing the sized form for backwards compatibility. This is not needed for the alignment feature, which is new.

Notes from the March, 2018 meeting:

CWG concurred with the recommendation.




2013. Pointer subtraction in large array

Section: 7.6.6  [expr.add]     Status: drafting     Submitter: Jason Merrill     Date: 2014-10-02

The common code sequence used by most implementations for pointer subtraction involves subtracting the pointer values to determine the number of bytes and then shifting to scale for the size of the array element. This produces incorrect results when the difference in bytes is larger than can be represented by a ptrdiff_t. For example, assuming a 32-bit ptrdiff_t:

  int *a, *b;
  a = malloc(0x21000000 * sizeof(int));
  b = a + 0x21000000;
  printf("%lx\n", (long)(b - a));

This will typically print e1000000 instead of 21000000.

Getting the right answer would require using a more expensive code sequence. It would be better to make this undefined behavior.




2182. Pointer arithmetic in array-like containers

Section: 7.6.6  [expr.add]     Status: drafting     Submitter: Jonathan Wakely     Date: 2015-10-20

The current direction for issue 1776 (see paper P0137) calls into question the validity of doing pointer arithmetic to address separately-allocated but contiguous objects in a container like std::vector. A related question is whether there should be some allowance made for allowing pointer arithmetic using a pointer to a base class if the derived class is a standard-layout class with no non-static data members. It is possible that std::launder could play a part in the resolution of this issue.

Notes from the February, 2016 meeting:

This issue is expected to be resolved by the resolution of issue 1776. The major problem is when the elements of the vector contain constant or reference members; 6.7.3 [basic.life] paragraph 7 implies that pointer arithmetic leading to such an object produces undefined behavior, and CWG expects this to continue. Some changes to the interface of std::vector may be required, perhaps using std::launder as part of iterator processing.




2023. Composite reference result type of conditional operator

Section: 7.6.16  [expr.cond]     Status: drafting     Submitter: Daniel Krügler     Date: 2014-10-16

The conditional operator converts pointer operands to their composite pointer type (7.6.16 [expr.cond] bullets 6.3 and 6.4). Similar treatment should be afforded to operands of reference type.

See also issue 2018.




2316. Simplifying class conversions in conditional expressions

Section: 7.6.16  [expr.cond]     Status: drafting     Submitter: S. B. Tam     Date: 2016-08-16

According to 7.6.16 [expr.cond] paragraph 4,

Attempts are made to form an implicit conversion sequence from an operand expression E1 of type T1 to a target type related to the type T2 of the operand expression E2 as follows:

It seems that to satisfy the conditions in the first two sub-bullets, T2 must be a class type, in which case T2 is the same as the type described in the third sub-bullet, since the lvalue-to-rvalue conversion does not change types and the other two conversions do not apply to a class type. Thus, this bullet and sub-bullets could be simplified to:

Notes from the August, 2020 teleconference:

This issue and suggested resolution predate the resolution of issue 2321, which added the second sub-bullet (the citation above reflects the wording after adoption of issue 2321), giving the result the cv-qualification of T1 instead of that of T2. The suggested resolution would revert that accepted resolution.




1542. Compound assignment of braced-init-list

Section: 7.6.19  [expr.ass]     Status: drafting     Submitter: Mike Miller     Date: 2012-08-21

The specification of 7.6.19 [expr.ass] paragraph 9 is presumably intended to allow use of a braced-init-list as the operand of a compound assignment operator as well as a simple assignment operator, although the normative wording does not explicitly say so. (The example in that paragraph does include

  complex<double> z;
  z += { 1, 2 };      // meaning z.operator+=({1,2})

for instance, which could be read to imply compound assignment operators for scalar types as well.)

However, the details of how this is to be implemented are not clear. Paragraph 7 says,

The behavior of an expression of the form E1 op = E2 is equivalent to E1 = E1 op E2 except that E1 is evaluated only once.

Applying this pattern literally to a braced-init-list yields invalid code: x += {1} would become x = x + {1}, which is non-syntactic.

Another problem is how to apply the prohibition against narrowing conversions to a compound assignment. For example,

  char c;
  c += {1};

would presumably always be a narrowing error, because after integral promotions, the type of c+1 is int. The similar issue 1078 was classified as "NAD" because the workaround was simply to add a cast to suppress the error; however, there is no place to put a similar cast in a compound assignment.

Notes from the October, 2012 meeting:

The incorrect description of the meaning of a compound assignment with a braced-init-list should be fixed by CWG. The question of whether it makes sense to apply narrowing rules to such assignments is better addressed by EWG.

See also issue 2399.




1255. Definition problems with constexpr functions

Section: 7.7  [expr.const]     Status: drafting     Submitter: Nikolay Ivchenkov     Date: 2011-03-08

The current wording of the Standard is not sufficiently clear regarding the interaction of class scope (which treats the bodies of member functions as effectively appearing after the class definition is complete) and the use of constexpr member functions within the class definition in contexts requiring constant expressions. For example, an array bound cannot use a constexpr member function that relies on the completeness of the class or on members that have not yet been declared, but the current wording does not appear to state that.

Additional note (October, 2013):

This question also affects function return type deduction (the auto specifier) in member functions. For example, the following should presumably be prohibited, but the current wording is not clear:

  struct S {
    static auto f() {
      return 42;
    }
    auto g() -> decltype(f()) {
      return f();
    }
  };



1626. constexpr member functions in brace-or-equal-initializers

Section: 7.7  [expr.const]     Status: drafting     Submitter: Daveed Vandevoorde     Date: 2013-02-19

The Standard should make clear that a constexpr member function cannot be used in a constant expression until its class is complete. For example:

  template<typename T> struct C {
    template<typename T2> static constexpr bool _S_chk() {
      return false;
    }
    static const bool __value = _S_chk<int>();
  }; 

  C<double> c;

Current implementations accept this, although they reject the corresponding non-template case:

  struct C {
    static constexpr bool _S_chk() { return false; }
    static const bool __value = _S_chk();
  };

  C c; 

Presumably the template case should be handled consistently with the non-template case.




2166. Unclear meaning of “undefined constexpr function”

Section: 7.7  [expr.const]     Status: drafting     Submitter: Howard Hinnant     Date: 2015-08-05

According to 7.7 [expr.const] bullet 2.3, an expression is a constant expression unless (among other reasons) it would evaluate

This does not address the question of the point at which a constexpr function must be defined. The intent, in order to allow mutually-recursive constexpr functions, was that the function must be defined prior to the outermost evaluation that eventually results in the invocation, but this is not clearly stated.




2186. Unclear point that “preceding initialization” must precede

Section: 7.7  [expr.const]     Status: drafting     Submitter: Hubert Tong     Date: 2015-10-24

Similar to the concern of issue 2166, the requirement of 7.7 [expr.const] bullet 2.7.1 for

does not specify the point at which the determination of “preceding initialization” is made: is it at the point at which the reference to the variable appears lexically, or is it the point at which the outermost constant evaluation occurs? There is implementation divergence on this point.




1680. Including <initializer_list> for range-based for

Section: 8.6.5  [stmt.ranged]     Status: drafting     Submitter: Richard Smith     Date: 2013-05-13

A simple example like

  int main() {
    int k = 0;
    for (auto x : { 1, 2, 3 })
      k += x;
    return k;
  }

requires that the <initializer_list> header be included, because the expansion of the range-based for involves a declaration of the form

  auto &&__range = { 1, 2, 3 };

and a braced-init-list causes auto to be deduced as a specialization of std::initializer_list. This seems unnecessary and could be eliminated by specifying that __range has an array type for cases like this.

(It should be noted that EWG is considering a proposal to change auto deduction for cases involving braced-init-lists, so resolution of this issue should be coordinated with that effort.)

Notes from the September, 2013 meeting:

CWG felt that this issue should be resolved by using the array variant of the range-based for implementation.




2115. Order of implicit destruction vs release of automatic storage

Section: 8.7  [stmt.jump]     Status: drafting     Submitter: Richard Smith     Date: 2015-04-16

The relative ordering between destruction of automatic variables on exit from a block and the release of the variables' storage is not specified by the Standard: are all the destructors executed first and then the storage released, or are they interleaved?

Notes from the February, 2016 meeting:

CWG agreed that the storage should persist until all destructions are complete, although the “as-if” rule would allow for unobservable optimizations of this ordering.




1223. Syntactic disambiguation and trailing-return-types

Section: 8.9  [stmt.ambig]     Status: drafting     Submitter: Michael Wong     Date: 2010-11-08

Because the restriction that a trailing-return-type can appear only in a declaration with “the single type-specifier auto” (9.3.4.6 [dcl.fct] paragraph 2) is a semantic, not a syntactic, restriction, it does not influence disambiguation, which is “purely syntactic” (8.9 [stmt.ambig] paragraph 3). Consequently, some previously unambiguous expressions are now ambiguous. For example:

struct A {
  A(int *);
  A *operator()(void);
  int B;
};
 
int *p;
typedef struct BB { int C[2]; } *B, C;
 
void foo() {
// The following line becomes invalid under C++0x:
  A (p)()->B;  // ill-formed function declaration
 
// In the following,
// - B()->C is either type-id or class member access expression
// - B()->C[1] is either type-id or subscripting expression
// N3126 subclause 8.2 [dcl.ambig.res] does not mention an ambiguity
// with these forms of expression
  A a(B ()->C);  // function declaration or object declaration
  sizeof(B ()->C[1]);  // sizeof(type-id) or sizeof on an expression
}

Notes from the March, 2011 meeting:

CWG agreed that the presence of auto should be considered in disambiguation, even though it is formally handled semantically rather than syntactically.




2117. Explicit specializations and constexpr function templates

Section: 9.2.6  [dcl.constexpr]     Status: drafting     Submitter: Faisal Vali     Date: 2015-04-26

According to 9.2.6 [dcl.constexpr] paragraph 6,

If no specialization of the template would satisfy the requirements for a constexpr function or constexpr constructor when considered as a non-template function or constructor, the template is ill-formed; no diagnostic required.

This should say “instantiated template specialization” instead of just “specialization” to clarify that an explicit specialization is not in view here.




1348. Use of auto in a trailing-return-type

Section: 9.2.9.6  [dcl.spec.auto]     Status: drafting     Submitter: Richard Smith     Date: 2011-08-16

It is not clear whether the auto specifier can appear in a trailing-return-type.




1670. auto as conversion-type-id

Section: 9.2.9.6  [dcl.spec.auto]     Status: drafting     Submitter: Richard Smith     Date: 2013-04-26

N3690 comment FI 4

The current wording allows something like

  struct S {
    operator auto() { return 0; }
  } s;

If it is intended to be permitted, the details of its handling are not clear. Also, a similar syntax has been discussed as a possible future extension for dealing with proxy types in deduction which, if adopted, could cause confusion.

Additional note, November, 2013:

Doubt was expressed during the 2013-11-25 drafting review teleconference as to the usefulness of this provision. It is therefore being left open for further consideration after C++14 is finalized.

Notes from the February, 2014 meeting:

CWG continued to express doubt as to the usefulness of this construct but felt that if it is permitted, the rules need clarification.




1868. Meaning of “placeholder type”

Section: 9.2.9.6  [dcl.spec.auto]     Status: drafting     Submitter: Dawn Perchik     Date: 2014-02-13

9.2.9 [dcl.type] paragraph 2 describes the auto specifier as “a placeholder for a type to be deduced.” Elsewhere, the Standard refers to the type represented by the auto specifier as a “placeholder type.” This usage has been deemed confusing by some, requiring either a definition of one or both terms or rewording to avoid them.




2476. placeholder-type-specifiers and function declarators

Section: 9.2.9.6.1  [dcl.spec.auto.general]     Status: drafting     Submitter: Davis Herring     Date: 2021-01-29

According to 9.2.9.6.1 [dcl.spec.auto.general] paragraph 3,

The placeholder type can appear with a function declarator in the decl-specifier-seq, type-specifier-seq, conversion-function-id, or trailing-return-type, in any context where such a declarator is valid. If the function declarator includes a trailing-return-type (9.3.4.6 [dcl.fct]), that trailing-return-type specifies the declared return type of the function. Otherwise, the function declarator shall declare a function.

This wording disallows a declaration like

   int f();
   auto (*fp)()=f;

The requirement to declare a function was introduced by the resolution of issue 1892.

Proposed resolution (April, 2021):

Change 9.2.9.6.1 [dcl.spec.auto.general] paragraph 3 as follows:

The placeholder type can appear with a function declarator in the decl-specifier-seq, type-specifier-seq, conversion-function-id, or trailing-return-type, in any context where such a declarator is valid if the function declarator includes a trailing-return-type T (9.3.4.6 [dcl.fct]) or declares a function. If the function declarator includes a trailing-return-type (9.3.4.6 [dcl.fct]), that trailing-return-type specifies In the former case, T is the declared return type of the function. Otherwise, the function declarator shall declare a function. If the declared return type of the a function contains a placeholder type, the return type of the function is deduced from non-discarded return statements, if any, in the body of the function (8.5.2 [stmt.if]).

Additional notes (May, 2021):

It was observed that the proposed resolution above does not address the example in the issue, since fp neither has a trailing-return-type nor declares a function. Presumably another case in which a function declarator with a placeholder return type should be permitted is in the declaration of a variable in which the type is deduced from its initializer.

It was also noted in passing that the deduction in the example is only partial: the parameter-type-list is specified by the declarator and only the return type is deduced from the initializer. Although this example is supported by current implementations, there is implementation divergence in the support of another case in which only part of the variable's type is deduced:

    auto (&ar)[2] = L"a";  // Array bound declared, element type deduced

This issue is related to issue 1892, which prohibited cases like

    std::vector<auto(*)()> v;

The ultimate outcome of the two issues should be:

    int f();
    auto (*fp1)() = f;       // OK
    auto (*fp2)()->int = f;  // OK
    auto (*fp3)()->auto = f; // OK

    template<typename T> struct C { };
    C<auto(*)()> c1;         // Not OK
    C<auto(*)()->int> c2;    // OK
    C<auto(*)()->auto> c3;   // Not OK



1342. Order of initialization with multiple declarators

Section: 9.3  [dcl.decl]     Status: drafting     Submitter: Alberto Ganesh Barbati     Date: 2011-08-11

It is not clear what, if anything, in the existing specification requires that the initialization of multiple init-declarators within a single declaration be performed in declaration order.




1488. abstract-pack-declarators in type-ids

Section: 9.3.2  [dcl.name]     Status: drafting     Submitter: Richard Smith     Date: 2012-03-28

The grammar for type-id in 11.3 [class.name] paragraph 1 has two problems. First, the fact that we allow an abstract-pack-declarator makes some uses of type-id (template arguments, alignment specifiers, exception-specifications) ambiguous: T... could be parsed either as a type-id, including the ellipsis, or as the type-id T with a following ellipsis. There does not appear to be any rule to disambiguate these parses.

The other problem is that we do not allow parentheses in an abstract-pack-declarator, which makes

  template<typename...Ts> void f(Ts (&...)[4]);

ill-formed because (&...)() is not an abstract-pack-declarator. There is implementation variance on this point.




453. References may only bind to “valid” objects

Section: 9.3.4.3  [dcl.ref]     Status: drafting     Submitter: Gennaro Prota     Date: 18 Jan 2004

9.3.4.3 [dcl.ref] paragraph 4 says:

A reference shall be initialized to refer to a valid object or function. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the "object" obtained by dereferencing a null pointer, which causes undefined behavior ...]

What is a "valid" object? In particular the expression "valid object" seems to exclude uninitialized objects, but the response to Core Issue 363 clearly says that's not the intent. This is an example (overloading construction on constness of *this) by John Potter, which I think is supposed to be legal C++ though it binds references to objects that are not initialized yet:

 struct Fun {
    int x, y;
    Fun (int x, Fun const&) : x(x), y(42) { }
    Fun (int x, Fun&) : x(x), y(0) { }
  };
  int main () {
    const Fun f1 (13, f1);
    Fun f2 (13, f2);
    cout << f1.y << " " << f2.y << "\n";
  }

Suggested resolution: Changing the final part of 9.3.4.3 [dcl.ref] paragraph 4 to:

A reference shall be initialized to refer to an object or function. From its point of declaration on (see 6.4.2 [basic.scope.pdecl]) its name is an lvalue which refers to that object or function. The reference may be initialized to refer to an uninitialized object but, in that case, it is usable in limited ways (6.7.3 [basic.life], paragraph 6) [Note: On the other hand, a declaration like this:
    int & ref = *(int*)0;
is ill-formed because ref will not refer to any object or function ]

I also think a "No diagnostic is required." would better be added (what about something like int& r = r; ?)

Proposed Resolution (October, 2004):

(Note: the following wording depends on the proposed resolution for issue 232.)

Change 9.3.4.3 [dcl.ref] paragraph 4 as follows:

A reference shall be initialized to refer to a valid object or function. If an lvalue to which a reference is directly bound designates neither an existing object or function of an appropriate type (9.4.4 [dcl.init.ref]), nor a region of memory of suitable size and alignment to contain an object of the reference's type (6.7.2 [intro.object], 6.7.3 [basic.life], 6.8 [basic.types]), the behavior is undefined. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” empty lvalue obtained by dereferencing a null pointer, which causes undefined behavior. As does not designate an object or function. Also, as described in 11.4.10 [class.bit], a reference cannot be bound directly to a bit-field. ]

The name of a reference shall not be used in its own initializer. Any other use of a reference before it is initialized results in undefined behavior. [Example:

  int& f(int&);
  int& g();

  extern int& ir3;
  int* ip = 0;

  int& ir1 = *ip;     // undefined behavior: null pointer
  int& ir2 = f(ir3);  // undefined behavior: ir3 not yet initialized
  int& ir3 = g();
  int& ir4 = f(ir4);  // ill-formed: ir4 used in its own initializer
end example]

Rationale: The proposed wording goes beyond the specific concerns of the issue. It was noted that, while the current wording makes cases like int& r = r; ill-formed (because r in the initializer does not "refer to a valid object"), an inappropriate initialization can only be detected, if at all, at runtime and thus "undefined behavior" is a more appropriate treatment. Nevertheless, it was deemed desirable to continue to require a diagnostic for obvious compile-time cases.

It was also noted that the current Standard does not say anything about using a reference before it is initialized. It seemed reasonable to address both of these concerns in the same wording proposed to resolve this issue.

Notes from the April, 2005 meeting:

The CWG decided that whether to require an implementation to diagnose initialization of a reference to itself should be handled as a separate issue (504) and also suggested referring to “storage” instead of “memory” (because 6.7.2 [intro.object] defines an object as a “region of storage”).

Proposed Resolution (April, 2005):

(Note: the following wording depends on the proposed resolution for issue 232.)

Change 9.3.4.3 [dcl.ref] paragraph 4 as follows:

A reference shall be initialized to refer to a valid object or function. If an lvalue to which a reference is directly bound designates neither an existing object or function of an appropriate type (9.4.4 [dcl.init.ref]), nor a region of storage of suitable size and alignment to contain an object of the reference's type (6.7.2 [intro.object], 6.7.3 [basic.life], 6.8 [basic.types]), the behavior is undefined. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” empty lvalue obtained by dereferencing a null pointer, which causes undefined behavior. As does not designate an object or function. Also, as described in 11.4.10 [class.bit], a reference cannot be bound directly to a bit-field. ]

Any use of a reference before it is initialized results in undefined behavior. [Example:

  int& f(int&);
  int& g();

  extern int& ir3;
  int* ip = 0;

  int& ir1 = *ip;     // undefined behavior: null pointer
  int& ir2 = f(ir3);  // undefined behavior: ir3 not yet initialized
  int& ir3 = g();
  int& ir4 = f(ir4);  // undefined behavior: ir4 used in its own initializer
end example]

Note (February, 2006):

The word “use” in the last paragraph of the proposed resolution was intended to refer to the description in 6.3 [basic.def.odr] paragraph 2. However, that section does not define what it means for a reference to be “used,” dealing only with objects and functions. Additional drafting is required to extend 6.3 [basic.def.odr] paragraph 2 to apply to references.

Additional note (May, 2008):

The proposed resolution for issue 570 adds wording to define “use” for references.

Note, January, 2012:

The resolution should also probably deal with the fact that the “one-past-the-end” address of an array does not designate a valid object (even if such a pointer might “point to” an object of the correct type, per 6.8.3 [basic.compound]) and thus is not suuitable for the lvalue-to-rvalue conversion.




1001. Parameter type adjustment in dependent parameter types

Section: 9.3.4.6  [dcl.fct]     Status: drafting     Submitter: Jason Merrill     Date: 2009-11-08

According to 9.3.4.6 [dcl.fct] paragraph 5, top-level cv-qualifiers on parameter types are deleted when determining the function type. It is not clear how or whether this adjustment should be applied to parameters of function templates when the parameter has a dependent type, however. For example:

    template<class T> struct A {
       typedef T arr[3];
    };

    template<class T> void f(const typename A<T>::arr) { } // #1

    template void f<int>(const A<int>::arr);

    template <class T> struct B {
       void g(T);
    };

    template <class T> void B<T>::g(const T) { } // #2

If the const in #1 is dropped, f<int> has a parameter type of A* rather than the const A* specified in the explicit instantiation. If the const in #2 is not dropped, we fail to match the definition of B::g to its declaration.

Rationale (November, 2010):

The CWG agreed that this behavior is intrinsic to the different ways cv-qualification applies to array types and non-array types.

Notes, January, 2012:

Additional discussion of this issue arose regarding the following example:

    template<class T> struct A {
      typedef double Point[2];
      virtual double calculate(const Point point) const = 0;
    };

    template<class T> struct B : public A<T> {
      virtual double calculate(const typename A<T>::Point point) const {
        return point[0];
      }
    };

    int main() {
      B<int> b;
      return 0;
    }

The question is whether the member function in B<int> has the same type as that in A<int>: is the parameter-type-list instantiated directly (i.e., using the adjusted types) or regenerated from the individual parameter types?

(See also issue 1322.)




1668. Parameter type determination still not clear enough

Section: 9.3.4.6  [dcl.fct]     Status: drafting     Submitter: Daniel Krügler     Date: 2013-04-25

According to 9.3.4.6 [dcl.fct] paragraph 5,

The type of a function is determined using the following rules. The type of each parameter (including function parameter packs) is determined from its own decl-specifier-seq and declarator. After determining the type of each parameter, any parameter of type “array of T” or “function returning T” is adjusted to be “pointer to T” or “pointer to function returning T,” respectively. After producing the list of parameter types, any top-level cv-qualifiers modifying a parameter type are deleted when forming the function type. The resulting list of transformed parameter types and the presence or absence of the ellipsis or a function parameter pack is the function's parameter-type-list. [Note: This transformation does not affect the types of the parameters. For example, int(*)(const int p, decltype(p)*) and int(*)(int, const int*) are identical types. —end note]

This is not sufficiently clear to specify the intended handling of an example like

  void f(int a[10], decltype(a) *p );

Should the type of p be int(*)[10] or int**? The latter is the intended result, but the phrase “after determining the type of each parameter” makes it sound as if the adjustments are performed after all the parameter types have been determined from the decl-specifier-seq and declarator instead of for each parameter individually.

See also issue 1444.




325. When are default arguments parsed?

Section: 9.3.4.7  [dcl.fct.default]     Status: drafting     Submitter: Nathan Sidwell     Date: 27 Nov 2001

The standard is not precise enough about when the default arguments of member functions are parsed. This leads to confusion over whether certain constructs are legal or not, and the validity of certain compiler implementation algorithms.

9.3.4.7 [dcl.fct.default] paragraph 5 says "names in the expression are bound, and the semantic constraints are checked, at the point where the default argument expression appears"

However, further on at paragraph 9 in the same section there is an example, where the salient parts are

  int b;
  class X {
    int mem2 (int i = b); // OK use X::b
    static int b;
  };
which appears to contradict the former constraint. At the point the default argument expression appears in the definition of X, X::b has not been declared, so one would expect ::b to be bound. This of course appears to violate 6.4.6 [basic.scope.class] paragraph 1(2) "A name N used in a class S shall refer to the same declaration in its context and when reevaluated in the complete scope of S. No diagnostic is required."

Furthermore 6.4.6 [basic.scope.class] paragraph 1(1) gives the scope of names declared in class to "consist not only of the declarative region following the name's declarator, but also of .. default arguments ...". Thus implying that X::b is in scope in the default argument of X::mem2 previously.

That previous paragraph hints at an implementation technique of saving the token stream of a default argument expression and parsing it at the end of the class definition (much like the bodies of functions defined in the class). This is a technique employed by GCC and, from its behaviour, in the EDG front end. The standard leaves two things unspecified. Firstly, is a default argument expression permitted to call a static member function declared later in the class in such a way as to require evaluation of that function's default arguments? I.e. is the following well formed?

  class A {
    static int Foo (int i = Baz ());
    static int Baz (int i = Bar ());
    static int Bar (int i = 5);
 };
If that is well formed, at what point does the non-sensicalness of
  class B {
    static int Foo (int i = Baz ());
    static int Baz (int i = Foo());
  };
become detected? Is it when B is complete? Is it when B::Foo or B::Baz is called in such a way to require default argument expansion? Or is no diagnostic required?

The other problem is with collecting the tokens that form the default argument expression. Default arguments which contain template-ids with more than one parameter present a difficulty in determining when the default argument finishes. Consider,

  template <int A, typename B> struct T { static int i;};
  class C {
    int Foo (int i = T<1, int>::i);
  };
The default argument contains a non-parenthesized comma. Is it required that this comma is seen as part of the default argument expression and not the beginning of another of argument declaration? To accept this as part of the default argument would require name lookup of T (to determine that the '<' was part of a template argument list and not a less-than operator) before C is complete. Furthermore, the more pathological
  class D {
    int Foo (int i = T<1, int>::i);
    template <int A, typename B> struct T {static int i;};
  };
would be very hard to accept. Even though T is declared after Foo, T is in scope within Foo's default argument expression.

Suggested resolution:

Append the following text to 9.3.4.7 [dcl.fct.default] paragraph 8.

The default argument expression of a member function declared in the class definition consists of the sequence of tokens up until the next non-parenthesized, non-bracketed comma or close parenthesis. Furthermore such default argument expressions shall not require evaluation of a default argument of a function declared later in the class.

This would make the above A, B, C and D ill formed and is in line with the existing compiler practice that I am aware of.

Notes from the October, 2005 meeting:

The CWG agreed that the first example (A) is currently well-formed and that it is not unreasonable to expect implementations to handle it by processing default arguments recursively.

Additional notes, May, 2009:

Presumably the following is ill-formed:

    int f(int = f());

However, it is not clear what in the Standard makes it so. Perhaps there needs to be a statement to the effect that a default argument only becomes usable after the complete declarator of which it is a part.

Notes from the August, 2011 meeting:

In addition to default arguments, commas in template argument lists also cause problems in initializers for nonstatic data members:

    struct S {
      int n = T<a,b>(c);  // ill-formed declarator for member b
                          // or template argument?
    };

(This is from #16 of the IssuesFoundImplementingC0x.pdf document on the Bloomington wiki.

Additional notes (August, 2011):

See also issues 1352 and 361.

Notes from the February, 2012 meeting:

It was decided to handle the question of parsing an initializer like T<a,b>(c) (a template-id or two declarators) in this issue and the remaining questions in issue 361. For this issue, a template-id will only be recognized if there is a preceding declaration of a template.

Additional note (November, 2020):

Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.




1580. Default arguments in explicit instantiations

Section: 9.3.4.7  [dcl.fct.default]     Status: drafting     Submitter: Daveed Vandevoorde     Date: 2012-10-29

It is not clear, either from 9.3.4.7 [dcl.fct.default] or 13.9.3 [temp.explicit], whether it is permitted to add a default argument in an explicit instantiation of a function template:

  template<typename T> void f(T, int) { }
  template void f<int>(int, int=0);  // Permitted?

Notes from the April, 2013 meeting:

The intent is to prohibit default arguments in explicit instantiations.




1997. Placement new and previous initialization

Section: 9.4  [dcl.init]     Status: drafting     Submitter: Jason Merrill     Date: 2014-09-08

Given the following example,

  #include <new>

  int main() {
    unsigned char buf[sizeof(int)] = {};
    int *ip = new (buf) int;
    return *ip; // 0 or undefined?
  }

Should the preceding initializsation of the buffer carry over to the value of *ip? According to 9.4 [dcl.init] paragraph 12,

When storage for an object with automatic or dynamic storage duration is obtained, the object has an indeterminate value, and if no initialization is performed for the object, that object retains an indeterminate value until that value is replaced (7.6.19 [expr.ass]).

In this case, no new storage is being obtained for the int object created by the new-expression.




2327. Copy elision for direct-initialization with a conversion function

Section: 9.4  [dcl.init]     Status: drafting     Submitter: Richard Smith     Date: 2016-09-30

Consider an example like:

  struct Cat {};
  struct Dog { operator Cat(); };

  Dog d;
  Cat c(d);

This goes to 9.4 [dcl.init] bullet 17.6.2:

Otherwise, if the initialization is direct-initialization, or if it is copy-initialization where the cv-unqualified version of the source type is the same class as, or a derived class of, the class of the destination, constructors are considered. The applicable constructors are enumerated (12.2.2.4 [over.match.ctor]), and the best one is chosen through overload resolution (12.2 [over.match]). The constructor so selected is called to initialize the object, with the initializer expression or expression-list as its argument(s). If no constructor applies, or the overload resolution is ambiguous, the initialization is ill-formed.

Overload resolution selects the move constructor of Cat. Initializing the Cat&& parameter of the constructor results in a temporary, per 9.4.4 [dcl.init.ref] bullet 5.2.1.2. This precludes the possitiblity of copy elision for this case.

This seems to be an oversight in the wording change for guaranteed copy elision. We should presumably be simultaneously considering both constructors and conversion functions in this case, as we would for copy-initialization, but we'll need to make sure that doesn't introduce any novel problems or ambiguities.




2116. Direct or copy initialization for omitted aggregate initializers

Section: 9.4.2  [dcl.init.aggr]     Status: drafting     Submitter: Richard Smith     Date: 2015-04-22

The Standard does not specify whether the initialization from {} that is done for omitted initializers in aggregate initialization is direct or copy initialization. There is divergence among implementations.

Proposed resolution (May, 2015) [SUPERSEDED]:

This issue is resolved by the resolution of issue 1630.

Notes from the October, 2015 meeting:

CWG agreed that copy initialization should be used; paragraph 7 should have wording similar to paragraph 2. See also issue 1518.




2128. Imprecise rule for reference member initializer

Section: 9.4.2  [dcl.init.aggr]     Status: drafting     Submitter: Richard Smith     Date: 2015-05-19

According to 11.9.3 [class.base.init] paragraph 11,

A temporary expression bound to a reference member from a brace-or-equal-initializer is ill-formed. [Example:

  struct A {
    A() = default;          // OK
    A(int v) : v(v) { }     // OK
    const int& v = 42;      // OK
  };
  A a1;                     // error: ill-formed binding of temporary to reference
  A a2(1);                  // OK, unfortunately

end example]

The rule is intended to apply only if an actual initialization results in such a binding, but it could be read as applying to the declaration of A::v itself. It would be clearer if the restriction were moved into bullet 9.1, e.g.,




2149. Brace elision and array length deduction

Section: 9.4.2  [dcl.init.aggr]     Status: drafting     Submitter: Vinny Romano     Date: 2015-06-25

According to 9.4.2 [dcl.init.aggr] paragraph 4,

An array of unknown size initialized with a brace-enclosed initializer-list containing n initializer-clauses, where n shall be greater than zero, is defined as having n elements (9.3.4.5 [dcl.array]).

However, the interaction of this with brace elision is not clear. For instance, in the example in paragraph 7,

  struct X { int i, j, k = 42; };
  X a[] = { 1, 2, 3, 4, 5, 6 };
  X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } };

a and b are said to have the same value, even though there are six initializer-clauses in the initializer list in a's initializer and two in b's initializer.

Similarly, 13.10.3.2 [temp.deduct.call] paragraph 1 says,

in the P'[N] case, if N is a non-type template parameter, N is deduced from the length of the initializer list

Should that take into account the underlying type of the array? For example,

  template<int N> void f1(const X(&)[N]);
  f1({ 1, 2, 3, 4, 5, 6 }); // Is N deduced to 2 or 6?

  template<int N> void f2(const X(&)[N][2]);
  f2({ 1, 2, 3, 4, 5, 6 }); // Is N deduced to 1 or 6?



1304. Omitted array bound with string initialization

Section: 9.4.3  [dcl.init.string]     Status: drafting     Submitter: Nikolay Ivchenkov     Date: 2011-04-26

The example in 9.4.3 [dcl.init.string] paragraph 1 says,

  char msg[] = "Syntax error on line %s\n";

shows a character array whose members are initialized with a string-literal. Note that because '\n' is a single character and because a trailing '\0' is appended, sizeof(msg) is 25.

However, there appears to be no normative specification of how the size of the array is to be calculated.




1414. Binding an rvalue reference to a reference-unrelated lvalue

Section: 9.4.4  [dcl.init.ref]     Status: drafting     Submitter: Mike Miller     Date: 2011-11-09

Currently an attempt to bind an rvalue reference to a reference-unrelated lvalue succeeds, binding the reference to a temporary initialized from the lvalue by copy-initialization. This appears to be intentional, as the accompanying example contains the lines

    int i3 = 2;
    double&& rrd3 = i3;  // rrd3 refers to temporary with value 2.0

This violates the expectations of some who expect that rvalue references can be initialized only with rvalues. On the other hand, it is parallel with the handling of an lvalue reference-to-const (and is handled by the same wording). It also can add efficiency without requiring existing code to be rewritten: the implicitly-created temporary can be moved from, just as if the call had been rewritten to create a prvalue temporary from the lvalue explicitly.

On a related note, assuming the binding is permitted, the intent of the overload tiebreaker found in 12.2.4.3 [over.ics.rank] paragraph 3 is not clear:

At question is what “to an rvalue” means here. If it is referring to the value category of the initializer itself, before conversions, then the supposed performance advantage of the binding under discussion does not occur because the competing rvalue and lvalue reference overloads will be ambiguous:

    void f(int&&);    // #1
    void f(const int&);
    void g(double d) {
        f(d);         // ambiguous: #1 does not bind to an rvalue
    }

On the other hand, if “to an rvalue” refers to the actual object to which the reference is bound, i.e., to the temporary in the case under discussion, the phrase would seem to be vacuous because an rvalue reference can never bind directly to an lvalue.

Notes from the February, 2012 meeting:

CWG agreed that the binding rules are correct, allowing creation of a temporary when binding an rvalue reference to a non-reference-related lvalue. The phrase “to an rvalue” in 12.2.4.3 [over.ics.rank] paragraph 3 is a leftover from before binding an rvalue reference to an lvalue was prohibited and should be removed. A change is also needed to handle the following case:

    void f(const char (&)[1]);         // #1
    template<typename T> void f(T&&);  // #2
    void g() {
      f("");                           //calls #2, should call #1
    }

Additional note (October, 2012):

Removing “to an rvalue,” as suggested, would have the effect of negating the preference for binding a function lvalue to an lvalue reference instead of an rvalue reference because the case would now fall under the preceding bullet of 12.2.4.3 [over.ics.rank] paragraph 3 bullet 1, sub-bullets 4 and 5:

Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:

Presumably if the suggested resolution is adopted, the order of these two bullets should be inverted.




1827. Reference binding with ambiguous conversions

Section: 9.4.4  [dcl.init.ref]     Status: drafting     Submitter: Hubert Tong     Date: 2014-01-07

In the following case,

  struct A {
    operator int &&() const;
    operator int &&() volatile;
    operator long();
  };

  int main() {
    int &&x = A();
  }

the conversion for direct binding cannot be used because of the ambiguity, so indirect binding is used, which allows the use of the conversion to long in creating the temporary.

Is this intended? There is implementation variation.

Notes from the February, 2014 meeting:

CWG agreed that an ambiguity like this should make the initialization ill-formed instead of falling through to do indirect binding.




2018. Qualification conversion vs reference binding

Section: 9.4.4  [dcl.init.ref]     Status: drafting     Submitter: Richard Smith     Date: 2014-10-07

Qualification conversions are not considered when doing reference binding, which leads to some unexpected results:

  template<typename T> T make();
  struct B {}; struct D : B {};

  const int *p1 = make<int*>();           // ok, qualification conversion
  const int *const *p2 = make<int**>();   // ok, qualification conversion
  const int **p3 = make<int**>();         // error, not type safe

  const int &r1 = make<int&>();           // ok, binds directly
  const int *const &r2 = make<int*&>();   // weird, binds to a temporary
  const int *&r3 = make<int*&>();         // error

  const int &&x1 = make<int&&>();         // ok, binds directly
  const int *const &&x2 = make<int*&&>(); // weird, binds to a temporary
  const int *&&x3 = make<int*&&>();       // weird, binds to a temporary

It might make sense to say that similar types are reference-related and if there is a qualification conversion they are reference-compatible.

See also issue 2023.




1996. Reference list-initialization ignores conversion functions

Section: 9.4.5  [dcl.init.list]     Status: drafting     Submitter: Richard Smith     Date: 2014-09-04

The specification for list-initialization of a reference does not consider the existence of conversion functions. Consequently, the following example is ill-formed:

  struct S { operator struct D &(); } s;
  D &d{s};



2144. Function/variable declaration ambiguity

Section: 9.5.1  [dcl.fct.def.general]     Status: drafting     Submitter: Richard Smith     Date: 2015-06-19

The following fragment,

  int f() {};

is syntactically ambiguous. It could be either a function-definition followed by an empty-declaration, or it could be a simple-declaration whose init-declarator has the brace-or-equal-initializer {}. The same is true of a variable declaration

  int a {};

since function-definition simply uses the term declarator in its production.




1854. Disallowing use of implicitly-deleted functions

Section: 9.5.2  [dcl.fct.def.default]     Status: drafting     Submitter: Richard Smith     Date: 2014-02-11

The resolution of issue 1778 means that whether an explicitly-defaulted function is deleted or not cannot be known until the end of the class definition. As a result, new rules are required to disallow references (in, e.g., decltype) to explicitly-defaulted functions that might later become deleted.

Notes from the June, 2014 meeting:

The approach favored by CWG was to make any reference to an explicitly-defaulted function ill-formed if it occurs prior to the end of the class definition.




1485. Out-of-class definition of member unscoped opaque enumeration

Section: 9.7.1  [dcl.enum]     Status: drafting     Submitter: Richard Smith     Date: 2012-03-26

The scope in which the names of enumerators are entered for a member unscoped opaque enumeration is not clear. According to 9.7.1 [dcl.enum] paragraph 10,

Each enum-name and each unscoped enumerator is declared in the scope that immediately contains the enum-specifier.

In the case of a member opaque enumeration defined outside its containing class, however, it is not clear whether the enumerator names are declared in the class scope or in the lexical scope containing the definition. Declaring them in the class scope would be a violation of 11.4 [class.mem] paragraph 1:

The member-specification in a class definition declares the full set of members of the class; no member can be added elsewhere.

Declaring the names in the lexical scope containing the definition would be contrary to the example in 13.7.2.6 [temp.mem.enum] paragraph 1:

  template<class T> struct A {
    enum E : T;
  };
  A<int> a;
  template<class T> enum A<T>::E : T { e1, e2 };
  A<int>::E e = A<int>::e1;

There also appear to be problems with the rules for dependent types and members of the current instantiation.

Notes from the October, 2012 meeting:

CWG agreed that an unscoped opaque enumeration in class scope should be forbidden.




2131. Ambiguity with opaque-enum-declaration

Section: 9.7.1  [dcl.enum]     Status: drafting     Submitter: Richard Smith     Date: 2015-05-28

The declaration

  enum E;

is ambiguous: it could be either a simple-declaration comprising the elaborated-type-specifier enum E and no init-declarator-list, or it could be an opaque-enum-declaration with an omitted enum-base (both of which are ill-formed, for different reasons).

(See also issue 2363.)




1817. Linkage specifications and nested scopes

Section: 9.11  [dcl.link]     Status: drafting     Submitter: Richard Smith     Date: 2013-12-04

According to Clause 9 [dcl.dcl] paragraph 2,

Unless otherwise stated, utterances in Clause Clause 9 [dcl.dcl] about components in, of, or contained by a declaration or subcomponent thereof refer only to those components of the declaration that are not nested within scopes nested within the declaration.

This contradicts the intent of 9.11 [dcl.link] paragraph 4, which says,

In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names with external linkage, and variable names with external linkage declared within the linkage-specification.

Also, one of the comments in the example in paragraph 4 is inconsistent with the intent:

  extern "C" {
    static void f4(); // the name of the function f4 has
                      // internal linkage (not C language
                      // linkage) and the function's type
                      // has C language linkage.
  }

  extern "C" void f5() {
    extern void f4(); // OK: Name linkage (internal)
                      // and function type linkage (C
                      // language linkage) gotten from
                      // previous declaration.
  }

The language linkage for the block-scope declaration of f4 is presumably determined by the fact that it appears in a C-linkage function, not by the previous declaration.

Proposed resolution (February, 2014):

Change 9.11 [dcl.link] paragraph 4 as follows:

Linkage specifications nest. When linkage specifications nest, the innermost one determines the language linkage. A linkage specification does not establish a scope. A linkage-specification shall occur only in namespace scope (6.4 [basic.scope]). In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names with external linkage, and variable names with external linkage declared within the linkage-specification, including those appearing in scopes nested inside the linkage specification and not inside a nested linkage-specification. [Example:

...

  extern "C" {
    static void f4(); // the name of the function f4 has
                      // internal linkage (not C language
                      // linkage) and the function's type
                      // has C language linkage.
  }

  extern "C" void f5() {
    extern void f4(); // OK: Name linkage (internal)
                      // and function type linkage (C
                      // language linkage) gotten from
                      // previous declaration.; function type
                      // linkage (C language
                      // linkage) gotten
                      // from linkage specification
  }

Additional note, November, 2014:

The issue has been returned to "drafting" status to clarify the circumstances under which a preceding declaration supplies the language linkage for a declaration (for example, not when the declaration uses a typedef, which carries the language linkage, but only when the declaration uses a function declarator).




1706. alignas pack expansion syntax

Section: 9.12.1  [dcl.attr.grammar]     Status: drafting     Submitter: Daveed Vandevoorde     Date: 2013-06-26

The grammar for alignment-specifier in 9.12.1 [dcl.attr.grammar] paragraph 1 is:

where the ellipsis indicates pack expansion. Naively, one would expect that the expansion would result in forms like

    alignas()
    alignas(1, 2)
    alignas(int, double) 

but none of those forms is given any meaning by the current wording. Instead, 13.7.4 [temp.variadic] paragraph 4 says,

In an alignment-specifier (9.12.2 [dcl.align]); the pattern is the alignment-specifier without the ellipsis.

Presumably this means that something like alignas(T...) would expand to something like

    alignas(int) alignas(double)

This is counterintuitive and should be reexamined.

See also messages 24016 through 24021.

Notes from the February, 2014 meeting:

CWG decided to change the pack expansion of alignas so that the type-id or assignment-expression is repeated inside the parentheses and to change the definition of alignas to accept multiple arguments with the same meaning as multiple alignas specifiers.




2223. Multiple alignas specifiers

Section: 9.12.2  [dcl.align]     Status: drafting     Submitter: Mike Herrick     Date: 2016-01-12

According to 9.12.2 [dcl.align] paragraph 4,

The alignment requirement of an entity is the strictest non-zero alignment specified by its alignment-specifiers, if any; otherwise, the alignment-specifiers have no effect.

It is not clear whether this applies to specifiers within a single declaration, or if it is intended to apply to the union of all declarations.

Similarly, paragraph 6 says,

If the defining declaration of an entity has an alignment-specifier, any non-defining declaration of that entity shall either specify equivalent alignment or have no alignment-specifier. Conversely, if any declaration of an entity has an alignment-specifier, every defining declaration of that entity shall specify an equivalent alignment. No diagnostic is required if declarations of an entity have different alignment-specifiers in different translation units.

This only talks about agreement between definitions and non-defining declarations. What about an example where an entity is not defined but is declared with different alignment-specifiers?

  struct alignas(16) A;
  struct alignas(32) A;

If A is not defined, is this, or should it be, ill-formed?

Notes from the February, 2017 meeting:

CWG agreed that the intent of the wording is that the “strictest” requirement is intended to apply to a single declaration, and the requirement for compatibility should apply to all declarations, whether the entity is defined or not.




2443. Meaningless template exports

Section: 10.2  [module.interface]     Status: drafting     Submitter: Davis Herring     Date: 2019-11-09

According to 10.2 [module.interface] paragraph 1, export does not interfere with other definitions; paragraph 3 merely requires that it appear in a declaration that declares at least one name. 13.1 [temp.pre] paragraph 4 prevents using an export-declaration as the declaration of a template-declaration.

With some interpretation, these rules appear to allow various useless constructs like:

   template export void f();
   export template void f();
   export template<> void g(int);
   template<> export void g(int);
   export template<class T> struct trait<T*>;

Simply forbidding them in 10.2 [module.interface] paragraph 3 would also prohibit their appearance in export blocks:

   export {
     template<class> struct A;
     template<class T> struct A<T*>;
   }

It is already the case that the closely-related example

   export {
     template<class T> struct A {A(non_deducible<T>);};
     template<class U> A(U) -> A<find_param<U>>;
   }

is disallowed, although a fix is pending in EWG.

Suggested resolution: Forbid the direct use of the export keyword in these contexts but continue to allow them (and perhaps more) in export { }.

Notes from the February, 2021 teleconference:

CWG agreed with the suggested direction.




1890. Member type depending on definition of member function

Section: 11.4  [class.mem]     Status: drafting     Submitter: Hubert Tong     Date: 2014-03-07

Consider an example like:

  struct A {
    struct B {
      auto foo() { return 0; }
    };
    decltype(B().foo()) x;
  };

There does not appear to be a prohibition of cases like this, where the type of a member depends on the definition of a member function.

(See also issues 1360 and 1397.)




1353. Array and variant members and deleted special member functions

Section: 11.4.5  [class.ctor]     Status: drafting     Submitter: Sean Hunt     Date: 2011-08-16

The specification of when a defaulted special member function is to be defined as deleted sometimes overlooks variant and array members.




1360. constexpr defaulted default constructors

Section: 11.4.5  [class.ctor]     Status: drafting     Submitter: Richard Smith     Date: 2011-08-16

According to 11.4.5 [class.ctor] paragraph 6, a defaulted default constructor is constexpr if the corresponding user-written constructor would satisfy the constexpr requirements. However, the requirements apply to the definition of a constructor, and a defaulted constructor is defined only if it is odr-used, leaving it indeterminate at declaration time whether the defaulted constructor is constexpr or not.

(See also issue 1358.)

Additional notes (February, 2013):

As an example of this issue, consider:

  struct S {
    int i = sizeof(S);
  };

You can't determine the value of the initializer, and thus whether the initializer is a constant expression, until the class is complete, but you can't complete the class without declaring the default constructor, and whether that constructor is constexpr or not depends on whether the member initializer is a constant expression.

A similar issue arises with the following example:

  struct A {
    int x = 37;
    struct B { int x = 37; } b;
    B b2[2][3] = { { } };
  };

This introduces an order dependency that is not specified in the current text: determining whether the default constructor of A is constexpr requires first determining the characteristics of the initializer of B::x and whether B::B() is constexpr or not.

The problem is exacerbated with class templates, since the current direction of CWG is to instantiate member initializers only when they are needed (see issue 1396). For a specific example:

  struct S;
  template<class T> struct X {
    int i = T().i;
  };
  unsigned n = sizeof(X<S>); // Error?
  struct S { int i; };

This also affects determining whether a class template specialization is a literal type or not; presumably getting the right answer to that requires instantiating the class and all its nonstatic data member initializers.

See also issues 1397 and 1594.

Notes from the September, 2013 meeting:

This issue should be resolved together with issue 1397.

Proposed resolution (May, 2014):

Change 11.4.5 [class.ctor] paragraphs 4-5 as follows:

A defaulted default constructor for class X is defined as deleted if:

An implicitly-declared default constructor is constexpr if:

A default constructor is trivial if it is not user-provided and if:

Otherwise, the default constructor is non-trivial.

A default constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used (6.3 [basic.def.odr]) to create an object of its class type (6.7.2 [intro.object]) or when it is explicitly defaulted after its first declaration. The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with no ctor-initializer (11.9.3 [class.base.init]) and an empty compound-statement. If that user-written default constructor would be ill-formed, the program is ill-formed. If that user-written default constructor would satisfy the requirements of a constexpr constructor (9.2.6 [dcl.constexpr]), the implicitly-defined default constructor is constexpr. Before the defaulted default constructor for a class is implicitly defined, all the non-user-provided default constructors for its base classes and its non-static data members shall have been implicitly defined. [Note:...

Additional notes, May, 2014:

The proposed resolution inadvertently allows a defaulted default constructor of a class with virtual bases to be constexpr. It has been updated with a change addressing that oversight and returned to "review" status.

See also issue 1890.




1623. Deleted default union constructor and member initializers

Section: 11.4.5  [class.ctor]     Status: drafting     Submitter: Vinny Romano     Date: 2013-02-15

According to 11.4.5 [class.ctor] paragraph 5,

A defaulted default constructor for class X is defined as deleted if:

Because the presence of a non-static data member initializer is the moral equivalent of a mem-initializer, these rules should probably be modified not to define the generated constructor as deleted when a union member has a non-static data member initializer. (Note the non-normative references in 11.5 [class.union] paragraphs 2-3 and 9.2.9.2 [dcl.type.cv] paragraph 2 that would also need to be updated if this restriction is changed.)

It would also be helpful to add a requirement to 11.5 [class.union] requiring either a non-static data member initializer or a user-provided constructor if all the members of the union have const-qualified types.

On a more general note, why is the default constructor defined as deleted just because a member has a non-trivial default constructor? The union itself doesn't know which member is the active one, and default construction won't initialize any members (assuming no brace-or-equal-initializer). It is up to the “owner” of the union to control the lifetime of the active member (if any), and requiring a user-provided constructor is forcing a design pattern that doesn't make sense. Along the same lines, why is the default destructor defined as deleted just because a member has a non-trivial destructor? I would agree with this restriction if it only applied when the union also has a user-provided constructor.

See also issues 1460, 1562, 1587, and 1621.




1808. Constructor templates vs default constructors

Section: 11.4.5  [class.ctor]     Status: drafting     Submitter: Richard Smith     Date: 2013-11-12

It is not clear when, if ever, a constructor template can be considered to provide a default constructor. For example:

  struct A {
    template<typename ...T> A(T...); // #1
    A(std::initializer_list<long>);  // #2
  };
  A a{};

According to 9.4.5 [dcl.init.list] paragraph 3, A will be value-initialized if it has a default constructor, and there is implementation divergence whether this example calls #1 or #2.

Similarly, for an example like

  struct B {
    template<typename T=int> B(T = 0);
  };

it is not completely clear whether a default constructor should be implicitly declared or not.

More generally, do utterances in the Standard concerning “constructors” also apply to constructor templates?

Notes from the February, 2014 meeting:

One possibility discussed was that we may need to change places that explicitly refer to a default constructor to use overload resolution, similar to the change that was made a few years ago with regard to copy construction vs “copy constructor.” One additional use of “default constructor” is in determining the triviality of a class, but it might be a good idea to remove the concept of a trivial class altogether. This possibility will be explored.

Notes from the February, 2016 meeting:

CWG reaffirmed the direction from the preceding note and also determined that the presence of a constructor template should suppress implicit declaration of a default constructor.




2329. Virtual base classes and generated assignment operators

Section: 11.4.6  [class.copy.assign]     Status: drafting     Submitter: Daveed Vandevoorde     Date: 2016-10-31

An example like the following,

  class A {
  private:
    A& operator=(const A&);
  };

  class B : virtual public A {
  public:
    B& operator = (const B& src);
  };

  class C: public B {
  public:
    void f(const C* psrc) {
      *this = *psrc;
    }
  }; 

is presumably well-formed, even though the copy assignment operator of A is inaccessible in C, because 11.4.6 [class.copy.assign] paragraph 12 says that only direct, not virtual, base class object assignment operators are invoked by the generated assignment operator (although there is implementation divergence on this question).

Should the example also be well-formed if A were a direct virtual base of C? That is, if a direct virtual base also has an indirect derivation path, its direct derivation can be ignored for generated assignment operators.

Possibly relevant to this question is the permission for an implementation to assign virtual base class objects more than once:

It is unspecified whether subobjects representing virtual base classes are assigned more than once by the implicitly-defined copy/move assignment operator.



1977. Contradictory results of failed destructor lookup

Section: 11.4.7  [class.dtor]     Status: drafting     Submitter: Gabriel Dos Reis     Date: 2014-07-21

According to 11.4.7 [class.dtor] paragraph 12,

At the point of definition of a virtual destructor (including an implicit definition (_N4750_.15.8 [class.copy])), the non-array deallocation function is looked up in the scope of the destructor's class (6.5.2 [class.member.lookup]), and, if no declaration is found, the function is looked up in the global scope. If the result of this lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function or a function with a deleted definition (9.5 [dcl.fct.def]), the program is ill-formed. [Note: This assures that a deallocation function corresponding to the dynamic type of an object is available for the delete-expression (11.11 [class.free]). —end note]

However, bullet 5.3 of that section says that such a lookup failure causes the destructor to be defined as deleted, rather than making the program ill-formed. It appears that paragraph 12 was overlooked when deleted functions were added to the language. See also 11.11 [class.free] paragraph 7.




2158. Polymorphic behavior during destruction

Section: 11.4.7  [class.dtor]     Status: drafting     Submitter: Richard Smith     Date: 2015-07-13

Consider the following example:

  #include <stdio.h>
  struct Base {
    Base *p;
    virtual void f() { puts("base"); }
    ~Base() {
      p->f();
    }
  };
  struct Derived : Base {
    Derived() { p = this; }
    void f() { puts("derived"); }
    void g() {
      p->f();
      delete this;
    }
  };
  void h() {
    Derived *p = new Derived;
    p->g();
  }

Should this have defined behavior? On the one hand, the Derived object is in its period of destruction, so the behavior of the p->f() call in the Base destructor should be to call Base::f(). On the other hand, p is a pointer to a Derived object whose lifetime has ended, and the rules in 6.7.3 [basic.life] don't appear to allow the call. (Calling this->f() from the Base destructor would be OK — the question is whether you can do that for a pointer that used to point to the derived object, or if you can only do it for a pointer that was “created” after the dynamic type of the object changed to be Base.)

If the above is valid, it has severe implications for devirtualization. The purpose of 6.7.3 [basic.life] paragraph 7 appears to be to allow an implementation to assume that if it will perform two loads of a constant field (for instance, a const member, the implicit pointer for a reference member, or a vptr), and the two loads are performed on the “same pointer value”, then they load the same value.

Should there be a rule for destructors similar to that of 11.4.5 [class.ctor] paragraph 12?

During the construction of a const object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified.



1726. Declarator operators and conversion function

Section: 11.4.8.3  [class.conv.fct]     Status: drafting     Submitter: James Widman     Date: 2013-08-02

Presumably the following example is intended to be ill-formed:

  struct A {
    (*operator int*());
  };
  A a;
  int *x = a; // Ok?

It is not clear, however, which rule is supposed to reject such a member-declaration.




1283. Static data members of classes with typedef name for linkage purposes

Section: 11.4.9.3  [class.static.data]     Status: drafting     Submitter: Mike Miller     Date: 2011-03-29

According to 11.4.9.3 [class.static.data] paragraph 4,

Unnamed classes and classes contained directly or indirectly within unnamed classes shall not contain static data members.

There is no such restriction on member functions, and there is no rationale for this difference, given that both static data members and member functions can be defined outside a unnamed class with a typedef name for linkage purposes. (Issue 406 acknowledged the lack of rationale by removing the specious note in 11.4.9.3 [class.static.data] that attempted to explain the restriction but left the normative prohibition in place.)

It would be more consistent to remove the restriction for classes with a typedef name for linkage purposes.

Additional note (August, 2012):

It was observed that, since no definition of a const static data member is required if it is not odr-used, there is no reason to prohibit such members in an unnamed class even without a typedef name for linkage purposes.




1721. Diagnosing ODR violations for static data members

Section: 11.4.9.3  [class.static.data]     Status: drafting     Submitter: Mike Miller     Date: 2013-07-31

Describing the handling of static data members with brace-or-equal-initializers, 11.4.9.3 [class.static.data] paragraph 3 says,

The member shall still be defined in a namespace scope if it is odr-used (6.3 [basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer.

The word “shall” implies a required diagnostic, but this is describing an ODR violation (the static data member might be defined in a different translation unit) and thus should be “no diagnostic required.”




2335. Deduced return types vs member types

Section: 11.4.9.3  [class.static.data]     Status: drafting     Submitter: John Spicer     Date: 2017-01-29

It is not clear how an example like the following should be treated:

  template <class ...> struct partition_indices {
    static auto compute_right () {}
    static constexpr auto right = compute_right;
  };
  auto foo () -> partition_indices<>;
  void f() {
    foo();
  };

The initialization of right is in a context that must be done during the initial parse of the class, but the function body of compute_right is not supposed to be evaluated until the class is complete. Current implementations appear to accept the template case but not the equivalent non-template case. It's not clear why those cases should be treated differently.

If you change the example to include a forward dependency in the body of compute_right, e.g.,

  template <int> struct X {};
  template <class T> struct partition_indices {
    static auto compute_right () { return X<I>(); }
    static constexpr auto right = compute_right;
    static constexpr int I = sizeof(T);
  };

  auto foo () -> partition_indices<int>;

  void f() {
    foo();
  }; 

current implementations reject the code, but it's not clear that there is a rationale for the different behavior.

Notes from the March, 2018 meeting:

It was proposed that one direction might be to disallow instantiating member functions while the containing class template is being instantiated. However, overnight implementation experience indicated that this approach breaks seemingly-innocuous and currently-accepted code like:

  template <class T> struct A {
    static constexpr int num() { return 42; }
    int ar[num()];
  };
  A<int> a;

There was divergence of opinion regarding whether the current rules describe the current behavior for the two original examples or whether additional explicit rules are needed to clarify the difference in behavior between template and non-template examples, as well as whether there should be a difference at all..

Notes from the June, 2018 meeting:

The consensus of CWG was to treat templates and classes the same by "instantiating" delayed-parse regions when they are needed instead of at the end of the class.




1404. Object reallocation in unions

Section: 11.5  [class.union]     Status: drafting     Submitter: Nikolay Ivchenkov     Date: 2011-10-19

According to 11.5 [class.union] paragraph 4,

[Note: In general, one must use explicit destructor calls and placement new operators to change the active member of a union. —end note] [Example: Consider an object u of a union type U having non-static data members m of type M and n of type N. If M has a non-trivial destructor and N has a non-trivial constructor (for instance, if they declare or inherit virtual functions), the active member of u can be safely switched from m to n using the destructor and placement new operator as follows:

  u.m.~M();
  new (&u.n)  N;

end example]

This pattern is only “safe” if the original object that is being destroyed does not involve any const-qualified or reference types, i.e., satisfies the requirements of 6.7.3 [basic.life] paragraph 7, bullet 3:

Although paragraph 4 of 11.5 [class.union] is a note and an example, it should at least refer to the lifetime issues described in 6.7.3 [basic.life].

Additional note (October, 2013):

See also issue 1776, which suggests possibly changing the restriction in 6.7.3 [basic.life]. If such a change is made, this issue may become moot.




1702. Rephrasing the definition of “anonymous union”

Section: 11.5  [class.union]     Status: drafting     Submitter: Richard Smith     Date: 2013-06-17

11.5 [class.union] paragraph 5 defines an anonymous union as follows:

A union of the form

is called an anonymous union; it defines an unnamed object of unnamed type.

It is obviously intended that a declaration like

    static union { int i; float f; };

is a declaration of that form (cf paragraph 6, which requires the static keyword for anonymous unions declared in namespace scope). However, it would be clearer if the definition were recast in more descriptive terms, e.g.,

An anonymous union is an unnamed class that is defined with the class-key union in a simple-declaration in which the init-declarator-list is omitted. Such a simple-declaration is treated as if it contained a single declarator declaring an unnamed variable of the union's type.

(Note that this definition would require some additional tweaking to apply to class member anonymous union declarations, since simple-declarations are not included as member-declarations.)

As a related point, it is not clear how the following examples are to be treated, and there is implementation variance on some:

   void f() { thread_local union { int a; }; }
   void g() { extern union { int b; }; }
   thread_local union { int c; }; // static is implied by thread_local
   static thread_local union { int d; };
   static const union { int e = 0; }; // is e const? Clang says yes, gcc says no
   static constexpr union { int f = 0; };



2246. Access of indirect virtual base class constructors

Section: 11.8.3  [class.access.base]     Status: drafting     Submitter: Vinny Romano     Date: 2016-03-08

[Detailed description pending.]

Notes from the December, 2016 teleconference:

The injected-class-name is irrelevant to the example, which is ill-formed. The access should be permitted only of conversion of the this pointer to a pointer to the base class would succeed.




472. Casting across protected inheritance

Section: 11.8.5  [class.protected]     Status: drafting     Submitter: Mike Miller     Date: 16 Jun 2004

Does the restriction in 11.8.5 [class.protected] apply to upcasts across protected inheritance, too? For instance,

    struct B {
        int i;
    };
    struct I: protected B { };
    struct D: I {
        void f(I* ip) {
            B* bp = ip;    // well-formed?
            bp->i = 5;     // aka "ip->i = 5;"
        }
    };

I think the rationale for the 11.8.5 [class.protected] restriction applies equally well here — you don't know whether ip points to a D object or not, so D::f can't be trusted to treat the protected B subobject consistently with the policies of its actual complete object type.

The current treatment of “accessible base class” in 11.8.3 [class.access.base] paragraph 4 clearly makes the conversion from I* to B* well-formed. I think that's wrong and needs to be fixed. The rationale for the accessibility of a base class is whether “an invented public member” of the base would be accessible at the point of reference, although we obscured that a bit in the reformulation; it seems to me that the invented member ought to be considered a non-static member for this purpose and thus subject to 11.8.5 [class.protected].

(See also issues 385 and 471.).

Notes from October 2004 meeting:

The CWG tentatively agreed that casting across protective inheritance should be subject to the additional restriction in 11.8.5 [class.protected].

Proposed resolution (April, 2011)

Change 11.8.3 [class.access.base] paragraph 4 as follows:

A base class B of N is accessible at R, if

[Example:

    class B {
    public:
      int m;
    };

    class S: private B {
      friend class N;
    };
    class N: private S {
      void f() {
        B* p = this;    // OK because class S satisfies the fourth condition
                        // above: B is a base class of N accessible in f() because
                        // B is an accessible base class of S and S is an accessible
                        // base class of N.
      }
    };

    class N2: protected B { };

    class P2: public N2 {
      void f2(N2* n2p) {
        B* bp = n2p;    // error: invented member would be protected and naming
                        // class N2 not the same as or derived from the referencing
                        // class P2
        n2p->m = 0;     // error (cf 11.8.5 [class.protected]) for the same reason
      }
    };

end example]




1883. Protected access to constructors in mem-initializers

Section: 11.8.5  [class.protected]     Status: drafting     Submitter: Daveed Vandevoorde     Date: 2014-02-26

According to 11.8.5 [class.protected] paragraph 1, except when forming a pointer to member,

All other accesses involve a (possibly implicit) object expression (7.6.1.5 [expr.ref]).

It is not clear that this is strictly true for the invocation of a base class constructor from a mem-initializer. A wording tweak may be advisable.




2187. Protected members and access via qualified-id

Section: 11.8.5  [class.protected]     Status: drafting     Submitter: Hubert Tong     Date: 2015-10-16

[Detailed description pending.]




2056. Member function calls in partially-initialized class objects

Section: 11.9.3  [class.base.init]     Status: drafting     Submitter: Richard Smith     Date: 2014-12-11

According to 11.9.3 [class.base.init] paragraph 16,

Member functions (including virtual member functions, 11.7.3 [class.virtual]) can be called for an object under construction. Similarly, an object under construction can be the operand of the typeid operator (7.6.1.8 [expr.typeid]) or of a dynamic_cast (7.6.1.7 [expr.dynamic.cast]). However, if these operations are performed in a ctor-initializer (or in a function called directly or indirectly from a ctor-initializer) before all the mem-initializers for base classes have completed, the result of the operation is undefined.

The example in that paragraph reads, in significant part,

  class B {
  public:
    int f();
  };

  class C {
  public:
    C(int);
  };

  class D : public B, C {
  public:
    D() : C(f())  // undefined: calls member function
                  // but base \tcode{C} not yet initialized
    {}
  };

However, the construction of B, the object for which the member function is being called) has completed its construction, so it is not clear why this should be undefined behavior.

(See also issue 1517.)




1517. Unclear/missing description of behavior during construction/destruction

Section: 11.9.5  [class.cdtor]     Status: drafting     Submitter: Daniel Krügler     Date: 2012-07-07

The current wording of 11.9.5 [class.cdtor] paragraph 4 does not describe the behavior of calling a virtual function in a mem-initializer for a base class, only for a non-static data member. Also, the changes for issue 1202 should have been, but were not, applied to the description of the behavior of typeid and dynamic_cast in paragraphs 5 and 6.

In addition, the resolution of issue 597 allowing the out-of-lifetime conversion of pointers/lvalues to non-virtual base classes, should have been, but were not, applied to paragraph 3.

(See also issue 2056.)

Proposed resolution (August, 2013):

  1. Change 11.9.5 [class.cdtor] paragraph 1 as follows:

  2. For an object with a non-trivial constructor, referring to any non-static member or virtual base class of the object before the constructor begins execution results in undefined behavior. For an object with a non-trivial destructor, referring to any non-static member or virtual base class of the object after the destructor finishes execution results in undefined behavior. [Example:
      struct X { int i; };
      struct Y : X { Y(); };                       // non-trivial
      struct A { int a; };
      struct B : public virtual A { int j; Y y; }; // non-trivial
    
      extern B bobj;
      B* pb = &bobj;                               // OK
      int* p1 = &bobj.a;                           // undefined, refers to base class member
      int* p2 = &bobj.y.i;                         // undefined, refers to member's member
    
      A* pa = &bobj;                               // undefined, upcast to a virtual base class type
      B bobj;                                      // definition of bobj
    
      extern X xobj;
      int* p3 = &xobj.i;                           //OK, X is a trivial class
      X xobj;
    
  3. Change 11.9.5 [class.cdtor] paragraphs 3-6 as follows:

  4. To explicitly or implicitly convert a pointer (a glvalue) referring to an object of class X to a pointer (reference) to a direct or indirect virtual base class B of X, the construction of X and the construction of all of its direct or indirect bases that directly or indirectly derive from for which B is a direct or indirect virtual base shall have started and the destruction of these classes shall not have completed, otherwise the conversion results in undefined behavior. To form a pointer to (or access the value of) a direct non-static member...

    Member functions, including virtual functions (11.7.3 [class.virtual]), can be called during construction or destruction (11.9.3 [class.base.init]). When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class's non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor's or destructor's class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access (7.6.1.5 [expr.ref]) and the object expression refers to the complete object of x or one of that object's base class subobjects but not to x or one of its base class subobjects, the behavior is undefined. The period of construction of an object or subobject whose type is a class type C begins immediately after the construction of all its base class subobjects is complete and concludes when the last constructor of class C exits. The period of destruction of an object or subobject whose type is a class type C begins when the destructor for C begins execution and concludes immediately before beginning the destruction of its base class subobjects. A polymorphic operation is a virtual function call (7.6.1.3 [expr.call]), the typeid operator (7.6.1.8 [expr.typeid]) when applied to a glvalue of polymorphic type, or the dynamic_cast operator (7.6.1.7 [expr.dynamic.cast]) when applied to a pointer to or glvalue of a polymorphic type. A polymorphic operand is the object expression in a virtual function call or the operand of a polymorphic typeid or dynamic_cast.

    During the period of construction or period of destruction of an object or subobject whose type is a class type C (call it x), the effect of performing a polymorphic operation in which the polymorphic operand designates x or a base class subobject thereof is as if the dynamic type of the object were class C. [Footnote: This is true even if C is an abstract class, which cannot be the type of a most-derived object. —end footnote] If a polymorphic operand refers to an object or subobject having class type C before its period of construction begins or after its period of destruction is complete, the behavior is undefined. [Note: This includes the evaluation of an expression appearing in a mem-initializer of C in which the mem-initializer-id designates C or one of its base classes. —end note] [Example:

      struct V {
        V();
        V(int);
        virtual void f();
        virtual void g();
      };
    
      struct A : virtual V {
        virtual void f();
        virtual int h();
        A() : V(h()) { }     // undefined behavior: virtual function h called
                             // before A's period of construction begins
      };
    
      struct B : virtual V {
        virtual void g();
        B(V*, A*);
      };
    
      struct D : A, B {
        virtual void f();
        virtual void g();
        D() : B((A*)this, this) { }
      };
    
      B::B(V* v, A* a) {
        f();                 // calls V::f, not A::f
        g();                 // calls B::g, not D::g
        v->g();              // v is base of B, the call is well-defined, calls B::g
        a->f();              // undefined behavior, a's type not a base of B
        typeid(*this);       // type_info for B
        typeid(*v);          // well-defined: *v has type V, a base of B,
                             // so its period of construction is complete;
                             // yields type_info for B
        typeid(*a);          // undefined behavior: A is not a base of B,
                             // so its period of construction has not begun
        dynamic_cast<B*>(v); // well-defined: v has type V*, V is a base of B,
                             // so its period of construction is complete;
                             // results in this
        dynamic_cast<B*>(a); // undefined behavior: A is not a base of B,
                             // so its period of construction has not begun
      }
    

    end example]

    The typeid operator (7.6.1.8 [expr.typeid]) can be used during construction or destruction (11.9.3 [class.base.init]). When typeid is used in a constructor (including the mem-initializer or brace-or-equal-initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of typeid refers to the object under construction or destruction, typeid yields the std::type_info object representing the constructor or destructor's class. If the operand of typeid refers to the object under construction or destruction and the static type of the operand is neither the constructor or destructor's class nor one of its bases, the result of typeid is undefined.

    dynamic_casts (7.6.1.7 [expr.dynamic.cast]) can be used during construction or destruction (11.9.3 [class.base.init]). When a dynamic_cast is used in a constructor (including the mem-initializer or brace-or-equal-initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of the dynamic_cast refers to the object under construction or destruction, this object is considered to be a most derived object that has the type of the constructor or destructor's class. If the operand of the dynamic_cast refers to the object under construction or destruction and the static type of the operand is not a pointer to or object of the constructor or destructor's own class or one of its bases, the dynamic_cast results in undefined behavior. [Example:

      struct V {
        virtual void f();
      };
    
      struct A : virtual V { };
    
      struct B : virtual V {
        B(V*, A*);
      };
    
      struct D : A, B {
        D() : B((A*)this, this) { }
      };
    
      B::B(V* v, A* a) {
        typeid(*this);       // type_info for B
        typeid(*v);          // well-defined: *v has type V, a base of B
                             // yields type_info for B
        typeid(*a);          // undefined behavior: type A not a base of B
        dynamic_cast<B*>(v); // well-defined: v of type V*, V base of B
                             // results in B*
        dynamic_cast<B*>(a); // undefined behavior,
                             // a has type A*, A not a base of B
    

    end example]




1278. Incorrect treatment of contrived object

Section: 12.2.2.2.2  [over.call.func]     Status: drafting     Submitter: Nikolay Ivchenkov     Date: 2011-03-27

Footnote 127 of 12.2.2.2.2 [over.call.func] paragraph 3 reads,

An implied object argument must be contrived to correspond to the implicit object parameter attributed to member functions during overload resolution. It is not used in the call to the selected function. Since the member functions all have the same implicit object parameter, the contrived object will not be the cause to select or reject a function.

It is not true that “the member functions all have the same implicit object parameter.” This statement does not take into account member functions brought into the class by using-declarations or cv-qualifiers and ref-qualifiers on the non-static member functions:

    struct B
    {
      char f();         // B &
    };

    struct D : B
    {
      using B::f;
      long f();         // D &

      char g() const;   // D const &
      long g();         // D &

      char h() &;       // D &
      long h() &&;      // D &&
    };

    int main()
    {
      // D::f() has better match than B::f()
      decltype(D().f()) *p1 = (long *)0;

      // D::g() has better match than D::g() const
      decltype(D().g()) *p2 = (long *)0;

      // D::h() & is not viable function
      // D::h() && is viable function
      decltype(D().h()) *p3 = (long *)0;
    }

The value category of a contrived object expression is not specified by the rules and, probably, cannot be properly specified in presence of ref-qualifiers, so the statement “the contrived object will not be the cause to select or reject a function” should be normative rather than informative:

    struct X
    {
      static void f(double) {}
      void f(int) & {}
      void f(int) && {}
    };

    int main()
    {
      X::f(0); // ???
    }



2089. Restricting selection of builtin overloaded operators

Section: 12.2.2.3  [over.match.oper]     Status: drafting     Submitter: Hubert Tong     Date: 2015-02-26

The candidates selected by 12.2.2.3 [over.match.oper] include built-in candidates that will result in an error if chosen; this was affirmed by issue 1687. As a result, t+u is ill-formed because it is resolved to the built-in operator+(int*,std::ptrdiff_t), although most implementations do not (yet) agree:

  struct Adaptor { Adaptor(int); };

  struct List { };
  void operator +(List &, Adaptor);

  struct DataType {
    operator int *() const = delete;
    operator List &() const;
  };

  struct Yea;
  struct Nay { int theNaysHaveIt; };

  template <typename T, typename U>
  Yea addCheck(int, T &&t, U &&u, char (*)[sizeof(t + u, 0)] = 0);

  template <typename T, typename U>
  Nay addCheck(void *, T &&t, U &&u);

  void test(DataType &data) { (void)sizeof(addCheck(0, data,
  0.).theNaysHaveIt); }

It might be better to adjust the candidate list in 12.2.2.4 [over.match.ctor] bullet 3.3.3 to allow conversion only on class types and exclude the second standard conversion sequence.




2028. Converting constructors in rvalue reference initialization

Section: 12.2.2.7  [over.match.ref]     Status: drafting     Submitter: Mitsuru Kariya     Date: 2014-10-25

Consider the following example:

  struct T {
    T() {}
    T(struct S&) {}
  };

  struct S {
    operator T() { return T(); }
  };

  int main()
  {
    S s;
    T&& t(s);  // #1
  }

Because there are two possible conversions from S to T, one by conversion function and the other by converting constructor, one might expect that the initialization at #1 would be ambiguous. However, 12.2.2.7 [over.match.ref] (used in the relevant bullet of 9.4.4 [dcl.init.ref], paragraph 5.2.1.2) only deals with conversion functions and ignores converting constructors.

Notes from the November, 2014 meeting:

CWG agreed that 9.4.4 [dcl.init.ref] should be changed to consider converting constructors in this case.




2108. Conversions to non-class prvalues in reference initialization

Section: 12.2.2.7  [over.match.ref]     Status: drafting     Submitter: Hubert Tong     Date: 2015-03-24

In 12.2.2.7 [over.match.ref], candidates that produce non-class prvalues are considered, although that seems to contradict what 9.4.4 [dcl.init.ref] says. See also issue 2077.




2471. Nested class template argument deduction

Section: 12.2.2.9  [over.match.class.deduct]     Status: drafting     Submitter: John Spicer     Date: 2021-01-26

Consider the following example:

  template<class T> struct S {
    template<class U> struct N {
      N(T) {}
      N(T, U) {}
      template<class V> N(V, U) {}
    };
  };
  S<int>::N x{2.0, 1};

The description of CTAD in 12.2.2.9 [over.match.class.deduct] doesn't really specify how nested classes work. If you are supposed to deduce all the enclosing class template arguments, the example is ill-formed because there is no way to deduce T. If you are supposed to consider S<int>::N as having a new constructor template, then it should probably be well-formed.

Notes from the March, 2021 teleconference:

CWG agreed that the intent is to use the partially-instantiated inner template with the explicitly-specified template argument int.




455. Partial ordering and non-deduced arguments

Section: 12.2.4  [over.match.best]     Status: drafting     Submitter: Rani Sharoni     Date: 19 Jan 2004

It's not clear how overloading and partial ordering handle non-deduced pairs of corresponding arguments. For example:

template<typename T>
struct A { typedef char* type; };

template<typename T> char* f1(T, typename A<T>::type);  // #1
template<typename T> long* f1(T*, typename A<T>::type*); // #2

long* p1 = f1(p1, 0); // #3

I thought that #3 is ambiguous but different compilers disagree on that. Comeau C/C++ 4.3.3 (EDG 3.0.3) accepted the code, GCC 3.2 and BCC 5.5 selected #1 while VC7.1+ yields ambiguity.

I intuitively thought that the second pair should prevent overloading from triggering partial ordering since both arguments are non-deduced and has different types - (char*, char**). Just like in the following:

template<typename T> char* f2(T, char*);   // #3
template<typename T> long* f2(T*, char**); // #4

long* p2 = f2(p2, 0); // #5

In this case all the compilers I checked found #5 to be ambiguous. The standard and DR 214 is not clear about how partial ordering handle such cases.

I think that overloading should not trigger partial ordering (in step 12.2.4 [over.match.best]/1/5) if some candidates have non-deduced pairs with different (specialized) types. In this stage the arguments are already adjusted so no need to mention it (i.e. array to pointer). In case that one of the arguments is non-deuced then partial ordering should only consider the type from the specialization:

template<typename T> struct B { typedef T type; };

template<typename T> char* f3(T, T);                   // #7
template<typename T> long* f3(T, typename B<T>::type); // #8

char* p3 = f3(p3, p3); // #9

According to my reasoning #9 should yield ambiguity since second pair is (T, long*). The second type (i.e. long*) was taken from the specialization candidate of #8. EDG and GCC accepted the code. VC and BCC found an ambiguity.

John Spicer: There may (or may not) be an issue concerning whether nondeduced contexts are handled properly in the partial ordering rules. In general, I think nondeduced contexts work, but we should walk through some examples to make sure we think they work properly.

Rani's description of the problem suggests that he believes that partial ordering is done on the specialized types. This is not correct. Partial ordering is done on the templates themselves, independent of type information from the specialization.

Notes from October 2004 meeting:

John Spicer will investigate further to see if any action is required.

(See also issue 885.)




2319. Nested brace initialization from same type

Section: 12.2.4.2  [over.best.ics]     Status: drafting     Submitter: Richard Smith     Date: 2016-09-06

[Detailed description pending.]

Notest from the July, 2017 meeting:

CWG agreed that the a2 example should be ill-formed but that the a1 example must remain for C compatibility.




2077. Overload resolution and invalid rvalue-reference initialization

Section: 12.2.4.2.5  [over.ics.ref]     Status: drafting     Submitter: Richard Smith     Date: 2015-01-29

The resolution of issue 1604 broke the following example:

  struct A {};
  struct B { operator const A() const; };
  void f(A const&);
  void f(A&&);

  int main() {
    B a;
    f(a);
  }

Overload resolution selects the A&& overload, but then initialization fails. This seems like a major regression; we're now required to reject

   std::vector<A> va;
   B b;
   va.push_back(b);

Should we update 12.2.4.2.5 [over.ics.ref] to match the changes made to 9.4.4 [dcl.init.ref]?

See also issue 2108.




1536. Overload resolution with temporary from initializer list

Section: 12.2.4.2.6  [over.ics.list]     Status: drafting     Submitter: Mike Miller     Date: 2012-08-14

In determining the implicit conversion sequence for an initializer list argument passed to a reference parameter, the intent is that a temporary of the appropriate type will be created and bound to the reference, as reflected in 12.2.4.2.6 [over.ics.list] paragraph 5:

Otherwise, if the parameter is a reference, see 12.2.4.2.5 [over.ics.ref]. [Note: The rules in this section will apply for initializing the underlying temporary for the reference. —end note]

However, 12.2.4.2.5 [over.ics.ref] deals only with expression arguments, not initializer lists:

When a parameter of reference type binds directly (9.4.4 [dcl.init.ref]) to an argument expression, the implicit conversion sequence is the identity conversion, unless the argument expression has a type that is a derived class of the parameter type, in which case the implicit conversion sequence is a derived-to-base Conversion (12.2.4.2 [over.best.ics])... If the parameter binds directly to the result of applying a conversion function to the argument expression, the implicit conversion sequence is a user-defined conversion sequence (12.2.4.2.3 [over.ics.user]), with the second standard conversion sequence either an identity conversion or, if the conversion function returns an entity of a type that is a derived class of the parameter type, a derived-to-base Conversion.

When a parameter of reference type is not bound directly to an argument expression, the conversion sequence is the one required to convert the argument expression to the underlying type of the reference according to 12.2.4.2 [over.best.ics]. Conceptually, this conversion sequence corresponds to copy-initializing a temporary of the underlying type with the argument expression. Any difference in top-level cv-qualification is subsumed by the initialization itself and does not constitute a conversion.

(Note in particular that the reference binding refers to 9.4.4 [dcl.init.ref], which also does not handle initializer lists, and not to 9.4.5 [dcl.init.list].)

Either 12.2.4.2.5 [over.ics.ref] needs to be revised to handle binding references to initializer list arguments or 12.2.4.2.6 [over.ics.list] paragraph 5 needs to be clearer on how the expression specification is intended to be applied to initializer lists.




2492. Comparing user-defined conversion sequences in list-initialization

Section: 12.2.4.2.6  [over.ics.list]     Status: drafting     Submitter: Jim X     Date: 2021-01-11

Consider the following example:

  #include <initializer_list>
  struct A{
    operator short(){
      return 0;
    }
  };
  struct B{
    operator bool(){
      return 0;
    }
  };
  void fun(std::initializer_list<int>){}
  void fun(std::initializer_list<bool>){}
  int main(){
    fun({A{},B{}});
  }

According to 12.2.4.2.6 [over.ics.list] paragraph 6,

Otherwise, if the parameter type is std::initializer_list<X> and all the elements of the initializer list can be implicitly converted to X, the implicit conversion sequence is the worst conversion necessary to convert an element of the list to X, or if the initializer list has no elements, the identity conversion. This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor.

In this example, all of the conversions from list elements to the initializer_list template argument type are user-defined conversions. According to 12.2.4.3 [over.ics.rank] bullet 3.3,

User-defined conversion sequence U1 is a better conversion sequence than another user-defined conversion sequence U2 if they contain the same user-defined conversion function or constructor or they initialize the same class in an aggregate initialization and in either case the second standard conversion sequence of U1 is better than the second standard conversion sequence of U2.

Since in both cases the two elements of the initializer-list argument involve different user-defined conversion functions, the two user-defined conversion sequences for the elements cannot be distinguished, so the determination of the “worst conversion” for the two candidates does not consider the second standard conversion sequence. This presumably makes it impossible to distinguish the conversion sequences for the two candidates in the function call, making the call ambiguous.

However, there is implementation divergence on the handling of this example, with g++ reporting an ambiguity and clang, MSVC, and EDG calling the int overload, presumably on the basis that short->int is a promotion while short->bool is a conversion.

Notes from the August, 2021 teleconference:

CWG agreed with the reasoning expressed in the analysis, that conversions involving different user-defined conversion functions cannot be compared, and thus the call is ambiguous. The use of the phrase “worst conversion” is insufficiently clear, however, and requires definition.

Proposed resolution, August, 2021:

Change 12.2.4.2.6 [over.ics.list] paragraphs 5 and 6 as follows:

Otherwise, if the parameter type is std::initializer_list<X> and either the initializer list is empty or all the elements of the initializer list can be implicitly converted to X, the implicit conversion sequence is the worst conversion worst conversion necessary to convert an element of the list to X, or if defined as follows. If the initializer list has no elements, the worst conversion is the identity conversion. Otherwise, the worst conversion is an implicit conversion sequence for a list element that is not better than any other implicit conversion sequence required by list elements, compared as described in 12.2.4.3 [over.ics.rank]. If more than one implicit conversion sequence satisfies this criterion, then if they are user-defined conversion sequences that do not all contain the same user-defined conversion function or constructor, the worst conversion sequence is the ambiguous conversion sequence (12.2.4.2.1 [over.best.ics.general]); otherwise, it is unspecified which of those conversion sequences is chosen as worst. This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor. [Example 2:

  void f(std::initializer_list<int>);
  f( {} );        // OK: f(initializer_list<int>) identity conversion
  f( {1,2,3} );   // OK: f(initializer_list<int>) identity conversion
  f( {'a','b'} ); // OK: f(initializer_list<int>) integral promotion
  f( {1.0} );     // error: narrowing

  struct A {
    A(std::initializer_list<double>);            // #1
    A(std::initializer_list<complex<double>>);   // #2
    A(std::initializer_list<std::string>);       // #3
  };
  A a{ 1.0,2.0 };        // OK, uses #1

  void g(A);
  g({ "foo", "bar" });   // OK, uses #3

  typedef int IA[3];
  void h(const IA&);
  h({ 1, 2, 3 });        // OK: identity conversion

  void x(std::initializer_list<int>);
  void x(std::initializer_list<bool>);
  struct S1 { operator short(); };
  struct S2 { operator bool(); };
  void y() {
    x({S1{}, S2{}});   // error: ambiguous. The ICSes for each list element are indistinguishable because
                       // they do not contain the same conversion function, so the worst conversion is
                       // the ambiguous conversion sequence.
  }

end example]

Otherwise, if the parameter type is “array of N X ” or “array of unknown bound of X”, if there exists an implicit conversion sequence from each element of the initializer list (and from {} in the former case if N exceeds the number of elements in the initializer list) to X, the implicit conversion sequence is the worst such implicit conversion sequence conversion necessary to convert an element of the list (including, if there are too few list elements, {}) to X, determined as described above for a std::initializer_list<X> with a non-empty initializer list.




2110. Overload resolution for base class conversion and reference/non-reference

Section: 12.2.4.3  [over.ics.rank]     Status: drafting     Submitter: Alexander Kulpin     Date: 2015-03-27

There are overload tiebreakers that order reference/nonreference and base/derived conversions, but how they relate is not specified. For example:

  struct A { A(); };
  struct B : A {};
  struct C : B {};

  void f1(B&);
  void f1(A);

  void f2(B);
  void f2(A&);

  int main()
  {
     C v;
     f1(v); // all compilers choose f1(B&)
     f2(v); // all compilers choose f2(B)
  }

The Standard does not appear to specify what happens in this case.




1989. Insufficient restrictions on parameters of postfix operators

Section: 12.4  [over.oper]     Status: drafting     Submitter: Richard Smith     Date: 2014-08-30

According to 12.4.7 [over.inc] paragraph 1,

The user-defined function called operator++ implements the prefix and postfix ++ operator. If this function is a non-static member function with no parameters, or a non-member function with one parameter, it defines the prefix increment operator ++ for objects of that type. If the function is a non-static member function with one parameter (which shall be of type int) or a non-member function with two parameters (the second of which shall be of type int), it defines the postfix increment operator ++ for objects of that type.

According to 12.4 [over.oper] paragraph 8,

Operator functions cannot have more or fewer parameters than the number required for the corresponding operator, as described in the rest of this subclause.

This does not rule out an operator++ with more than two parameters, however, since there is no corresponding operator.

One possibility might be to add a sentence like,

A function named operator++ shall declare either a prefix or postfix increment operator.



1444. Type adjustments of non-type template parameters

Section: 13.2  [temp.param]     Status: drafting     Submitter: Johannes Schaub     Date: 2012-01-15

The type adjustment of template non-type parameters described in 13.2 [temp.param] paragraph 8 appears to be underspecified. For example, implementations vary in their treatment of

  template<typename T, T[T::size]> struct A {};
  int dummy;
  A<int, &dummy> a;

and

  template<typename T, T[1]> struct A;
  template<typename T, T*> struct A {};
  int dummy;
  A<int, &dummy> a;

See also issues 1322 and 1668.

Additional note, February, 2021:

See the discussion regarding top-level cv-qualifiers on template parameters when determining the type in this compiler bug report.




1635. How similar are template default arguments to function default arguments?

Section: 13.2  [temp.param]     Status: drafting     Submitter: Richard Smith     Date: 2013-03-06

Default function arguments are instantiated only when needed. Is the same true of default template arguments? For example, is the following well-formed?

  #include <type_traits>

  template<class T>
  struct X {
    template<class U = typename T::type>
    static void foo(int){}
    static void foo(...){}
  };

  int main(){
    X<std::enable_if<false>>::foo(0);
  }

Also, is the effect on lookup the same? E.g.,

  struct S {
    template<typename T = U> void f();
    struct U {};
  };

Additional note (November, 2020):

Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.




2450. braced-init-list as a template-argument

Section: 13.3  [temp.names]     Status: drafting     Submitter: Marek Polacek     Date: 2019-01-07

Since non-type template parameters can now have class types, it would seem to make sense to allow a braced-init-list as a template-argument, but the grammar does not permit it.

See also issues 2049 and 2459.




2043. Generalized template arguments and array-to-pointer decay

Section: 13.4.3  [temp.arg.nontype]     Status: drafting     Submitter: Richard Smith     Date: 2014-11-13

According to 13.4.3 [temp.arg.nontype] paragraph 1 (newly revised by the adoption of paper N4268),

For a non-type template-parameter of reference or pointer type, the value of the constant expression shall not refer to (or for a pointer type, shall not be the address of):

This change breaks an example like

   template<int *p> struct X {};
   int arr[32];
   X<arr> x;

because the array-to-pointer decay produces a pointer to the first element, which is a subobject.

Suggested resolution:

Change the referenced bullet to read:

Note that this resolution also allows an example like

    template<char &p> struct S { };
    char arr[2];
    S<arr[0]> s_arr;

which may not be exactly what we want.




2049. List initializer in non-type template default argument

Section: 13.4.3  [temp.arg.nontype]     Status: drafting     Submitter: Ville Voutilainen     Date: 2014-11-20

According to 13.4.3 [temp.arg.nontype] paragraph 1,

A template-argument for a non-type template-parameter shall be a converted constant expression (7.7 [expr.const]) of the type of the template-parameter.

This does not permit an example like:

  template <int* x = {}> struct X {};

which seems inconsistent.

See also issues 2450 and 2459.




2459. Template parameter initialization

Section: 13.4.3  [temp.arg.nontype]     Status: drafting     Submitter: Davis Herring     Date: 2020-09-21

The initialization of template parameters is severely underspecified. The only descriptions in the existing wording that apply are that the argument is “[converted] to the type of the template-parameter” (13.6 [temp.type] bullet 1.3) and, in 13.4.3 [temp.arg.nontype] paragraph 2,

A template-argument for a non-type template-parameter shall be a converted constant expression (7.7 [expr.const]) of the type of the template-parameter.

This omission is particularly important for template parameters of class type with lvalue template parameter objects whose addresses can be examined during construction. See also issue 2450.

Suggested resolution:

To avoid address-based paradoxes, template arguments for a template parameter of class type C that are not of that type or a derived type are converted to C to produce an exemplar. No restrictions are imposed on the conversion from a template argument to a constructor parameter, since explicit and list-initialization may already be used to limit conversions in a similar fashion. Template arguments that are of such a type are used directly as the exemplar (potentially after a materialization conversion); the effect is as if the template parameter were of type const C& (except that temporaries are allowed). (In the latter case, we must impose some restrictions on glvalue template parameters to interpret them.) Each exemplar is used to copy-initialize the template parameter object to which it is (to be) template-argument-equivalent; the initialization is required to produce a template-argument-equivalent value. The multiple initializations of the template parameter object are (required to be) all equivalent and produce no side effects, so it is unobservable which happen.




2057. Template template arguments with default arguments

Section: 13.4.4  [temp.arg.template]     Status: drafting     Submitter: Jonathan Caves     Date: 2014-12-12

It is not clear how to handle an example like:

  template<typename T1, typename T2 = char> class A { };

  template<template<typename... T> class X> class S {
    X<int> x;
  };

  S<A> a;

Issue 184 dealt with a similar question but did so in the era before variadic templates. This usage should be permitted in modern C++.

Notes from the February, 2016 meeting:

CWG felt that this usage should be permitted, but only for template template parameters with a parameter pack.. Furthermore, if the template template parameter has a default argument followed by a parameter pack, the parameter's default argument would be used, followed by any remaining default arguments from the template template argument.




2037. Alias templates and template declaration matching

Section: 13.6  [temp.type]     Status: drafting     Submitter: Richard Smith     Date: 2014-11-06

For the following example,

  template<int N> struct A {};
  template<short N> using B = A<N>;
  template<int N> void f(B<N>) {} // #1
  template<int N> void f(A<N>) {} // #2

There is implementation variance as to whether there is one f or two. As with previously-discussed cases, these have different SFINAE effects, perhaps equivalent but not functionally equivalent. Should the argument to #1 be treated as something like A<(int)(short)N> and not just A<N>.

See also issues 1668 and 1979.




1730. Can a variable template have an unnamed type?

Section: 13.7  [temp.decls]     Status: drafting     Submitter: Larisse Voufo     Date: 2013-08-05

Is it permitted for a variable template to have an unnamed type?




1432. Newly-ambiguous variadic template expansions

Section: 13.7.4  [temp.variadic]     Status: drafting     Submitter: Daniel Krügler     Date: 2011-12-17

With the new core rules in regard to variadic pack expansions the library specification of the traits template common_type is now broken, the reason is that it is defined as a series of specializations of the primary template

    template <class ...T>
    struct common_type;

The broken one is this pair:

  template <class T, class U>
  struct common_type<T, U> {
   typedef decltype(true ? declval<T>() : declval<U>()) type;
  };

  template <class T, class U, class... V>
  struct common_type<T, U, V...> {
   typedef typename common_type<typename common_type<T, U>::type, V...>::type type;
  };

With the new rules both specializations would now be ambiguous for an instantiation like common_type<X, Y>.

(See also issue 1395.)

Notes from the October, 2012 meeting:

It is possible that _N4868_.13.7.6.3 [temp.class.order] may resolve this problem.




1286. Equivalence of alias templates

Section: 13.7.8  [temp.alias]     Status: drafting     Submitter: Gabriel Dos Reis     Date: 2011-04-03

Issue 1244 was resolved by changing the example in 13.6 [temp.type] paragraph 1 from

  template<template<class> class TT> struct X { };
  template<class> struct Y { };
  template<class T> using Z = Y<T>;
  X<Y> y;
  X<Z> z;

to

  template<class T> struct X { };
  template<class> struct Y { };
  template<class T> using Z = Y<T>;
  X<Y<int> > y;
  X<Z<int> > z;

In fact, the original intent was that the example should have been correct as written; however, the normative wording to make it so was missing. The current wording of 13.7.8 [temp.alias] deals only with the equivalence of a specialization of an alias template with the type-id after substitution. Wording needs to be added specifying under what circumstances an alias template itself is equivalent to a class template.

Proposed resolution (September, 2012):

  1. Add the following as a new paragraph following 13.7.8 [temp.alias] paragraph 2:

  2. When the type-id in the declaration of alias template (call it A) consists of a simple-template-id in which the template-argument-list consists of a list of identifiers naming each template-parameter of A exactly once in the same order in which they appear in A's template-parameter-list, the alias template is equivalent to the template named in the simple-template-id (call it T) if A and T have the same number of template-parameters. [Footnote: This rule is transitive: if an alias template A is equivalent to another alias template B that is equivalent to a class template C, then A is also equivalent to C, and A and B are also equivalent to each other. —end footnote] [Example:

      template<typename T, U = T> struct A;
    
      template<typename V, typename W>
        using B = A<V, W>;                // equivalent to A
    
      template<typename V, typename W>
        using C = A<V>;                   // not equivalent to A:
                                          // not all parameters used
    
      template<typename V>
        using D = A<V>;                   // not equivalent to A:
                                          // different number of parameters
    
      template<typename V, typename W>
        using E = A<W, V>;                // not equivalent to A:
                                          // template-arguments in wrong order
    
      template<typename V, typename W = int>
        using F = A<V, W>;                // equivalent to A:
                                          // default arguments not considered
    
      template<typename V, typename W>
        using G = A<V, W>;                // equivalent to A and B
    
      template<typename V, typename W>
        using H = E<V, W>;                // equivalent to E
    
      template<typename V, typename W>
        using I = A<V, typename W::type>; // not equivalent to A:
                                          // argument not identifier
    
    

    end example]

  3. Change 13.6 [temp.type] paragraph 1 as follows:

  4. Two template-ids refer to the same class or function if

    [Example:

    ...declares x2 and x3 to be of the same type. Their type differs from the types of x1 and x4.

      template<class T template<class> class TT> struct X { };
      template<class> struct Y { };
      template<class T> using Z = Y<T>;
      X<Y<int> Y> y;
      X<Z<int> Z> z;
    

    declares y and z to be of the same type. —end example]

Additional note, November, 2014:

Concern has been expressed over the proposed resolution with regard to its handling of default template arguments that differ between the template and its alias, e.g.,

   template<typename T, typename U = int> struct A {};
   template<typename T, typename U = char> using B = A<T, U>;
   template<template<typename...> typename C> struct X { C<int> c; };

Notes from the May, 2015 meeting:

See also issue 1979, which CWG is suggesting to be resolved by defining a “simple” alias, one in which the SFINAE conditions are the same as the referenced template and that uses all template parameters.




1430. Pack expansion into fixed alias template parameter list

Section: 13.7.8  [temp.alias]     Status: drafting     Submitter: Jason Merrill     Date: 2011-12-13

Originally, a pack expansion could not expand into a fixed-length template parameter list, but this was changed in N2555. This works fine for most templates, but causes issues with alias templates.

In most cases, an alias template is transparent; when it's used in a template we can just substitute in the dependent template arguments. But this doesn't work if the template-id uses a pack expansion for non-variadic parameters. For example:

    template<class T, class U, class V>
    struct S {};

    template<class T, class V>
    using A = S<T, int, V>;

    template<class... Ts>
    void foo(A<Ts...>);

There is no way to express A<Ts...> in terms of S, so we need to hold onto the A until we have the Ts to substitute in, and therefore it needs to be handled in mangling.

Currently, EDG and Clang reject this testcase, complaining about too few template arguments for A. G++ did as well, but I thought that was a bug. However, on the ABI list John Spicer argued that it should be rejected.

(See also issue 1558.)

Notes from the October, 2012 meeting:

The consensus of CWG was that this usage should be prohibited, disallowing use of an alias template when a dependent argument can't simply be substituted directly into the type-id.

Additional note, April, 2013:

For another example, consider:

  template<class... x> class list{};
  template<class a, class... b> using tail=list<b...>;
  template <class...T> void f(tail<T...>);

  int main() {
    f<int,int>({});
  }

There is implementation variance in the handling of this example.




1554. Access and alias templates

Section: 13.7.8  [temp.alias]     Status: drafting     Submitter: Jason Merrill     Date: 2012-09-17

The interaction of alias templates and access control is not clear from the current wording of 13.7.8 [temp.alias]. For example:

  template <class T> using foo = typename T::foo;

  class B {
    typedef int foo;
    friend struct C;
  };

  struct C {
    foo<B> f;    // Well-formed?
  };

Is the substitution of B::foo for foo<B> done in the context of the befriended class C, making the reference well-formed, or is the access determined independently of the context in which the alias template specialization appears?

If the answer to this question is that the access is determined independently from the context, care must be taken to ensure that an access failure is still considered to be “in the immediate context of the function type” (13.10.3 [temp.deduct] paragraph 8) so that it results in a deduction failure rather than a hard error.

Notes from the October, 2012 meeting:

The consensus of CWG was that instantiation (lookup and access) for alias templates should be as for other templates, in the definition context rather than in the context where they are used. They should still be expanded immediately, however.

Additional note (February, 2014):

A related problem is raised by the definition of std::enable_if_t (20.15.3 [meta.type.synop]):

  template <bool b, class T = void>
  using enable_if_t = typename enable_if<b,T>::type;

If b is false, there will be no type member. The intent is that such a substitution failure is to be considered as being “in the immediate context” where the alias template specialization is used, but the existing wording does not seem to accomplish that goal.

Additional note, November, 2014:

Concern has been expressed that the intent to analyze access in the context of the alias template definition is at odds with the fact that friendship cannot be granted to alias templates; if it could, the access violation in the original example could be avoided by making foo a friend of class B, but that is not possible.

Additional node, February, 2016:

The issue has been returned to "open" status to facilitate further discussion by CWG as to whether the direction in the October, 2012 note is still desirable.

Notes from the February, 2016 meeting:

CWG reaffirmed the direction described in the October, 2012 note above. With regard to the November, 2014 note regarding granting of friendship, it was observed that the same problem occurs with enumerators, which might refer to inaccessible names in the enumerator volue. The solution in both cases is to embed the declaration in a class and grant the class friendship. See issue 1844, dealing with the definition of “immediate context.”




1979. Alias template specialization in template member definition

Section: 13.7.8  [temp.alias]     Status: drafting     Submitter: Gabriel Dos Reis     Date: 2014-07-31

In an example like

  template<typename T> struct A {
    struct B {
      void f();
    };
  };

  template<typename T> using X = typename A<T>::B;

  template<typename T> void X<T>::f() { }       // #1

should #1 be considered a definition of A<T>::B::f()?

Analogy with alias-declarations would suggest that it should, but alias template specializations involve issues like SFINAE on unused template parameters (see issue 1558) and possibly other complications.

(See also issues 1980, 2021, 2025, and 2037.)

Notes from the May, 2015 meeting:

CWG felt that this kind of usage should be permitted only via a “simple” alias, in which the SFINAE is the same as the template to which it refers and all the template parameters are used. See also issue 1286.




1980. Equivalent but not functionally-equivalent redeclarations

Section: 13.7.8  [temp.alias]     Status: drafting     Submitter: Richard Smith     Date: 2014-08-04

In an example like

  template<typename T, typename U> using X = T;
  template<typename T> X<void, typename T::type> f();
  template<typename T> X<void, typename T::other> f();

it appears that the second declaration of f is a redeclaration of the first but distinguishable by SFINAE, i.e., equivalent but not functionally equivalent.

Notes from the November, 2014 meeting:

CWG felt that these two declarations should not be equivalent.




2236. When is an alias template specialization dependent?

Section: 13.7.8  [temp.alias]     Status: drafting     Submitter: Maxim Kartashev     Date: 2016-03-01

[Detailed description pending.]




2462. Problems with the omission of the typename keyword

Section: 13.8.1  [temp.res.general]     Status: drafting     Submitter: Mark Hall     Date: 2020-12-03

According to 13.8.2 [temp.local] paragraph 5,

A qualified-id is assumed to name a type if

There are two possible problems with this specification. First, consider an example like

   template<typename T> struct S {
     static void (*pfunc)(T::name);                               // Omitted typename okay because it is a
                                                                  // member-declaration
   };
   template<typename T> void (*S<T>::pfunc)(T::name) = nullptr;   // Omitted typename ill-formed because not a function
                                                                  // or function template declaration

Should bullet 5.2.4 be extended to include function pointer and member function pointer declarations, as well as function and function template declarations?

Second, given an example like

   template<typename T> struct Y {};
   template<typename T> struct S {
     Y<int(T::type)> m;  // Omitted typename okay because it is in a member-declaration?
  };

Should bullet 5.2.3 be restricted to parameter-declarations of the member being declared, rather than simply “in” such a member-declaration?

Notes from the December, 2020 teleconference:

The second issue was split off into issue 2468 to allow the resolutions to proceed independently.




2468. Omission of the typename keyword in a member template parameter list

Section: 13.8.1  [temp.res.general]     Status: drafting     Submitter: Mark Hall     Date: 2020-12-03

According to 13.8.2 [temp.local] paragraph 5,

A qualified-id is assumed to name a type if

This specification would appear to allow an example like:

   template<typename T> struct Y {};
   template<typename T> struct S {
     Y<int(T::type)> m;  // Omitted typename okay because it is in a member-declaration?
  };

The affected parameter-declarations should be only those of the member declarator, not in a member template's template parameter list.

(Note: this issue was spun off from issue 2462 to allow the resolutions to proceed independently.)




1390. Dependency of alias template specializations

Section: 13.8.3.2  [temp.dep.type]     Status: drafting     Submitter: Johannes Schaub     Date: 2011-09-04

According to 13.8.3.2 [temp.dep.type] paragraph 8, a type is dependent (among other things) if it is

This applies to alias template specializations, even if the resulting type does not depend on the template argument:

    struct B { typedef int type; };
    template<typename> using foo = B;
    template<typename T> void f() {
      foo<T>::type * x;  //error: typename required
    }

Is a change to the rules for cases like this warranted?

Notes from the October, 2012 meeting:

CWG agreed that no typename should be required in this case. In some ways, an alias template specialization is like the current instantiation and can be known at template definition time.




1524. Incompletely-defined class template base

Section: 13.8.3.2  [temp.dep.type]     Status: drafting     Submitter: Jason Merrill     Date: 2012-07-17

The correct handling of an example like the following is unclear:

  template<typename T> struct A {
    struct B: A { };
  };

A type used as a base must be complete (11.7 [class.derived] paragraph 2). The fact that the base class in this example is the current instantiation could be interpreted as indicating that it should be available for lookup, and thus the normal rule should apply, as members declared after the nested class would not be visible.

On the other hand, 13.8.3 [temp.dep] paragraph 3 says,

In the definition of a class or class template, if a base class depends on a template-parameter, the base class scope is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.

This wording refers not to a dependent type, which would permit lookup in the current instantiation, but simply to a type that “depends on a template-parameter,” and the current instantiation is such a type.

Implementations vary on the handling of this example.

(See also issue 1526 for another case related to the distinction between a “dependent type” and a “type that depends on a template-parameter.”)

Notes from the October, 2012 meeting:

CWG determined that the example should be ill-formed.




2074. Type-dependence of local class of function template

Section: 13.8.3.2  [temp.dep.type]     Status: drafting     Submitter: Richard Smith     Date: 2015-01-20

According to 13.8.3.2 [temp.dep.type] paragraph 9, a local class in a function template is dependent if and only if it contains a subobject of a dependent type. However, given an example like

  template<typename T> void f() {
    struct X {
      typedef int type;
  #ifdef DEPENDENT
      T x;
  #endif
    };
  X::type y;    // #1
  }
  void g() { f<int>(); }

there is implementation variance in the treatment of #1, but whether or not DEPENDENT is defined appears to make no difference.

In a related question, should a value-dependent alignas specifier cause a type to be dependent? Given

  template<int N> struct Y { typedef int type; };
  template<int N> void h() {
    struct alignas(N) X {};
    Y<alignof(X)>::type z;   // #2
  }
  void i() { h<4>(); }

Most/all implementations issue an error for a missing typename in #2.

Perhaps the right answer is that the types should be dependent but a member of the current instantiation, permitting name lookup without typename.




2275. Type-dependence of function template

Section: 13.8.3.3  [temp.dep.expr]     Status: drafting     Submitter: Jason Merrill     Date: 2016-06-21

[Detailed description pending.]




2405. Additional type-dependent expressions

Section: 13.8.3.3  [temp.dep.expr]     Status: drafting     Submitter: Andrey Davydov     Date: 2018-08-20

According to 13.8.3.3 [temp.dep.expr] paragraph 3,

...Expressions of the following forms are type-dependent only if the type specified by the type-id, simple-type-specifier or new-type-id is dependent, even if any subexpression is type-dependent:

This list is missing cases for:




2487. Type dependence of function-style cast to incomplete array type

Section: 13.8.3.3  [temp.dep.expr]     Status: drafting     Submitter: Richard Smith     Date: 2021-03-12

Consider:

  using T = int[];
  using U = int[2];
  template<auto M, int ...N> void f() {
    auto &&arr1 = T(N...);
    auto &&arr2 = T{N...};
    auto &&arr3 = U(M, M);
    auto &&arr4 = U{M, M};
  };

I think here T(N...) is not type-dependent, per 13.8.3.3 [temp.dep.expr] paragraph 3, but should be. (I think T{N...} is type-dependent.) Conversely, I think U{M, M} is type-dependent, per 13.8.3.3 [temp.dep.expr] paragraph 6, but should not be. (U(M, M) is not type-dependent.)

I think we should say that

are type-dependent if the type specifier names a dependent type, or if it names an array of unknown bound and the braced-init-list or expression-list is type-dependent.

(I think we could be a little more precise than that in the case where there is no top-level pack expansion: T{M, M} needs to be type-dependent for a general array of unknown bound T due to brace elision, but not in the case where the array element type is a scalar type. And T(M, M) does not need to be type-dependent because direct aggregate initialization can't perform brace elision. But I think the simpler rule is probably good enough.)

Notes from the August, 2021 teleconference:

CWG agreed with the suggested change. There was some support for the “more precise” approach mentioned in the description.




2090. Dependency via non-dependent base class

Section: 13.8.3.5  [temp.dep.temp]     Status: drafting     Submitter: Maxim Kartashev     Date: 2015-02-27

According to 13.8.3.5 [temp.dep.temp] paragraph 3,

a non-type template-argument is dependent if the corresponding non-type template-parameter is of reference or pointer type and the template-argument designates or points to a member of the current instantiation or a member of a dependent type.

Members of non-dependent base classes are members of the current instantiation, but using one as a non-type template argument should not be considered dependent.




2. How can dependent names be used in member declarations that appear outside of the class template definition?

Section: 13.8.4  [temp.dep.res]     Status: drafting     Submitter: unknown     Date: unknown
    template <class T> class Foo {
    
       public:
       typedef int Bar;
       Bar f();
    };
    template <class T> typename Foo<T>::Bar Foo<T>::f() { return 1;}
                       --------------------
In the class template definition, the declaration of the member function is interpreted as:
   int Foo<T>::f();
In the definition of the member function that appears outside of the class template, the return type is not known until the member function is instantiated. Must the return type of the member function be known when this out-of-line definition is seen (in which case the definition above is ill-formed)? Or is it OK to wait until the member function is instantiated to see if the type of the return type matches the return type in the class template definition (in which case the definition above is well-formed)?

Suggested resolution: (John Spicer)

My opinion (which I think matches several posted on the reflector recently) is that the out-of-class definition must match the declaration in the template. In your example they do match, so it is well formed.

I've added some additional cases that illustrate cases that I think either are allowed or should be allowed, and some cases that I don't think are allowed.

    template <class T> class A { typedef int X; };
    
    
    template <class T> class Foo {
     public:
       typedef int Bar;
       typedef typename A<T>::X X;
       Bar f();
       Bar g1();
       int g2();
       X h();
       X i();
       int j();
     };
    
     // Declarations that are okay
     template <class T> typename Foo<T>::Bar Foo<T>::f()
                                                     { return 1;}
     template <class T> typename Foo<T>::Bar Foo<T>::g1()
                                                     { return 1;}
     template <class T> int Foo<T>::g2() { return 1;}
     template <class T> typename Foo<T>::X Foo<T>::h() { return 1;}
    
     // Declarations that are not okay
     template <class T> int Foo<T>::i() { return 1;}
     template <class T> typename Foo<T>::X Foo<T>::j() { return 1;}
In general, if you can match the declarations up using only information from the template, then the declaration is valid.

Declarations like Foo::i and Foo::j are invalid because for a given instance of A<T>, A<T>::X may not actually be int if the class is specialized.

This is not a problem for Foo::g1 and Foo::g2 because for any instance of Foo<T> that is generated from the template you know that Bar will always be int. If an instance of Foo is specialized, the template member definitions are not used so it doesn't matter whether a specialization defines Bar as int or not.




287. Order dependencies in template instantiation

Section: 13.8.4.1  [temp.point]     Status: drafting     Submitter: Martin Sebor     Date: 17 May 2001

Implementations differ in their treatment of the following code:

    template <class T>
    struct A {
	typename T::X x;
    };

    template <class T>
    struct B {
	typedef T* X;
	A<B> a;
    };

    int main ()
    {
	B<int> b;
    }

Some implementations accept it. At least one rejects it because the instantiation of A<B<int> > requires that B<int> be complete, and it is not at the point at which A<B<int> > is being instantiated.

Erwin Unruh:

In my view the programm is ill-formed. My reasoning:

So each class needs the other to be complete.

The problem can be seen much easier if you replace the typedef with

    typedef T (*X) [sizeof(B::a)];

Now you have a true recursion. The compiler cannot easily distinguish between a true recursion and a potential recursion.

John Spicer:

Using a class to form a qualified name does not require the class to be complete, it only requires that the named member already have been declared. In other words, this kind of usage is permitted:

    class A {
        typedef int B;
        A::B ab;
    };

In the same way, once B has been declared in A, it is also visible to any template that uses A through a template parameter.

The standard could be more clear in this regard, but there are two notes that make this point. Both 6.5.5.2 [class.qual] and _N4567_.5.1.1 [expr.prim.general] paragraph 7 contain a note that says "a class member can be referred to using a qualified-id at any point in its potential scope (6.4.6 [basic.scope.class])." A member's potential scope begins at its point of declaration.

In other words, a class has three states: incomplete, being completed, and complete. The standard permits a qualified name to be used once a name has been declared. The quotation of the notes about the potential scope was intended to support that.

So, in the original example, class A does not require the type of T to be complete, only that it have already declared a member X.

Bill Gibbons:

The template and non-template cases are different. In the non-template case the order in which the members become declared is clear. In the template case the members of the instantiation are conceptually all created at the same time. The standard does not say anything about trying to mimic the non-template case during the instantiation of a class template.

Mike Miller:

I think the relevant specification is 13.8.4.1 [temp.point] paragraph 3, dealing with the point of instantiation:

For a class template specialization... if the specialization is implicitly instantiated because it is referenced from within another template specialization, if the context from which the specialization is referenced depends on a template parameter, and if the specialization is not instantiated previous to the instantiation of the enclosing template, the point of instantiation is immediately before the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.

That means that the point of instantiation of A<B<int> > is before that of B<int>, not in the middle of B<int> after the declaration of B::X, and consequently a reference to B<int>::X from A<B<int> > is ill-formed.

To put it another way, I believe John's approach requires that there be an instantiation stack, with the results of partially-instantiated templates on the stack being available to instantiations above them. I don't think the Standard mandates that approach; as far as I can see, simply determining the implicit instantiations that need to be done, rewriting the definitions at their respective points of instantiation with parameters substituted (with appropriate "forward declarations" to allow for non-instantiating references), and compiling the result normally should be an acceptable implementation technique as well. That is, the implicit instantiation of the example (using, e.g., B_int to represent the generated name of the B<int> specialization) could be something like

        struct B_int;

        struct A_B_int {
            B_int::X x;    // error, incomplete type
        };

        struct B_int {
            typedef int* X;
            A_B_int a;
        };

Notes from 10/01 meeting:

This was discussed at length. The consensus was that the template case should be treated the same as the non-template class case it terms of the order in which members get declared/defined and classes get completed.

Proposed resolution:

In 13.8.4.1 [temp.point] paragraph 3 change:

the point of instantiation is immediately before the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.

To:

the point of instantiation is the same as the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the nearest enclosing declaration. [Note: The point of instantiation is still at namespace scope but any declarations preceding the point of instantiation, even if not at namespace scope, are considered to have been seen.]

Add following paragraph 3:

If an implicitly instantiated class template specialization, class member specialization, or specialization of a class template references a class, class template specialization, class member specialization, or specialization of a class template containing a specialization reference that directly or indirectly caused the instantiation, the requirements of completeness and ordering of the class reference are applied in the context of the specialization reference.

and the following example

  template <class T> struct A {
          typename T::X x;
  };

  struct B {
          typedef int X;
          A<B> a;
  };

  template <class T> struct C {
          typedef T* X;
          A<C> a;
  };

  int main ()
  {
          C<int> c;
  }

Notes from the October 2002 meeting:

This needs work. Moved back to drafting status.

See also issues 595 and 1330.




1258. “Instantiation context” differs from dependent lookup rules

Section: 13.8.4.1  [temp.point]     Status: drafting     Submitter: Nikolay Ivchenkov     Date: 2011-03-10

C++11 expanded the lookup rules for dependent function calls (13.8.4.2 [temp.dep.candidate] paragraph 1 bullet 2) to include functions with internal linkage; previously only functions with external linkage were considered. However, 13.8.4.1 [temp.point] paragraph 6 still says,

The instantiation context of an expression that depends on the template arguments is the set of declarations with external linkage declared prior to the point of instantiation of the template specialization in the same translation unit.

Presumably this wording was overlooked and should be harmonized with the new specification.




1845. Point of instantiation of a variable template specialization

Section: 13.8.4.1  [temp.point]     Status: drafting     Submitter: Richard Smith     Date: 2014-01-28

The current wording of 13.8.4.1 [temp.point] does not define the point of instantiation of a variable template specialization. Presumably replacing the references to “static data member of a class template” with “variable template” in paragraphs 1 and 8 would be sufficient.

Additional note, July, 2017:

It has also been observed that there is no definition of the point of instantiation for an alias template. It is not clear that there is a need for normative wording for the point of instantiation of an alias template, but if not, a note explaining its absence would be helpful.




2245. Point of instantiation of incomplete class template

Section: 13.8.4.1  [temp.point]     Status: drafting     Submitter: Richard Smith     Date: 2016-03-08

[Detailed description pending.]

Notes from the December, 2016 teleconference:

The consensus was that references to specializations before the template definition is seen are not points of instantiation.




2497. Points of instantiation for constexpr function templates

Section: 13.8.4.1  [temp.point]     Status: drafting     Submitter: Richard Smith     Date: 2019-07-20

Consider:

  template<typename T> constexpr T f();
  constexpr int g() { return f<int>(); } // #1
  template<typename T> constexpr T f() { return 123; }
  int k[g()];
  // #2

There are two points of instantiation for f<int>. At #1, the template isn't defined, so it cannot be instantiated there. At #2, it's too late, as the definition was needed when parsing the type of k.

Should we also treat the point of definition of (at least) a constexpr function template as a point of instantiation for all specializations that have a point of instantiation before that point? Note the possible interaction of such a resolution with 13.8.4.1 [temp.point] paragraph 7:

If two different points of instantiation give a template specialization different meanings according to the one-definition rule (6.3 [basic.def.odr]), the program is ill-formed, no diagnostic required.

Notes from the November, 2021 teleconference:

Another possibility for a point of instantiation, other than the definition of the template, would be the point at which the function is called. Similar questions have been raised regarding the points at which variables are initialized (issue 2186) and constexpr functions are defined (issue 2166).




1253. Generic non-template members

Section: 13.9  [temp.spec]     Status: drafting     Submitter: Nikolay Ivchenkov     Date: 2011-03-06

Many statements in the Standard apply only to templates, for example, 13.8 [temp.res] paragraph 8:

If no valid specialization can be generated for a template definition, and that template is not instantiated, the template definition is ill-formed, no diagnostic required.

This clearly should apply to non-template member functions of class templates, not just to templates per se. Terminology should be established to refer to these generic entities that are not actually templates.

Additional notes (August, 2012):

Among the generic entities that should be covered by such a term are default function arguments, as they can be instantiated independently. If issue 1330 is resolved as expected, exception-specifications should also be covered by the same term.

See also issue 1484.




1396. Deferred instantiation and checking of non-static data member initializers

Section: 13.9.2  [temp.inst]     Status: drafting     Submitter: Jason Merrill     Date: 2011-09-22

Non-static data member initializers get the same late parsing as member functions and default arguments, but are they also instantiated as needed like them? And when is their validity checked?

Notes from the October, 2012 meeting:

CWG agreed that non-static data member initializers should be handled like default arguments.

Additional note (March, 2013):

Determining whether a defaulted constructor is constexpr or not requires parsing the class's non-static data member initializers; see also issue 1360.




2072. Default argument instantiation for member functions of templates

Section: 13.9.2  [temp.inst]     Status: drafting     Submitter: Maxim Kartashev     Date: 2015-01-19

Default argument instantiation is described in 13.9.2 [temp.inst] paragraph 13, and although instantiation of default arguments for member functions of class templates is mentioned elsewhere a number of times, this paragraph only describes default argument instantiation for function templates.




2202. When does default argument instantiation occur?

Section: 13.9.2  [temp.inst]     Status: drafting     Submitter: Richard Smith     Date: 2015-11-19

According to 13.9.2 [temp.inst] paragraph 11,

If a function template f is called in a way that requires a default argument to be used, the dependent names are looked up, the semantics constraints are checked, and the instantiation of any template used in the default argument is done as if the default argument had been an initializer used in a function template specialization with the same scope, the same template parameters and the same access as that of the function template f used at that point, except that the scope in which a closure type is declared (7.5.5.2 [expr.prim.lambda.closure]) — and therefore its associated namespaces — remain as determined from the context of the definition for the default argument. This analysis is called default argument instantiation. The instantiated default argument is then used as the argument of f.

Some details are not clear from this description. For example, given

  #include <type_traits>
  template<class T> struct Foo { Foo(T = nullptr) {} };
  bool b = std::is_constructible<Foo<int>>::value;
  int main() {}

does “used” mean odr-used or used in any way? Is a failure of default argument instantiation in the immediate context of the call or is a failure a hard error? And does it apply only to function templates, as it says, or should it apply to member functions of class templates? There is implementation divergence on these questions.

Notes from the March, 2018 meeting:

CWG felt that such errors should be substitution failures, not hard errors.




2222. Additional contexts where instantiation is not required

Section: 13.9.2  [temp.inst]     Status: drafting     Submitter: CWG     Date: 2016-01-11

According to 13.9.2 [temp.inst] paragraph 6,

If the function selected by overload resolution (12.2 [over.match]) can be determined without instantiating a class template definition, it is unspecified whether that instantiation actually takes place.

There are other contexts in which a smart implementation could presumably avoid instantiations, such as when doing argument-dependent lookup involving a class template specialization when the template definition contains no friend declarations or checking base/derived relationships involving incomplete class template definitions. It would be helpful to enumerate such contexts.




2263. Default argument instantiation for friends

Section: 13.9.2  [temp.inst]     Status: drafting     Submitter: Hubert Tong     Date: 2016-05-04

[Detailed description pending.]

Notes from the December, 2016 teleconference:

This issue should be resolved by the resolution of issue 2174.




2265. Delayed pack expansion and member redeclarations

Section: 13.9.2  [temp.inst]     Status: drafting     Submitter: Hubert Tong     Date: 2016-05-11

It is not clear how to handle parameter packs that are expanded during instantiation in parallel with those that are not yet concrete. In particular, does the following example require a diagnostic?

  template<typename ...T> struct Tuple; 
  template<class T, class U> struct Outer; 
  template<class ...T, class ...U> 
  struct Outer<Tuple<T ...>, Tuple<U ...> > { 
    template<class X, class Y> struct Inner; 
    template<class ...Y> struct Inner<Tuple<T, Y> ...> { }; 
    template<class ...Y> struct Inner<Tuple<U, Y> ...> { }; 
  }; 
  Outer<Tuple<int, void>, Tuple<int, void> > outer; 

Notes from the March, 2018 meeting:

CWG felt that ill-formed, no diagnostic required was the correct approach.




1665. Declaration matching in explicit instantiations

Section: 13.9.3  [temp.explicit]     Status: drafting     Submitter: Richard Smith     Date: 2013-04-19

Consider a case like

  struct X {
    template<typename T> void f(T);
    void f(int);
  };
  template void X::f(int);

or

  template<typename T> void f(T) {}
  void f(int);
  template void f(int);

Presumably in both these cases the explicit instantiation should refer to the template and not to the non-template; however, 13.7.3 [temp.mem] paragraph 2 says,

A normal (non-template) member function with a given name and type and a member function template of the same name, which could be used to generate a specialization of the same type, can both be declared in a class. When both exist, a use of that name and type refers to the non-template member unless an explicit template argument list is supplied.

This would appear to give the wrong answer for the first example. It's not clearly stated, but consistency would suggest a similar wrong answer for the second. Presumably a statement is needed somewhere that an explicit instantiation directive applies to a template and not a non-template function if both are visible.

Additional note, January, 2014:

A related example has been raised:

  template<typename T> class Matrix {
  public:
    Matrix(){}
    Matrix(const Matrix&){}
    template<typename U>
      Matrix(const Matrix<U>&);
  };

  template Matrix<int>::Matrix(const Matrix&);

  Matrix<int> m;
  Matrix<int> mm(m);

If the explicit instantiation directive applies to the constructor template, there is no way to explicitly instantiate the copy constructor.




2421. Explicit instantiation of constrained member functions

Section: 13.9.3  [temp.explicit]     Status: drafting     Submitter: Casey Carter     Date: 2019-07-16

An explicit instantiation of a class template specialization also explicitly instantiates member functions of that class template specialization whose constraints are satisfied, even those that are not callable because a more-constrained overload exists which would always be selected by overload resolution. Ideally, we would not explicitly instantiate definitions of such uncallable functions.

Notes from the August, 2020 teleconference:

CWG felt that the concept of “eligible” might form a basis for the resolution of this issue.




529. Use of template<> with “explicitly-specialized” class templates

Section: 13.9.4  [temp.expl.spec]     Status: drafting     Submitter: James Widman     Date: 16 August 2005

Paragraph 17 of 13.9.4 [temp.expl.spec] says,

A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized.

This is curious, because paragraph 3 only allows explicit specialization of members of implicitly-instantiated class specializations, not explicit specializations. Furthermore, paragraph 4 says,

Definitions of members of an explicitly specialized class are defined in the same manner as members of normal classes, and not using the explicit specialization syntax.

Paragraph 18 provides a clue for resolving the apparent contradiction:

In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. In such explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding the explicit specialization declaration of the member.

It appears from this and the following example that the phrase “explicitly specialized” in paragraphs 17 and 18, when referring to enclosing class templates, does not mean that explicit specializations have been declared for them but that their names in the qualified-id are followed by template argument lists. This terminology is confusing and should be changed.

Proposed resolution (October, 2005):

  1. Change 13.9.4 [temp.expl.spec] paragraph 17 as indicated:

  2. A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized specialization. [Example:...
  3. Change 13.9.4 [temp.expl.spec] paragraph 18 as indicated:

  4. In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well that is, the template-id naming the template may be composed of template parameter names rather than template-arguments. In For each unspecialized template in such an explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding the explicit specialization declaration of the member. The types of the template-parameters in the template-parameter-list shall be the same as those specified in the primary template definition. In such declarations, an unspecialized template-id shall not precede the name of a template specialization in the qualified-id naming the member. [Example:...

Notes from the April, 2006 meeting:

The revised wording describing “unspecialized” templates needs more work to ensure that the parameter names in the template-id are in the correct order; the distinction between template arguments and parameters is also probably not clear enough. It might be better to replace this paragraph completely and avoid the “unspecialized” wording altogether.

Proposed resolution (February, 2010):

  1. Change 13.9.4 [temp.expl.spec] paragraph 17 as follows:

  2. A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized specialization. [Example:...
  3. Change 13.9.4 [temp.expl.spec] paragraph 18 as follows:

  4. In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. In such explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding the explicit specialization declaration of the member. The types of the template-parameters in the template-parameter-list shall be the same as those specified in the primary template definition. that is, the corresponding template prefix may specify a template-parameter-list instead of template<> and the template-id naming the template be written using those template-parameters as template-arguments. In such a declaration, the number, kinds, and types of the template-parameters shall be the same as those specified in the primary template definition, and the template-parameters shall be named in the template-id in the same order that they appear in the template-parameter-list. An unspecialized template-id shall not precede the name of a template specialization in the qualified-id naming the member. [Example:...



1840. Non-deleted explicit specialization of deleted function template

Section: 13.9.4  [temp.expl.spec]     Status: drafting     Submitter: Richard Smith     Date: 2014-01-19

The resolution of issue 941 permits a non-deleted explicit specialization of a deleted function template. For example:

  template<typename T> void f() = delete;
  decltype(f<int>()) *p;
  template<> void f<int>();

However, the existing normative wording is not adequate to handle this usage. For one thing, =delete is formally, at least, a function definition, and an implementation is not permitted to instantiate a function definition unless it is used; presumably, then, an implementation could not reject the decltype above as a reference to a deleted specialization. Furthermore, there should be a requirement that a non-deleted explicit specialization of a deleted function template must precede any reference to that specialization. (I.e., the example should be ill-formed as written but well-formed if the last two lines were interchanged.)




1993. Use of template<> defining member of explicit specialization

Section: 13.9.4  [temp.expl.spec]     Status: drafting     Submitter: Richard Smith     Date: 2014-08-31

Issue 531 surveyed existing practice at the time and determined that the most common syntax for defining a member of an explicit specialization used the template<> prefix. This approach, however, does not seem consistent, since such a definition is not itself an explicit specialization.




2409. Explicit specializations of constexpr static data members

Section: 13.9.4  [temp.expl.spec]     Status: drafting     Submitter: Mike Miller     Date: 2019-04-29

The status of an example like the following is not clear:

  struct S {
    template <int N> static constexpr inline int m = N;
  };
  template <> constexpr inline int S::m<5>;

Some implementations accept this, apparently on the basis of allowing and ignoring a redeclaration of a constexpr static data member outside its class, although there is implementation divergence. Most or all implementations, however, diagnose an attempt to use such a specialization in a constant context.

Should it be required to have a definition of the explicit specialization in order to declare it outside the class in such cases?

In addition, most or all implementations accept a version of the example in which the explicit specialization contains an initializer, including allowing its use in constant contexts:

  template <> constexpr inline int S::m<5> = 2;

This would seem to be disallowed both by 11.4.9.3 [class.static.data] paragraph 3,

An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer. If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see _N4778_.D.4 [depr.static_constexpr]).

which prohibits an initializer, and 13.9.4 [temp.expl.spec] paragraph 2,

An explicit specialization may be declared in any scope in which the corresponding primary template may be defined (_N4868_.9.8.2.3 [namespace.memdef], 11.4 [class.mem], 13.7.3 [temp.mem]).

since the definition of a constexpr static data member is inside the class.

Notes from the May, 2019 teleconference:

These examples should behave in the same way as if the class were templated: instantiate the declaration and the definition of the static data member separately. The first example should be ill-formed, because the explicit specializaation does not have an initializer.




2055. Explicitly-specified non-deduced parameter packs

Section: 13.10.2  [temp.arg.explicit]     Status: drafting     Submitter: Jonathan Caves     Date: 2014-12-09

According to 13.10.2 [temp.arg.explicit] paragraph 3,

Trailing template arguments that can be deduced (13.10.3 [temp.deduct]) or obtained from default template-arguments may be omitted from the list of explicit template-arguments. A trailing template parameter pack (13.7.4 [temp.variadic]) not otherwise deduced will be deduced to an empty sequence of template arguments. If all of the template arguments can be deduced, they may all be omitted; in this case, the empty template argument list <> itself may also be omitted. In contexts where deduction is done and fails, or in contexts where deduction is not done, if a template argument list is specified and it, along with any default template arguments, identifies a single function template specialization, then the template-id is an lvalue for the function template specialization.

It is not clear that this permits an example like:

  template<typename... T> void f(typename T::type...)   {
  }

  int main() {
    f<>();
  }

See also issue 2105.




1172. “instantiation-dependent” constructs

Section: 13.10.3  [temp.deduct]     Status: drafting     Submitter: Adamczyk     Date: 2010-08-05

There are certain constructs that are not covered by the existing categories of “type dependent” and “value dependent.” For example, the expression sizeof(sizeof(T())) is neither type-dependent nor value-dependent, but its validity depends on whether T can be value-constructed. We should be able to overload on such characteristics and select via deduction failure, but we need a term like “instantiation-dependent” to describe these cases in the Standard. The phrase “expression involving a template parameter” seems to come pretty close to capturing this idea.

Notes from the November, 2010 meeting:

The CWG favored extending the concepts of “type-dependent” and “value-dependent” to cover these additional cases, rather than adding a new concept.

Notes from the March, 2011 meeting:

The CWG reconsidered the direction from the November, 2010 meeting, as it would make more constructs dependent, thus requiring more template and typename keywords, resulting in worse error messages, etc.

Notes from the August, 2011 meeting:

The following example (from issue 1273) was deemed relevant for this issue:

    template <class T> struct C;

    class A {
       int i;
       friend struct C<int>;
    } a;

    class B {
       int i;
       friend struct C<float>;
    } b;

    template <class T>
    struct C {
       template <class U> decltype (a.i) f() { } // #1
       template <class U> decltype (b.i) f() { } // #2
    };

    int main() {
       C<int>().f<int>();     // calls #1
       C<float>().f<float>(); // calls #2
    }



1322. Function parameter type decay in templates

Section: 13.10.3  [temp.deduct]     Status: drafting     Submitter: Jason Merrill     Date: 2011-05-19

The discussion of issue 1001 seemed to have settled on the approach of doing the 9.3.4.6 [dcl.fct] transformations immediately to the function template declaration, so that the original form need not be remembered. However, the example in 13.10.3 [temp.deduct] paragraph 8 suggests otherwise:

  template <class T> int f(T[5]);
  int I = f<int>(0);
  int j = f<void>(0); // invalid array

One way that might be addressed would be to separate the concepts of the type of the template that participates in overload resolution and function matching from the type of the template that is the source for template argument substitution. (See also the example in paragraph 3 of the same section.)

Notes, January, 2012:




1582. Template default arguments and deduction failure

Section: 13.10.3  [temp.deduct]     Status: drafting     Submitter: John Spicer     Date: 2012-10-31

According to 13.10.3 [temp.deduct] paragraph 5,

The resulting substituted and adjusted function type is used as the type of the function template for template argument deduction. If a template argument has not been deduced and its corresponding template parameter has a default argument, the template argument is determined by substituting the template arguments determined for preceding template parameters into the default argument. If the substitution results in an invalid type, as described above, type deduction fails.

This leaves the impression that default arguments are used after deduction failure leaves an argument undeduced. For example,

  template<typename T> struct Wrapper;
  template<typename T = int> void f(Wrapper<T>*);
  void g() {
    f(0);
  }

Deduction fails for T, so presumably int is used. However, some implementations reject this code. It appears that the intent would be better expressed as something like

...If a template argument is used only in a non-deduced context and its corresponding template parameter has a default argument...

Rationale (November, 2013):

CWG felt that this issue should be considered by EWG in a broader context before being resolved.

Additional note, April, 2015:

EWG has requested that CWG resolve this issue along the lines discussed above.

Notes from the May, 2015 meeting:

CWG agreed that a default template argument should only be used if the parameter is not used in a deducible context. See also issue 2092.




1844. Defining “immediate context”

Section: 13.10.3  [temp.deduct]     Status: drafting     Submitter: Richard Smith     Date: 2014-01-28

The handling of an example like

  template<typename T, std::size_t S = sizeof(T)> struct X {};
  template<typename T> X<T> foo(T*);
  void foo(...);

  void test() { struct S *s; foo(s); }

varies among implementations, presumably because the meaning of “immediate context” in determining whether an error is a substitution failure or a hard error is not clearly defined.

Notes from the February, 2016 meeting:

See also issue 1554; the resolution of this issue should also deal with alias templates.




1513. initializer_list deduction failure

Section: 13.10.3.2  [temp.deduct.call]     Status: drafting     Submitter: Steve Adamczyk     Date: 2012-06-28

According to 13.10.3.2 [temp.deduct.call] paragraph 1,

If removing references and cv-qualifiers from P gives std::initializer_list<P'> for some P' and the argument is an initializer list (9.4.5 [dcl.init.list]), then deduction is performed instead for each element of the initializer list, taking P' as a function template parameter type and the initializer element as its argument. Otherwise, an initializer list argument causes the parameter to be considered a non-deduced context (13.10.3.6 [temp.deduct.type]).

It is not entirely clear whether the deduction for an initializer list meeting a std::initializer_list<T> is a recursive subcase, or part of the primary deduction. A relevant question is: if the deduction on that part fails, does the entire deduction fail, or is the parameter to be considered non-deduced?

See also issue 2326.

Notes from the October, 2012 meeting:

CWG determined that the entire deduction fails in this case.




1584. Deducing function types from cv-qualified types

Section: 13.10.3.2  [temp.deduct.call]     Status: drafting     Submitter: Daniel Krügler     Date: 2012-11-04

It is not clear whether the following is well-formed or not:

  void foo(){}

  template<class T>
  void deduce(const T*) { }

  int main() {
    deduce(foo);
  }

Implementations vary in their treatment of this example.

Proposed resolution (April, 2013):

Change 13.10.3.6 [temp.deduct.type] paragraph 18 as follows:

A template-argument can be deduced from a function, pointer to function, or pointer to member function type. [Note: cv-qualification of a deduced function type is ignored; see 9.3.4.6 [dcl.fct]. —end note] [Example:

  template<class T> void f(void(*)(T,int));
  template<class T> void f2(const T*);
  template<class T> void foo(T,int);
  void g(int,int);
  void g(char,int);
  void g2();

  void h(int,int,int);
  void h(char,int);
  int m() {
    f(&g);     // error: ambiguous
    f(&h);     // OK: void h(char,int) is a unique match
    f(&foo);   // error: type deduction fails because foo is a template
    f2(g2);    // OK: cv-qualification of deduced function type ignored
  }

end example]

Additional note, November, 2014:

Concern was expressed regarding the proposed resolution over its treatment of an example like the following:

  template<typename T> struct tuple_size {};
  template<typename T> struct tuple_size<T const>: tuple_size<T> {};

  tuple_size<void()> t;

In this case T const is always considered to be more specialized for void(), leading to infinite self-derivation.

The issue has been returned to "open" status for further consideration.

Notes from the May, 2015 meeting:

The consensus of CWG was that the cv-qualification of the argument and parameter must match, so the original example should be rejected.




1939. Argument conversions to nondeduced parameter types revisited

Section: 13.10.3.2  [temp.deduct.call]     Status: drafting     Submitter: Richard Smith     Date: 2014-06-11

The intent of the resolution of issue 1184 appears not to have been completely realized. In particular, the phrase, “contains no template-parameters that participate in template argument deduction” in both the note in 13.10.3.2 [temp.deduct.call] paragraph 4 and the normative wording in 13.10.2 [temp.arg.explicit] paragraph 6 is potentially misleading and probably should say something like, “contains no template-parameters outside non-deduced contexts.” Also, the normative wording should be moved to 13.10.3.2 [temp.deduct.call] paragraph 4, since it applies when there are no explicitly-specified template arguments. For example,

  template<typename T>
  void f(T, typename identity<T>::type*);

Presumably the second parameter should allow pointer conversions, even though it does contain a template-parameter that participates in deduction (via the first function parameter).

Additional note, October, 2015:

See also issue 1391.




1486. Base-derived conversion in member pointer deduction

Section: 13.10.3.3  [temp.deduct.funcaddr]     Status: drafting     Submitter: John Spicer     Date: 2012-03-26

The rules for deducing template arguments when taking the address of a function template in 13.10.3.3 [temp.deduct.funcaddr] do not appear to allow for a base-to-derived conversion in a case like:

  struct Base {
    template<class U> void f(U);
  };

  struct Derived : Base { };

  int main() {
    void (Derived::*pmf)(int) = &Derived::f;
  } 

Most implementations appear to allow this adjustment, however.




1610. Cv-qualification in deduction of reference to array

Section: 13.10.3.5  [temp.deduct.partial]     Status: drafting     Submitter: Richard Smith     Date: 2013-01-28

Given

   template<class C> void foo(const C* val) {}
   template<int N> void foo(const char (&t)[N]) {}

it is intuitive that the second template is more specialized than the first. However, the current rules make them unordered. In 13.10.3.5 [temp.deduct.partial] paragraph 4, we have P as const C* and A as const char (&)[N]. Paragraph 5 transforms A to const char[N]. Finally, paragraph 7 removes top-level cv-qualification; since a cv-qualified array element type is considered to be cv-qualification of the array (6.8.4 [basic.type.qualifier] paragraph 5, cf issue 1059), A becomes char[N]. P remains const C*, so deduction fails because of the missing const in A.

Notes from the April, 2013 meeting:

CWG agreed that the const should be preserved in the array type.




2328. Unclear presentation style of template argument deduction rules

Section: 13.10.3.6  [temp.deduct.type]     Status: drafting     Submitter: Richard Smith     Date: 2016-10-11

The presentation style of 13.10.3.6 [temp.deduct.type] paragraph 8 results in a specification that is unclear, needlessly verbose, and incomplete. Specific problems include:




2172. Multiple exceptions with one exception object

Section: 14.4  [except.handle]     Status: drafting     Submitter: Richard Smith     Date: 2015-09-14

During the discussion of issue 2098 it was observed that multiple exceptions may share a single exception object via std::exception_ptr. It is not clear that the current wording handles that case correctly.




2219. Dynamically-unreachable handlers

Section: 14.4  [except.handle]     Status: drafting     Submitter: 2016-01-04     Date: Richard Smith

Consider the following example:

  #include <cstdio>
  #include <cstdlib>

  void f() {
    struct X {
     ~X() {
       std::puts("unwound");
       std::exit(0);
     }
    } x;
    throw 0;
  }

  int main(int argc, char**) {
    try {
      f();
    } catch (int) {
      std::puts("caught");
    }
  }

According to the Standard, this should print unwound and exit. Current optimizing implementations call terminate(), because:

More abstractly, before calling terminate, we're required to check whether there is an active handler for an exception of type int, and in some sense there is not (because the handler in main is dynamically unreachable).

There seem to be three possible solutions:

  1. Change the standard to say that terminate() is a valid response to this situation [this seems problematic, as any non-returning destructor now risks program termination, but is in fact the status quo on multiple implementations and does not seem to have resulted in any bug reports]

  2. Always fully unwind before calling terminate() [this significantly harms debugability of exceptions]

  3. Teach the compilers to not optimize out unreachable exception handlers [for some implementations, this is “remove, redesign and reimplement middle-end support for EH”-level difficult, and harms the ability to optimize code involving catch handlers]




1436. Interaction of constant expression changes with preprocessor expressions

Section: 15.2  [cpp.cond]     Status: drafting     Submitter: Richard Smith     Date: 2012-01-02

It appears that some of the recent changes to the description of constant expressions have allowed constructs into preprocessor expressions that do not belong there. Some changes are required to restrict the current capabilities of constant expressions to what is intended to be allowed in preprocessor expressions.

Proposed resolution (February, 2012):

  1. Change 15.2 [cpp.cond] paragraph 2 as follows:

  2. Each preprocessing token that remains (in the list of preprocessing tokens that will become the controlling expression) after all macro replacements have occurred shall be in the lexical form of a token (5.6 [lex.token]). Any such token that is a literal (5.13.1 [lex.literal.kinds]) shall be an integer-literal, a character-literal, or a boolean-literal.
  3. Change 15.2 [cpp.cond] paragraph 4 as follows:

  4. ...using arithmetic that has at least the ranges specified in 17.3 [support.limits]. The only operators permitted in the controlling constant expression are ?:, ||, &&, |, ^, &, ==, !=, <, <=, >, >=, <<, >>, -, +, *, /, %, !, and ~. For the purposes of this token conversion...



1718. Macro invocation spanning end-of-file

Section: 15.6  [cpp.replace]     Status: drafting     Submitter: David Krauss     Date: 2013-07-23

Although it seems to be common implementation practice to reject a macro invocation that begins in a header file and whose closing right parenthesis appears in the file that included it, there does not seem to be a prohibition of this case in the specification of function-style macros. Should this be accepted?

Notes from the February, 2014 meeting:

CWG agreed that macro invocations spanning file boundaries should be prohibited. Resolution of this issue should be coordinated with WG14.




2003. Zero-argument macros incorrectly specified

Section: 15.6  [cpp.replace]     Status: drafting     Submitter: Richard Smith     Date: 2014-09-12

According to 15.6 [cpp.replace] paragraph 4,

If the identifier-list in the macro definition does not end with an ellipsis, the number of arguments (including those arguments consisting of no preprocessing tokens) in an invocation of a function-like macro shall equal the number of parameters in the macro definition.

That is, a sequence of no preprocessing tokens counts as an argument. That phrasing has problems with zero-argument function-like macros, e.g.,

  #define M()
  M();

M is defined as having no parameters but the invocation has one (empty) argument, which does not match the number of parameters in the definition.




1335. Stringizing, extended characters, and universal-character-names

Section: 15.6.3  [cpp.stringize]     Status: drafting     Submitter: Johannes Schaub     Date: 2011-07-03

When a string literal containing an extended character is stringized (15.6.3 [cpp.stringize]), the result contains a universal-character-name instead of the original extended character. The reason is that the extended character is translated to a universal-character-name in translation phase 1 (5.2 [lex.phases]), so that the string literal "@" (where @ represents an extended character) becomes "\uXXXX". Because the preprocessing token is a string literal, when the stringizing occurs in translation phase 4, the \ is doubled, and the resulting string literal is "\"\\uXXXX\"". As a result, the universal-character-name is not recognized as such when the translation to the execution character set occurs in translation phase 5. (Note that phase 5 translation does occur if the stringized extended character does not appear in a string literal.) Existing practice appears to ignore these rules and preserve extended characters in stringized string literals, however.

See also issue 578.

Additional note (August, 2013):

Implementations are granted substantial latitude in their handling of extended characters and universal-character-names in 5.2 [lex.phases] paragraph 1 phase 1, i.e.,

(An implementation may use any internal encoding, so long as an actual extended character encountered in the source file, and the same extended character expressed in the source file as a universal-character-name (i.e., using the \uXXXX notation), are handled equivalently except where this replacement is reverted in a raw string literal.)

However, this freedom is mostly nullified by the requirements of stringizing in 15.6.3 [cpp.stringize] paragraph 2:

If, in the replacement list, a parameter is immediately preceded by a # preprocessing token, both are replaced by a single character string literal preprocessing token that contains the spelling of the preprocessing token sequence for the corresponding argument.

This means that, in order to handle a construct like

  #define STRINGIZE_LITERAL( X ) # X
  #define STRINGIZE( X ) STRINGIZE_LITERAL( X )

  STRINGIZE( STRINGIZE( identifier_\u00fC\U000000Fc ) ) 

an implementation must recall the original spelling, including the form of UCN and the capitalization of any non-numeric hexadecimal digits, rather than simply translating the characters into a convenient internal representation.

To effect the freedom asserted in 5.2 [lex.phases], the description of stringizing should make the spelling of a universal-character-name implementation-defined.




1709. Stringizing raw string literals containing newline

Section: 15.6.3  [cpp.stringize]     Status: drafting     Submitter: David Krauss     Date: 2013-07-01

Stringizing a raw string literal containing a newline produces an invalid (unterminated) string literal and hence results in undefined behavior. It should be specified that a newline in a string literal is transformed to the two characters '\' 'n' in the resulting string literal.

A slightly related case involves stringizing a bare backslash character: because backslashes are only escaped within a string or character literal, a stringized bare backslash becomes "\", which is invalid and hence results in undefined behavior.




1889. Unclear effect of #pragma on conformance

Section: 15.9  [cpp.pragma]     Status: drafting     Submitter: James Widman     Date: 2014-03-05

According to 15.9 [cpp.pragma] paragraph 1, the effect of a #pragma is to cause

the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner.

It should be clarified that the extent of the non-conformance is limited to the implementation-defined behavior.




2181. Normative requirements in an informative Annex

Section: Annex B  [implimits]     Status: drafting     Submitter: Sean Hunt     Date: 2015-10-18

According to Annex B [implimits] paragraph 1,

Because computers are finite, C++ implementations are inevitably limited in the size of the programs they can successfully process. Every implementation shall document those limitations where known.

Because Annex Annex B [implimits] is informative, not normative, it should not use “shall.”




1279. Additional differences between C++ 2003 and C++ 2011

Section: C.4  [diff.cpp03]     Status: drafting     Submitter: Nikolay Ivchenkov     Date: 2011-03-27

A number of differences between C++03 and C++11 were omitted from C.4 [diff.cpp03]:

Additional note (January, 2012):

In addition to the items previously mentioned, access declarations were removed from C++11 but are not mentioned in C.4 [diff.cpp03].

Proposed (partial) resolution (February, 2012):

Add the following as a new section in C.4 [diff.cpp03]:

C.2.5 Clause 11.8 [class.access]: member access control pdiff.cpp03.class.access

Change: Remove access declarations.

Rationale: Removal of feature deprecated since C++ 1998.

Effect on original feature: Valid C++ 2003 code that uses access declarations is ill-formed in this International Standard. Instead, using-declarations (9.9 [namespace.udecl]) can be used.




205. Templates and static data members

Section: Clause 13  [temp]     Status: drafting     Submitter: Mike Miller     Date: 11 Feb 2000

Static data members of template classes and of nested classes of template classes are not themselves templates but receive much the same treatment as template. For instance, Clause 13 [temp] paragraph 1 says that templates are only "classes or functions" but implies that "a static data member of a class template or of a class nested within a class template" is defined using the template-declaration syntax.

There are many places in the clause, however, where static data members of one sort or another are overlooked. For instance, Clause 13 [temp] paragraph 6 allows static data members of class templates to be declared with the export keyword. I would expect that static data members of (non-template) classes nested within class templates could also be exported, but they are not mentioned here.

Paragraph 8, however, overlooks static data members altogether and deals only with "templates" in defining the effect of the export keyword; there is no description of the semantics of defining a static data member of a template to be exported.

These are just two instances of a systematic problem. The entire clause needs to be examined to determine which statements about "templates" apply to static data members, and which statements about "static data members of class templates" also apply to static data members of non-template classes nested within class templates.

(The question also applies to member functions of template classes; see issue 217, where the phrase "non-template function" in 9.3.4.7 [dcl.fct.default] paragraph 4 is apparently intended not to include non-template member functions of template classes. See also issue 108, which would benefit from understanding nested classes of class templates as templates. Also, see issue 249, in which the usage of the phrase "member function template" is questioned.)

Notes from the 4/02 meeting:

Daveed Vandevoorde will propose appropriate terminology.




1529. Nomenclature for variable vs reference non-static data member

Section: Clause 6  [basic]     Status: drafting     Submitter: Daniel Krügler     Date: 2012-07-24

According to Clause 6 [basic] paragraph 6,

A variable is introduced by the declaration of a reference other than a non-static data member or of an object.

In other words, non-static data members of reference type are not variables. This complicates the wording in a number of places, where the text refers to “variable or data member,” presumably to cover the reference case, but that phrasing could lead to the mistaken impression that all data members are not variables. It would be better if either there were a term for the current phrase “variable or data member” or if there were a less-unwieldy term for “non-static data member of reference type” that could be used in place of “data member” in the current phrasing.






Issues with "Open" Status


916. Does a reference type have a destructor?

Section: _N2914_.14.10.2.1  [concept.map.fct]     Status: open     Submitter: James Widman     Date: 12 June, 2009

Is the following well-formed?

    auto concept HasDestructor<typename T> {
      T::~T();
    }

    concept_map HasDestructor<int&> { }

According to _N2914_.14.10.2.1 [concept.map.fct] paragraph 4, the destructor requirement in the concept map results in an expression x.~X(), where X is the type int&. According to _N4778_.7.6.1.4 [expr.pseudo], this expression is ill-formed because the object type and the type-name must be the same type, but the object type cannot be a reference type (references are dropped from types used in expressions, Clause 7 [expr] paragraph 5).

It is not clear whether this should be addressed by changing _N4778_.7.6.1.4 [expr.pseudo] or _N2914_.14.10.2.1 [concept.map.fct].




6. Should the optimization that allows a class object to alias another object also allow the case of a parameter in an inline function to alias its argument?

Section: _N4750_.15.8  [class.copy]     Status: open     Submitter: unknown     Date: unknown

[Picked up by evolution group at October 2002 meeting.]

(See also paper J16/99-0005 = WG21 N1182.)

At the London meeting, _N4750_.15.8 [class.copy] paragraph 31 was changed to limit the optimization described to only the following cases:

One other case was deemed desirable as well: However, there are cases when this aliasing was deemed undesirable and, at the London meeting, the committee was not able to clearly delimit which cases should be allowed and which ones should be prohibited.

Can we find an appropriate description for the desired cases?

Rationale (04/99): The absence of this optimization does not constitute a defect in the Standard, although the proposed resolution in the paper should be considered when the Standard is revised.

Note (March, 2008):

The Evolution Working Group has accepted the intent of this issue and referred it to CWG for action (not for C++0x). See paper J16/07-0033 = WG21 N2173.

Notes from the June, 2008 meeting:

The CWG decided to take no action on this issue until an interested party produces a paper with analysis and a proposal.




1049. Copy elision through reference parameters of inline functions

Section: _N4750_.15.8  [class.copy]     Status: open     Submitter: Jason Merrill     Date: 2010-03-10

Consider the following example:

    int c;

    struct A {
       A() { ++c; }
       A(const A&) { ++c; }
    };

    struct B {
       A a;
       B(const A& a): a(a) { }
    };

    int main() {
       (B(A()));
       return c - 1;
    }

Here we would like to be able to avoid the copy and just construct the A() directly into the A subobject of B. But we can't, because it isn't allowed by _N4750_.15.8 [class.copy] paragraph 34 bullet 3:

The part about not being bound to a reference was added for an unrelated reason by issue 185. If that resolution were recast to require that the temporary object is not accessed after the copy, rather than banning the reference binding, this optimization could be applied.

The similar example using pass by value is also not one of the allowed cases, which could be considered part of issue 6.




708. Partial specialization of member templates of class templates

Section: _N4868_.13.7.6  [temp.class.spec]     Status: open     Submitter: James Widman     Date: 8 Aug, 2008

The Standard does not appear to specify clearly the effect of a partial specialization of a member template of a class template. For example:

    template<class T> struct B {
         template<class U> struct A { // #1
             void h() {}
         };
         template<class U> struct A<U*> {  // #2
             void f() {}
         };
    };

    template<> template<class U> struct B<int>::A { // #3
         void g() {}
    };

    void q(B<int>::A<char*>& p) {
         p.f();  // #4
    }

The explicit specialization at #3 replaces the primary member template #1 of B<int>; however, it is not clear whether the partial specialization #2 should be considered to apply to the explicitly-specialized member template of A<int> (thus allowing the call to p.f() at #4) or whether the partial specialization will be used only for specializations of B that are implicitly instantiated (meaning that #4 could call p.g() but not p.f()).




2173. Partial specialization with non-deduced contexts

Section: _N4868_.13.7.6  [temp.class.spec]     Status: open     Submitter: Mike Miller     Date: 2015-09-14

During the discussion of issue 1315, it was observed that the example

  template <int I, int J> struct B {};
  template <int I> struct B<I, I*2> {};

is ill-formed because the deduction succeeds in both directions. This seems surprising. It was suggested that perhaps a non-deduced context should be considered more specialized than a deduced context.




2118. Stateful metaprogramming via friend injection

Section: _N4868_.13.8.6  [temp.inject]     Status: open     Submitter: Richard Smith     Date: 2015-04-27

Defining a friend function in a template, then referencing that function later provides a means of capturing and retrieving metaprogramming state. This technique is arcane and should be made ill-formed.

Notes from the May, 2015 meeting:

CWG agreed that such techniques should be ill-formed, although the mechanism for prohibiting them is as yet undetermined.




949. Requirements for freestanding implementations

Section: 4.1  [intro.compliance]     Status: open     Submitter: Detlef Vollman     Date: 2 August, 2009

According to 4.1 [intro.compliance] paragraph 7,

A freestanding implementation is one in which execution may take place without the benefit of an operating system, and has an implementation-defined set of libraries that includes certain language-support libraries (16.4.2.4 [compliance]).

This definition links two relatively separate topics: the lack of an operating system and the minimal set of libraries. Furthermore, 6.9.3.1 [basic.start.main] paragraph 1 says:

[Note: in a freestanding environment, start-up and termination is implementation-defined; start-up contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration. —end note]

It would be helpful if the two characteristics (lack of an operating system and restricted set of libraries) were named separately and if these statements were clarified to identify exactly what is implementation-defined.

Notes from the October, 2009 meeting:

The CWG felt that it needed a specific proposal in a paper before attempting to resolve this issue.




578. Phase 1 replacement of characters with universal-character-names

Section: 5.2  [lex.phases]     Status: open     Submitter: Martin Vejnár     Date: 7 May 2006

According to 5.2 [lex.phases] paragraph 1, in translation phase 1,

Any source file character not in the basic source character set (5.3 [lex.charset]) is replaced by the universal-character-name that designates that character.

If a character that is not in the basic character set is preceded by a backslash character, for example

    "\á"

the result is equivalent to

    "\\u00e1"

that is, a backslash character followed by the spelling of the universal-character-name. This is different from the result in C99, which accepts characters from the extended source character set without replacing them with universal-character-names.

See also issue 1335.




1698. Files ending in \

Section: 5.2  [lex.phases]     Status: open     Submitter: David Krauss     Date: 2013-06-10

The description of how to handle file not ending in a newline in 5.2 [lex.phases] paragraph 1, phase 2, is:

  1. Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. If, as a result, a character sequence that matches the syntax of a universal-character-name is produced, the behavior is undefined. A source file that is not empty and that does not end in a new-line character, or that ends in a new-line character immediately preceded by a backslash character before any such splicing takes place, shall be processed as if an additional new-line character were appended to the file.

This is not clear regarding what happens if the last character in the file is a backslash. In such a case, presumably the result of adding the newline should not be a line splice but rather a backslash preprocessing-token (that will be diagnosed as an invalid token in phase 7), but that should be spelled out.




1403. Universal-character-names in comments

Section: 5.7  [lex.comment]     Status: open     Submitter: David Krauss     Date: 2011-10-05

According to 5.3 [lex.charset] paragraph 2,

If the hexadecimal value for a universal-character-name corresponds to a surrogate code point (in the range 0xD800-0xDFFF, inclusive), the program is ill-formed. Additionally, if the hexadecimal value for a universal-character-name outside the c-char-sequence, s-char-sequence, or r-char-sequence of a character or string literal corresponds to a control character (in either of the ranges 0x00-0x1F or 0x7F-0x9F, both inclusive) or to a character in the basic source character set, the program is ill-formed.

These restrictions should not apply to comment text. Arguably the prohibitions of control characters and characters in the basic character set already do not apply, as they require that the preprocessing tokens for literals have already been recognized; this occurs in phase 3, which also replaces comments with single spaces. However, the prohibition of surrogate code points is not so limited and might conceivably be applied within comments.

Probably the most straightforward way of addressing this problem would be simply to state in 5.7 [lex.comment] that character sequences that resemble universal-character-names are not recognized as such within comment text.




1972. Identifier character restrictions in non-identifiers

Section: 5.10  [lex.name]     Status: open     Submitter: Richard Smith     Date: 2014-07-15

According to 5.10 [lex.name] paragraph 1,

Each universal-character-name in an identifier shall designate a character whose encoding in ISO 10646 falls into one of the ranges specified in _N4606_.E.1 [charname.allowed].

However, identifier-nondigit is also used in the grammar for pp-number. Should this restriction also be understood to apply in that non-identifier context?




1266. user-defined-integer-literal overflow

Section: 5.13.8  [lex.ext]     Status: open     Submitter: Michael Wong     Date: 2011-03-20

The decimal-literal in a user-defined-integer-literal might be too large for an unsigned long long to represent (in implementations with extended integer types). In such cases, the original intent appears to have been to call a raw literal operator or a literal operator template; however, the existing wording of 5.13.8 [lex.ext] paragraph 3 always calls the unsigned long long literal operator if it exists, regardless of the value of the decimal-literal.




1209. Is a potentially-evaluated expression in a template definition a “use?”

Section: 6.3  [basic.def.odr]     Status: open     Submitter: Johannes Schaub     Date: 2010-10-08

Consider the following complete program:

    void f();
    template<typename T> void g() { f(); }
    int main() { }

Must f() be defined to make this program well-formed? The current wording of 6.3 [basic.def.odr] does not make any special provision for expressions that appear only in uninstantiated template definitions.

(See also issue 1254.)


2488. Overloading virtual functions and functions with trailing requires-clauses

Section: 6.4.1  [basic.scope.scope]     Status: open     Submitter: Jiang An     Date: 2020-08-19

According to 6.4.1 [basic.scope.scope] paragraph 3,

Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unless

This would indicate that a virtual function (which cannot have a trailing requires-clause, per 11.7.3 [class.virtual] paragraph 6) can be overloaded with a non-virtual member function with the same parameter type list but with a trailing requires-clause. However, this is not implementable on some ABIs, since the mangling of the two functions would be the same. For example:


  #include <type_traits>
  template<class T>
  struct Foo {
     virtual void fun() const {}
     void fun() const requires std::is_object_v<T> {}
  };
  int main() {
    Foo<int>{}.fun();
  }

Should such overloading be ill-formed or conditionally-supported, or should the current rules be kept?

Rationale (August, 2021):

CWG felt that the current rules are correct; it simply means that only the virtual function can be called, and all other references are simply ambiguous. (See also issue 2501 for a related question dealing with explicit instantiation.

Notes from the November, 2021 teleconference:

The issue has been reopened in response to additional discussion.




380. Definition of "ambiguous base class" missing

Section: 6.5.2  [class.member.lookup]     Status: open     Submitter: Jason Merrill     Date: 22 Oct 2002

The term "ambiguous base class" doesn't seem to be actually defined anywhere. 6.5.2 [class.member.lookup] paragraph 7 seems like the place to do it.




1953. Data races and common initial sequence

Section: 6.7.1  [intro.memory]     Status: open     Submitter: Faisal Vali     Date: 2014-06-23

According to 6.7.1 [intro.memory] paragraph 3,

A memory location is either an object of scalar type or a maximal sequence of adjacent bit-fields all having non-zero width. [Note: Various features of the language, such as references and virtual functions, might involve additional memory locations that are not accessible to programs but are managed by the implementation. —end note] Two or more threads of execution (6.9.2 [intro.multithread]) can update and access separate memory locations without interfering with each other.

It is not clear how this relates to the permission granted in 11.4 [class.mem] paragraph 18 to inspect the common initial sequence of standard-layout structs that are members of a standard-layout union. If one thread is writing to the common initial sequence and another is reading from it via a different struct, that should constitute a data race, but the current wording does not clearly state that.




2334. Creation of objects by typeid

Section: 6.7.2  [intro.object]     Status: open     Submitter: Chris Hallock     Date: 2017-01-30

The list of ways that an object may be created in 6.7.2 [intro.object] paragraph 1 does not include creation of type_info objects by typeid expressions, but 7.6.1.8 [expr.typeid] does not appear to require that such objects exist before they are referenced. Should the list in 6.7.2 [intro.object] be extended to include this case?




2489. Storage provided by array of char

Section: 6.7.2  [intro.object]     Status: open     Submitter: Jiang An     Date: 2021-04-15

According to 6.7.2 [intro.object] paragraph 3,

If a complete object is created (7.6.2.8 [expr.new]) in storage associated with another object e of type “array of N unsigned char” or of type “array of N std::byte” (17.2.1 [cstddef.syn]), that array provides storage for the created object if...

However, note 4 in paragraph 13 indicates that a char array can also provide storage:

An operation that begins the lifetime of an array of char, unsigned char, or std::byte implicitly creates objects within the region of storage occupied by the array.

[Note 4: The array object provides storage for these objects. —end note]

The normative text and the note should be reconciled.




419. Can cast to virtual base class be done on partially-constructed object?

Section: 6.7.3  [basic.life]     Status: open     Submitter: Judy Ward     Date: 2 June 2003

Consider

  extern "C" int printf (const char *,...);

  struct Base { Base();};
  struct Derived: virtual public Base {
     Derived() {;}
  };

  Derived d;
  extern Derived& obj = d;

  int i;

  Base::Base() {
    if ((Base *) &obj) i = 4;
    printf ("i=%d\n", i);
  }

  int main() { return 0; }

11.9.5 [class.cdtor] paragraph 2 makes this valid, but 6.7.3 [basic.life] paragraph 5 implies that it isn't valid.

Steve Adamczyk: A second issue:

  extern "C" int printf(const char *,...);
  struct A                      { virtual ~A(); int x; };
  struct B : public virtual A   { };
  struct C : public B           { C(int); };
  struct D : public C           { D(); };
 
  int main()                    { D t; printf("passed\n");return 0; }
 
  A::~A()                       {} 
  C::C(int)                     {} 
  D::D() : C(this->x)           {}

Core issue 52 almost, but not quite, says that in evaluating "this->x" you do a cast to the virtual base class A, which would be an error according to 11.9.5 [class.cdtor] paragraph 2 because the base class B constructor hasn't started yet. 7.6.1.5 [expr.ref] should be clarified to say that the cast does need to get done.

James Kanze submitted the same issue via comp.std.c++ on 11 July 2003:

Richard Smith: Nonsense. You can use "this" perfectly happily in a constructor, just be careful that (a) you're not using any members that are not fully initialised, and (b) if you're calling virtual functions you know exactly what you're doing.

In practice, and I think in intent, you are right. However, the standard makes some pretty stringent restrictions in 6.7.3 [basic.life]. To start with, it says (in paragraph 1):

The lifetime of an object is a runtime property of the object. The lifetime of an object of type T begins when: The lifetime of an object of type T ends when:
(Emphasis added.) Then when we get down to paragraph 5, it says:

Before the lifetime of an object has started but after the storage which the object will occupy has been allocated [which sounds to me like it would include in the constructor, given the text above] or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any pointer that refers to the storage location where the object will be or was located may be used but only in limited ways. [...] If the object will be or was of a non-POD class type, the program has undefined behavior if:

[...]

I can't find any exceptions for the this pointer.

Note that calling a non-static function in the base class, or even constructing the base class in initializer list, involves an implicit conversion of this to a pointer to the base class. Thus undefined behavior. I'm sure that this wasn't the intent, but it would seem to be what this paragraph is saying.




2258. Storage deallocation during period of destruction

Section: 6.7.3  [basic.life]     Status: open     Submitter: Richard Smith     Date: 2016-04-12

[Detailed description pending.]

Notes from the December, 2016 teleconference:

The consensus view was that this should be undefined behavior.




365. Storage duration and temporaries

Section: 6.7.5  [basic.stc]     Status: open     Submitter: James Kanze     Date: 24 July 2002

There are several problems with 6.7.5 [basic.stc]:

Steve Adamczyk: There may well be an issue here, but one should bear in mind the difference between storage duration and object lifetime. As far as I can see, there is no particular problem with temporaries having automatic or static storage duration, as appropriate. The point of 6.7.7 [class.temporary] is that they have an unusual object lifetime.

Notes from Ocrober 2002 meeting:

It might be desirable to shorten the storage duration of temporaries to allow reuse of them. The as-if rule allows some reuse, but such reuse requires analysis, including noting whether the addresses of such temporaries have been taken.

Notes from the August, 2011 meeting:

The CWG decided that further consideration of this issue would be deferred until someone produces a paper explaining the need for action and proposing specific changes.




1682. Overly-restrictive rules on function templates as allocation functions

Section: 6.7.5.5.2  [basic.stc.dynamic.allocation]     Status: open     Submitter: Jason Merrill     Date: 2009-03-03

Requirements for allocation functions are given in 6.7.5.5.2 [basic.stc.dynamic.allocation] paragraph 1:

An allocation function can be a function template. Such a template shall declare its return type and first parameter as specified above (that is, template parameter types shall not be used in the return type and first parameter type). Template allocation functions shall have two or more parameters.

There are a couple of problems with this description. First, it is instances of function templates that can be allocation functions, not the templates themselves (cf 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 2, which uses the correct terminology regarding deallocation functions).

More importantly, this specification was written before template metaprogramming was understood and hence prevents use of SFINAE on the return type or parameter type to select among function template specializations. (The parallel passage for deallocation functions in 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 2 shares this deficit.)

(See also issue 1628.)




523. Can a one-past-the-end pointer be invalidated by deleting an adjacent object?

Section: 6.7.5.5.3  [basic.stc.dynamic.deallocation]     Status: open     Submitter: comp.std.c++     Date: 8 July 2005

When an object is deleted, 6.7.5.5.3 [basic.stc.dynamic.deallocation] says that the deallocation “[renders] invalid all pointers referring to any part of the deallocated storage.” According to 6.8.3 [basic.compound] paragraph 3, a pointer whose address is one past the end of an array is considered to point to an unrelated object that happens to reside at that address. Does this need to be clarified to specify that the one-past-the-end pointer of an array is not invalidated by deleting the following object? (See also 7.6.2.9 [expr.delete] paragraph 4, which also mentions that the system deallocation function renders a pointer invalid.)




2434. Mandatory copy elision vs non-class objects

Section: 6.7.7  [class.temporary]     Status: open     Submitter: Richard Smith     Date: 2019-09-30

In the following example,

  int f() {
    X x;
    return 4;
  }
  int a = f();

a must be directly initialized in the return statement of f() because the exception permitting temporaries for function arguments and return types in 6.7.7 [class.temporary] paragraph 3 applies only to certain class types:

When an object of class type X is passed to or returned from a function, if X has at least one eligible copy or move constructor (11.4.4 [special]), each such constructor is trivial, and the destructor of X is either trivial or deleted, implementations are permitted to create a temporary object to hold the function parameter or result object. The temporary object is constructed from the function argument or return value, respectively, and the function's parameter or return object is initialized as if by using the eligible trivial constructor to copy the temporary (even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object). [Note: This latitude is granted to allow objects of class type to be passed to or returned from functions in registers. —end note]

This requirement is observable, since the destructor of X in the example could inspect the value of a.

The permissions in this paragraph should also apply to all non-class types.




350. signed char underlying representation for objects

Section: 6.8  [basic.types]     Status: open     Submitter: Noah Stein     Date: 16 April 2002

Sent in by David Abrahams:

Yes, and to add to this tangent, 6.8.2 [basic.fundamental] paragraph 1 states "Plain char, signed char, and unsigned char are three distinct types." Strangely, 6.8 [basic.types] paragraph 2 talks about how "... the underlying bytes making up the object can be copied into an array of char or unsigned char. If the content of the array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value." I guess there's no requirement that this copying work properly with signed chars!

Notes from October 2002 meeting:

We should do whatever C99 does. 6.5p6 of the C99 standard says "array of character type", and "character type" includes signed char (6.2.5p15), and 6.5p7 says "character type". But see also 6.2.6.1p4, which mentions (only) an array of unsigned char.

Proposed resolution (April 2003):

Change 6.7.3 [basic.life] paragraph 5 bullet 3 from

to

Change 6.7.3 [basic.life] paragraph 6 bullet 3 from

to

Change the beginning of 6.8 [basic.types] paragraph 2 from

For any object (other than a base-class subobject) of POD type T, whether or not the object holds a valid value of type T, the underlying bytes (6.7.1 [intro.memory]) making up the object can be copied into an array of char or unsigned char.

to

For any object (other than a base-class subobject) of POD type T, whether or not the object holds a valid value of type T, the underlying bytes (6.7.1 [intro.memory]) making up the object can be copied into an array of byte-character type.

Add the indicated text to 6.8.2 [basic.fundamental] paragraph 1:

Objects declared as characters (char) shall be large enough to store any member of the implementation's basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types, called the byte-character types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (6.8 [basic.types]); that is, they have the same object representation. For byte-character types, all bits of the object representation participate in the value representation. For unsigned byte-character types, all possible bit patterns of the value representation represent numbers. These requirements do not hold for other types. In any particular implementation, a plain char object can take on either the same values as a signed char or an unsigned char; which one is implementation-defined.

Change 7.2.1 [basic.lval] paragraph 15 last bullet from

to

Notes from October 2003 meeting:

It appears that in C99 signed char may have padding bits but no trap representation, whereas in C++ signed char has no padding bits but may have -0. A memcpy in C++ would have to copy the array preserving the actual representation and not just the value.

March 2004: The liaisons to the C committee have been asked to tell us whether this change would introduce any unnecessary incompatibilities with C.

Notes from October 2004 meeting:

The C99 Standard appears to be inconsistent in its requirements. For example, 6.2.6.1 paragraph 4 says:

The value may be copied into an object of type unsigned char [n] (e.g., by memcpy); the resulting set of bytes is called the object representation of the value.

On the other hand, 6.2 paragraph 6 says,

If a value is copied into an object having no declared type using memcpy or memmove, or is copied as an array of character type, then the effective type of the modified object for that access and for subsequent accesses that do not modify the value is the effective type of the object from which the value is copied, if it has one.

Mike Miller will investigate further.

Proposed resolution (February, 2010):

  1. Change 6.7.3 [basic.life] paragraph 5 bullet 4 as follows:

  2. ...The program has undefined behavior if:

  3. Change 6.7.3 [basic.life] paragraph 6 bullet 4 as follows:

  4. ...The program has undefined behavior if:

  5. Change 6.8 [basic.types] paragraph 2 as follows:

  6. For any object (other than a base-class subobject) of trivially copyable type T, whether or not the object holds a valid value of type T, the underlying bytes (6.7.1 [intro.memory]) making up the object can be copied into an array of char or unsigned char a byte-character type (6.8.2 [basic.fundamental]).39 If the content of the that array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value. [Example:...
  7. Change 6.8.2 [basic.fundamental] paragraph 1 as follows:

  8. ...Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types, called the byte-character types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (6.7.6 [basic.align]); that is, they have the same object representation. For byte-character types, all bits of the object representation participate in the value representation. For unsigned character types unsigned char, all possible bit patterns of the value representation represent numbers...
  9. Change 7.2.1 [basic.lval] paragraph 15 final bullet as follows:

  10. If a program attempts to access the stored value of an object through an lvalue of other than one of the following types the behavior is undefined 52

  11. Change 6.7.6 [basic.align] paragraph 6 as follows:

  12. The alignment requirement of a complete type can be queried using an alignof expression (7.6.2.6 [expr.alignof]). Furthermore, the byte-character types (6.8.2 [basic.fundamental]) char, signed char, and unsigned char shall have the weakest alignment requirement. [Note: this enables the byte-character types to be used as the underlying type for an aligned memory area (9.12.2 [dcl.align]). —end note]
  13. Change 7.6.2.8 [expr.new] paragraph 10 as follows:

  14. ...For arrays of char and unsigned char a byte-character type (6.8.2 [basic.fundamental]), the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the strictest fundamental alignment requirement (6.7.6 [basic.align]) of any object type whose size is no greater than the size of the array being created. [Note: Because allocation functions are assumed to return pointers to storage that is appropriately aligned for objects of any type with fundamental alignment, this constraint on array allocation overhead permits the common idiom of allocating byte-character arrays into which objects of other types will later be placed. —end note]

Notes from the March, 2010 meeting:

The CWG was not convinced that there was a need to change the existing specification at this time. Some were concerned that there might be implementation difficulties with giving signed char the requisite semantics; implementations for which that is true can currently make char equivalent to unsigned char and avoid those problems, but the suggested change would undermine that strategy.

Additional note, November, 2014:

There is now the term “narrow character type” that should be used instead of “byte-character type”.




146. Floating-point zero

Section: 6.8.2  [basic.fundamental]     Status: open     Submitter: Andy Sawyer     Date: 23 Jul 1999

6.8.2 [basic.fundamental] does not impose a requirement on the floating point types that there be an exact representation of the value zero. This omission is significant in 7.3.14 [conv.fctptr] paragraph 1, in which any non-zero value converts to the bool value true.

Suggested resolution: require that all floating point types have an exact representation of the value zero.




251. How many signed integer types are there?

Section: 6.8.2  [basic.fundamental]     Status: open     Submitter: Beman Dawes     Date: 18 Oct 2000

6.8.2 [basic.fundamental] paragraph 2 says that

There are four signed integer types: "signed char", "short int", "int", and "long int."

This would indicate that const int is not a signed integer type.

Notes from the June, 2016 meeting:

See issue 2185.




689. Maximum values of signed and unsigned integers

Section: 6.8.2  [basic.fundamental]     Status: open     Submitter: James Kanze     Date: 30 March, 2008

The relationship between the values representable by corresponding signed and unsigned integer types is not completely described, but 6.8 [basic.types] paragraph 4 says,

The value representation of an object is the set of bits that hold the value of type T.

and 6.8.2 [basic.fundamental] paragraph 3 says,

The range of nonnegative values of a signed integer type is a subrange of the corresponding unsigned integer type, and the value representation of each corresponding signed/unsigned type shall be the same.

I.e., the maximum value of each unsigned type must be larger than the maximum value of the corresponding signed type.

C90 doesn't have this restriction, and C99 explicitly says (6.2.6.2, paragraph 2),

For signed integer types, the bits of the object representation shall be divided into three groups: value bits, padding bits, and the sign bit. There need not be any padding bits; there shall be exactly one sign bit. Each bit that is a value bit shall have the same value as the same bit in the object representation of the corresponding unsigned type (if there are M value bits in the signed type and N in the unsigned type, then M <= N).

Unlike C++, the sign bit is not part of the value, and on an architecture that does not have native support of unsigned types, an implementation can emulate unsigned integers by simply ignoring what would be the sign bit in the signed type and be conforming.

The question is whether we intend to make a conforming implementation on such an architecture impossible. More generally, what range of architectures do we intend to support? And to what degree do we want to follow C99 in its evolution since C89?

(See paper J16/08-0141 = WG21 N2631.)




2185. Cv-qualified numeric types

Section: 6.8.2  [basic.fundamental]     Status: open     Submitter: CWG     Date: 2015-10-21

The definitions of integral, floating, and arithmetic types in 6.8.2 [basic.fundamental] paragraphs 7-8 do not, but presumably should, include cv-qualified versions of those fundamental types.

Notes from the June, 2016 meeting:

This issue subsumes issue 251.




2475. Object declarations of type cv void

Section: 6.8.2  [basic.fundamental]     Status: open     Submitter: Krystian Stasiowski     Date: 2020-04-22

(From editorial issue 3953.)

Although an object cannot be defined with a type of cv void, there is nothing preventing a non-defining declaration of an object with that type. Should it be disallowed?

Notes from the December, 2020 teleconference:

Such declarations are permitted in C, so this question was referred to the C liaison for investigation.




698. The definition of “sequenced before” is too narrow

Section: 6.9.1  [intro.execution]     Status: open     Submitter: Jens Maurer     Date: 13 July, 2008

According to 6.9.1 [intro.execution] paragraph 14, “sequenced before” is a relation between “evaluations.” However, 6.9.3.3 [basic.start.dynamic] paragraph 3 says,

If the completion of the initialization of a non-local object with static storage duration is sequenced before a call to std::atexit (see <cstdlib>, 17.5 [support.start.term]), the call to the function passed to std::atexit is sequenced before the call to the destructor for the object. If a call to std::atexit is sequenced before the completion of the initialization of a non-local object with static storage duration, the call to the destructor for the object is sequenced before the call to the function passed to std::atexit. If a call to std::atexit is sequenced before another call to std::atexit, the call to the function passed to the second std::atexit call is sequenced before the call to the function passed to the first std::atexit call.

Except for the calls to std::atexit, these events do not correspond to “evaluation” of expressions that appear in the program. If the “sequenced before” relation is to be applied to them, a more comprehensive definition is needed.




2297. Unclear specification of atomic operations

Section: 6.9.2.2  [intro.races]     Status: open     Submitter: Kazutoshi Satoda     Date: 2016-01-21

It is not sufficiently clear that the only atomic operations are the ones defined in clause Clause 31 [atomics] by the library. The intent is that no accesses are atomic unless the Standard describes them as such.

An additional problem is that, e.g., new and delete are defined to be synchronization operations, but they are not defined in Clauses Clause 31 [atomics] and Clause 32 [thread].

Suggested resolution:

Change 6.9.2.2 [intro.races] paragraph 3 as follows:

The library defines a number the set of atomic operations (Clause Clause 31 [atomics]) and operations on mutexes (Clause Clause 32 [thread]) that. Some of these, and some other library operations, such as those on mutexes (Clause Clause 32 [thread]) are specially identified as synchronization operations. These operations...

Notes from the April, 2017 teleconference:

CWG determined that this issue should be handled editorially; it will be in "review" status until the change has been made and verified. See editorial issue 1611.

Additional notes, October, 2018:

This is also library issue 2506. SG1 has requested a paper to deal with this issue, so it is no longer considered editorial.




371. Interleaving of constructor calls

Section: 6.9.3.2  [basic.start.static]     Status: open     Submitter: Matt Austern     Date: 7 August 2002

Is a compiler allowed to interleave constructor calls when performing dynamic initialization of nonlocal objects? What I mean by interleaving is: beginning to execute a particular constructor, then going off and doing something else, then going back to the original constructor. I can't find anything explicit about this in clause 6.9.3.2 [basic.start.static].

I'll present a few different examples, some of which get a bit wild. But a lot of what this comes down to is exactly what the standard means when it talks about the order of initialization. If it says that some object x must be initialized before a particular event takes place, does that mean that x's constructor must be entered before that event, or does it mean that it must be exited before that event? If object x must be initialized before object y, does that mean that x's constructor must exit before y's constructor is entered?

(The answer to that question might just be common sense, but I couldn't find an answer in clause 6.9.3.2 [basic.start.static]. Actually, when I read 6.9.3.2 [basic.start.static] carefully, I find there are a lot of things I took for granted that aren't there.)

OK, so a few specific scenerios.

  1. We have a translation unit with nonlocal objects A and B, both of which require dynamic initialization. A comes before B. A must be initialized before B. May the compiler start to construct A, get partway through the constructor, then construct B, and then go back to finishing A?
  2. We have a translation unit with nonlocal object A and function f. Construction of A is deferred until after the first statement of main. A must be constructed before the first use of f. Is the compiler permitted to start constructing A, then execute f, then go back to constructing A?
  3. We have nonlocal objects A and B, in two different translation units. The order in which A and B are constructed is unspecified by the Standard. Is the compiler permitted to begin constructing A, then construct B, then finish A's constructor? Note the implications of a 'yes' answer. If A's and B's constructor both call some function f, then the call stack might look like this:
       <runtime gunk>
         <Enter A's constructor>
            <Enter f>
               <runtime gunk>
                  <Enter B's constructor>
                     <Enter f>
                     <Leave f>
                  <Leave B's constructor>
            <Leave f>
         <Leave A's constructor>
    
    The implication of a 'yes' answer for users is that any function called by a constructor, directly or indirectly, must be reentrant.
  4. This last example is to show why a 'no' answer to #3 might be a problem too. New scenerio: we've got one translation unit containing a nonlocal object A and a function f1, and another translation unit containing a nonlocal object B and a function f2. A's constructor calls f2. Initialization of A and B is deferred until after the first statement of main(). Someone in main calls f1. Question: is the compiler permitted to start constructing A, then go off and construct B at some point before f2 gets called, then go back and finish constructing A? In fact, is the compiler required to do that? We've got an unpleasant tension here between the bad implications of a 'yes' answer to #3, and the explicit requirement in 6.9.3.2 [basic.start.static] paragraph 3.

At this point, you might be thinking we could avoid all of this nonsense by removing compilers' freedom to defer initialization until after the beginning of main(). I'd resist that, for two reasons. First, it would be a huge change to make after the standard has been out. Second, that freedom is necessary if we want to have support for dynamic libraries. I realize we don't yet say anything about dynamic libraries, but I'd hate to make decisions that would make such support even harder.




1659. Initialization order of thread_local template static data members

Section: 6.9.3.2  [basic.start.static]     Status: open     Submitter: Richard Smith     Date: 2013-04-14

According to 6.9.3.2 [basic.start.static] paragraph 5,

It is implementation-defined whether the dynamic initialization of a non-local variable with static or thread storage duration is done before the first statement of the initial function of the thread. If the initialization is deferred to some point in time after the first statement of the initial function of the thread, it shall occur before the first odr-use (6.3 [basic.def.odr]) of any variable with thread storage duration defined in the same translation unit as the variable to be initialized.

This doesn't consider that initialization of instantiations of static data members of class templates (which can be thread_local) are unordered. Presumably odr-use of such a static data member should not trigger the initialization of any thread_local variable other than that one?




640. Accessing destroyed local objects of static storage duration

Section: 6.9.3.3  [basic.start.dynamic]     Status: open     Submitter: Howard Hinnant     Date: 30 July 2007

6.9.3.3 [basic.start.dynamic] paragraph 2 says,

If a function contains a local object of static storage duration that has been destroyed and the function is called during the destruction of an object with static storage duration, the program has undefined behavior if the flow of control passes through the definition of the previously destroyed local object.

I would like to turn this behavior from undefined to well-defined behavior for the purpose of achieving a graceful shutdown, especially in a multi-threaded world.

Background: Alexandrescu describes the “phoenix singleton” in Modern C++ Design. This is a class used as a function local static, that will reconstruct itself, and reapply itself to the atexit chain, if the program attempts to use it after it is destructed in the atexit chain. It achieves this by setting a “destructed flag” in its own state in its destructor. If the object is later accessed (and a member function is called on it), the member function notes the state of the “destructed flag” and does the reconstruction dance. The phoenix singleton pattern was designed to address issues only in single-threaded code where accesses among static objects can have a non-scoped pattern. When we throw in multi-threading, and the possibility that threads can be running after main returns, the chances of accessing a destroyed static significantly increase.

The very least that I would like to see happen is to standardize what I believe is existing practice: When an object is destroyed in the atexit chain, the memory the object occupied is left in whatever state the destructor put it in. If this can only be reliably done for objects with standard layout, that would be an acceptable compromise. This would allow objects to set “I'm destructed” flags in their state and then do something well-defined if accessed, such as throw an exception.

A possible refinement of this idea is to have the compiler set up a 3-state flag around function-local statics instead of the current 2-state flag:

We have the first two states today. We might choose to add the third state, and if execution passes over a function-local static with “destroyed” state, an exception could be thrown. This would mean that we would not have to guarantee memory stability in destroyed objects of static duration.

This refinement would break phoenix singletons, and is not required for the ~mutex()/~condition() I've described and prototyped. But it might make it easier for Joe Coder to apply this kind of guarantee to his own types.




2438. Problems in the specification of qualification conversions

Section: 7.3.6  [conv.qual]     Status: open     Submitter: Richard Smith     Date: 2019-08-14
  1. A type has multiple cv-decompositions, and 7.3.6 [conv.qual] paragraph 3 does not say which one to use when determining the cv-combined type. Should this be the longest decomposition that works, i.e., the greatest n for which you can decompose both types? (We used to refer to the cv-qualification signature, which implicitly meant to take the longest decomposition.)

  2. When computing the cv-combined types of two types T1 and T2, if U1 and U2 are different, shouldn't we add const to all layers above that in the type?

  3. cv03 is left unspecified by the wording in paragraph 3.

  4. We are too eager to replace a Pi3 with “array of unknown bound of”. That should only happen if both Pi1 and Pi2 are array types, or we end up not forming a type T3 that is similar to T1. For example, the cv-combined type of int** and const int (*)[], when decomposed with n == 2, is required to be const int (*)[] by the bulleted rules, and that type is not similar to the original T1.

  5. In various places, we have operators that say, “if one operand is of pointer type, apply array-to-pointer conversions, pointer conversions, and qualification conversions to bring the two operands to their composite pointer type,” but that doesn't work, because the definition of composite pointer type can't cope with one operand being a pointer and the other being an array. We either need to define the composite pointer type of a pointer and an array (and, if that's done in terms of computing the cv-combined type, be careful to ensure that computing the cv-combined type actually works in that case) or to perform the array-to-pointer conversion before considering the composite pointer type.




2485. Bit-fields in integral promotions

Section: 7.3.7  [conv.prom]     Status: open     Submitter: Richard Smith     Date: 2021-04-01

According to 7.3.7 [conv.prom] paragraph 5,

A prvalue for an integral bit-field (11.4.10 [class.bit]) can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field. If the bit-field is larger yet, no integral promotion applies to it. If the bit-field has an enumerated type, it is treated as any other value of that type for promotion purposes.

This description has several problems. First, the “bit-field” semantic property only makes sense for glvalue expressions, so it's unclear why these rules are described as applying to a prvalue. Perhaps this should be rephrased as something like “An expression that was a bit-field glvalue prior to the application of the lvalue-to-rvalue conversion”?

Second, suppose that char32_t is wider than int. Per paragraph 2, a char32_t prvalue promotes to unsigned long (because unsigned long is necessarily at least 32 bits wide). But per paragraph 5, a char32_t : 32 bitfield does not promote. This seems inconsistent.

Finally, it is not clear that the usual integral promotions are not applied to bit-fields. This should be made explicit.




2284. Sequencing of braced-init-list arguments

Section: 7.6.1.3  [expr.call]     Status: open     Submitter: Richard Smith     Date: 2016-06-30

[Detailed description pending.]




742. Postfix increment/decrement with long bit-field operands

Section: 7.6.1.6  [expr.post.incr]     Status: open     Submitter: Mike Miller     Date: 11 November, 2008

Given the following declarations:

    struct S {
        signed long long sll: 3;
    };
    S s = { -1 };

the expressions s.sll-- < 0u and s.sll < 0u have different results. The reason for this is that s.sll-- is an rvalue of type signed long long (7.6.1.6 [expr.post.incr]), which means that the usual arithmetic conversions (Clause 7 [expr] paragraph 10) convert 0u to signed long long and the result is true. s.sll, on the other hand, is a bit-field lvalue, which is promoted (7.3.7 [conv.prom] paragraph 3) to int; both operands of < have the same rank, so s.sll is converted to unsigned int to match the type of 0u and the result is false. This disparity seems undesirable.




282. Namespace for extended_type_info

Section: 7.6.1.8  [expr.typeid]     Status: open     Submitter: Jens Maurer     Date: 01 May 2001

The original proposed resolution for issue 160 included changing extended_type_info (7.6.1.8 [expr.typeid] paragraph 1, footnote 61) to std::extended_type_info. There was no consensus on whether this name ought to be part of namespace std or in a vendor-specific namespace, so the question was moved into a separate issue.




528. Why are incomplete class types not allowed with typeid?

Section: 7.6.1.8  [expr.typeid]     Status: open     Submitter: Dave Abrahams     Date: 18 May 2005

7.6.1.8 [expr.typeid] paragraph 4 says,

When typeid is applied to a type-id, the result refers to a std::type_info object representing the type of the type-id. If the type of the type-id is a reference type, the result of the typeid expression refers to a std::type_info object representing the referenced type. If the type of the type-id is a class type or a reference to a class type, the class shall be completely-defined.

I'm wondering whether this is not overly restrictive. I can't think of a reason to require that T be completely-defined in typeid(T) when T is a class type. In fact, several popular compilers enforce that restriction for typeid(T), but not for typeid(T&). Can anyone explain this?

Nathan Sidwell: I think this restriction is so that whenever the compiler has to emit a typeid object of a class type, it knows what the base classes are, and can therefore emit an array of pointers-to-base-class typeids. Such a tree is necessary to implement dynamic_cast and exception catching (in a commonly implemented and obvious manner). If the class could be incomplete, the compiler might have to emit a typeid for incomplete Foo in one object file and a typeid for complete Foo in another object file. The compilation system will then have to make sure that (a) those compare equal and (b) the complete Foo gets priority, if that is applicable.

Unfortunately, there is a problem with exceptions that means there still can be a need to emit typeids for incomplete class. Namely one can throw a pointer-to-pointer-to-incomplete. To implement the matching of pointer-to-derived being caught by pointer-to-base, it is necessary for the typeid of a pointer type to contain a pointer to the typeid of the pointed-to type. In order to do the qualification matching on a multi-level pointer type, one has a chain of pointer typeids that can terminate in the typeid of an incomplete type. You cannot simply NULL-terminate the chain, because one must distinguish between different incomplete types.

Dave Abrahams: So if implementations are still required to be able to do it, for all practical purposes, why aren't we letting the user have the benefits?

Notes from the April, 2006 meeting:

There was some concern expressed that this might be difficult under the IA64 ABI. It was also observed that while it is necessary to handle exceptions involving incomplete types, there is no requirement that the RTTI data structures be used for exception handling.




1954. typeid null dereference check in subexpressions

Section: 7.6.1.8  [expr.typeid]     Status: open     Submitter: David Majnemer     Date: 2014-06-23

According to 7.6.1.8 [expr.typeid] paragraph 2,

If the glvalue expression is obtained by applying the unary * operator to a pointer69 and the pointer is a null pointer value (7.3.12 [conv.ptr]), the typeid expression throws an exception (14.2 [except.throw]) of a type that would match a handler of type std::bad_typeid exception (17.7.5 [bad.typeid]).

The footnote makes clear that this requirement applies without regard to parentheses, but it is unspecified whether it applies when the dereference occurs in a subexpression of the operand (e.g., in the second operand of the comma operator or the second or third operand of a conditional operator). There is implementation divergence on this question.




2048. C-style casts that cast away constness vs static_cast

Section: 7.6.1.9  [expr.static.cast]     Status: open     Submitter: Richard Smith     Date: 2014-11-19

According to 7.6.1.9 [expr.static.cast] paragraph 1,

The static_cast operator shall not cast away constness (7.6.1.11 [expr.const.cast]).

However, this phrasing is problematic in the context of a C-style cast like the following:

   const void *p;
   int *q = (int*)p;

The intent of 7.6.3 [expr.cast] is that this should be interpreted as a static_cast followed by a const_cast. However, because int* to const void* is a valid standard conversion, and 7.6.1.9 [expr.static.cast] paragraph 7 allows static_cast to perform the inverse of a standard conversion sequence, the C-style cast is interpreted as just a static_cast without a const_cast and is thus ill-formed.




267. Alignment requirement for new-expressions

Section: 7.6.2.8  [expr.new]     Status: open     Submitter: James Kuyper     Date: 4 Dec 2000

Requirements for the alignment of pointers returned by new-expressions are given in 7.6.2.8 [expr.new] paragraph 10:

For arrays of char and unsigned char, the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the most stringent alignment requirement (6.8 [basic.types]) of any object type whose size is no greater than the size of the array being created.

The intent of this wording is that the pointer returned by the new-expression will be suitably aligned for any data type that might be placed into the allocated storage (since the allocation function is constrained to return a pointer to maximally-aligned storage). However, there is an implicit assumption that each alignment requirement is an integral multiple of all smaller alignment requirements. While this is probably a valid assumption for all real architectures, there's no reason that the Standard should require it.

For example, assume that int has an alignment requirement of 3 bytes and double has an alignment requirement of 4 bytes. The current wording only requires that a buffer that is big enough for an int or a double be aligned on a 4-byte boundary (the more stringent requirement), but that would allow the buffer to be allocated on an 8-byte boundary — which might not be an acceptable location for an int.

Suggested resolution: Change "of any object type" to "of every object type."

A similar assumption can be found in 7.6.1.10 [expr.reinterpret.cast] paragraph 7:

...converting an rvalue of type "pointer to T1" to the type "pointer to T2" (where ... the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer value...

Suggested resolution: Change the wording to

...converting an rvalue of type "pointer to T1" to the type "pointer to T2" (where ... the alignment requirements of T1 are an integer multiple of those of T2) and back to its original type yields the original pointer value...

The same change would also be needed in paragraph 9.




473. Block-scope declarations of allocator functions

Section: 7.6.2.8  [expr.new]     Status: open     Submitter: John Spicer     Date: 12 Jul 2004

Looking up operator new in a new-expression uses a different mechanism from ordinary lookup. According to 7.6.2.8 [expr.new] paragraph 9,

If the new-expression begins with a unary :: operator, the allocation function's name is looked up in the global scope. Otherwise, if the allocated type is a class type T or array thereof, the allocation function's name is looked up in the scope of T. If this lookup fails to find the name, or if the allocated type is not a class type, the allocation function's name is looked up in the global scope.

Note in particular that the scope in which the new-expression occurs is not considered. For example,

    void f() {
        void* operator new(std::size_t, void*);
        int* i = new int;    // okay?
    }

In this example, the implicit reference to operator new(std::size_t) finds the global declaration, even though the block-scope declaration of operator new with a different signature would hide it from an ordinary reference.

This seems strange; either the block-scope declaration should be ill-formed or it should be found by the lookup.

Notes from October 2004 meeting:

The CWG agreed that the block-scope declaration should not be found by the lookup in a new-expression. It would, however, be found by ordinary lookup if the allocation function were invoked explicitly.




1628. Deallocation function templates

Section: 7.6.2.8  [expr.new]     Status: open     Submitter: Richard Smith     Date: 2013-02-22

According to 7.6.2.8 [expr.new] paragraphs 18-20, an exception thrown during the initialization of an object allocated by a new-expression will cause a deallocation function to be called for the object's storage if a matching deallocation function can be found. The rules deal only with functions, however; nothing is said regarding a mechanism by which a deallocation function template might be instantiated to free the storage, although 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 2 indicates that a deallocation function can be an instance of a function template.

One possibility for this processing might be to perform template argument deduction on any deallocation function templates; if there is a specialization that matches the allocation function, by the criteria listed in paragraph 20, that function template would be instantiated and used, although a matching non-template function would take precedence as is the usual outcome of overloading between function template specializations and non-template functions.

Another possibility might be to match non-template deallocation functions with non-template allocation functions and template deallocation functions with template allocation functions.

There is a slightly related wording problem in 7.6.2.8 [expr.new] paragraph 21:

If a placement deallocation function is called, it is passed the same additional arguments as were passed to the placement allocation function, that is, the same arguments as those specified with the new-placement syntax.

This wording ignores the possibility of default arguments in the allocation function, in which case the arguments passed to the deallocation function might be a superset of those specified in the new-placement.

(See also issue 1682.)




196. Arguments to deallocation functions

Section: 7.6.2.9  [expr.delete]     Status: open     Submitter: Matt Austern     Date: 20 Jan 2000

7.6.2.8 [expr.new] paragraph 10 says that the result of an array allocation function and the value of the array new-expression from which it was invoked may be different, allowing for space preceding the array to be used for implementation purposes such as saving the number of elements in the array. However, there is no corresponding description of the relationship between the operand of an array delete-expression and the argument passed to its deallocation function.

6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 3 does state that

the value supplied to operator delete[](void*) in the standard library shall be one of the values returned by a previous invocation of either operator new[](std::size_t) or operator new[](std::size_t, const std::nothrow_t&) in the standard library.

This statement might be read as requiring an implementation, when processing an array delete-expression and calling the deallocation function, to perform the inverse of the calculation applied to the result of the allocation function to produce the value of the new-expression. (7.6.2.9 [expr.delete] paragraph 2 requires that the operand of an array delete-expression "be the pointer value which resulted from a previous array new-expression.") However, it is not completely clear whether the "shall" expresses an implementation requirement or a program requirement (or both). Furthermore, there is no direct statement about user-defined deallocation functions.

Suggested resolution: A note should be added to 7.6.2.9 [expr.delete] to clarify that any offset added in an array new-expression must be subtracted in the array delete-expression.




1256. Unevaluated operands are not necessarily constant expressions

Section: 7.7  [expr.const]     Status: open     Submitter: Nikolay Ivchenkov     Date: 2011-03-08

The current definition of constant expressions appears to make unevaluated operands constant expressions; for example, new char[10] would seem to be a constant expression if it appears as the operand of sizeof. This seems wrong.




2192. Constant expressions and order-of-eval undefined behavior

Section: 7.7  [expr.const]     Status: open     Submitter: Peter Sommerlad     Date: 2015-10-27

Notes from the November, 2016 meeting:

CWG did not wish to require implementations to detect this kind of undefined behavior in determining whether an expression is constant or not, but an implementation should be permitted to reject such expressions. These should be indeterminately sequenced, not unsequenced.




2301. Value-initialization and constexpr constructor evaluation

Section: 7.7  [expr.const]     Status: open     Submitter: Daveed Vandevoorde     Date: 2016-04-18

Consider the following example:

  union A {
    constexpr A(int) : x(++x) { }
    int x;
    char* y;
  };
  union B {
    A a = 5;
  };
  int arr[B().a.x];

Value-initialization of the object created by B() zero-initializes the object (9.4 [dcl.init] bullet 8.2), which should mean that the ++x in the mem-initilizer for A operates on a zero-initialized object, but current implementations reject this code as non-constant. It is not clear what in the current wording justifies this treatment.




2392. new-expression size check and constant evaluation

Section: 7.7  [expr.const]     Status: open     Submitter: Tam S. B     Date: 2018-12-05

According to 7.6.2.8 [expr.new] paragraph 8, if the expression in a noptr-new-declarator is a core constant expression, the program is ill-formed if the expression is erroneous, e.g., negative. However, consider the following example:

  template<class T = void> constexpr int f() { T t; return 1; }
  using _ = decltype(new int[f()]);

f() is a core constant expression, so it must be evaluated to determine its value. However, because the expression appears in an unevaluated operand, it is not “potentially constant evaluated” and thus f is not “needed for constant evaluation”, so the template is not instantiated (13.9.2 [temp.inst] paragraph 7). There is implementation divergence on the handling of this example.




2440. Allocation in core constant expressions

Section: 7.7  [expr.const]     Status: open     Submitter: Davis Herring     Date: 2019-08-28

7.7 [expr.const] paragraph 5 attempts to describe allowable allocation/deallocation calls in terms of what could be called “core constant subexpressions,” but the actual definition of a core constant expression in paragraph 4 is in terms of evaluation.

Suggested resolution:

Replace the entirety of 7.7 [expr.const] paragraph 5 with the following:

For the purposes of determining whether an expression is a core constant expression, the execution of the body of a member function of std::allocator<T> as defined in 20.10.10.2 [allocator.members], where T is a literal type, is ignored. Similarly, the execution of the body of std::destroy_at, std::ranges::destroy_at, std::construct_at, or std::ranges::construct_at is considered to include only the underlying constructor (for the functions construct_at) or destructor (for the functions destroy_at) call if the first argument (of type T*) points to storage allocated with std::allocator<T>).



2456. Viable user-defined conversions in converted constant expressions

Section: 7.7  [expr.const]     Status: open     Submitter: Mike Miller     Date: 2020-08-17

Consider an example like the following:

  struct A {
    constexpr A(int i) : val(i) { }
    constexpr operator int() const { return val; }
    constexpr operator float() const { return val; }
  private:
    int val;
  };
  constexpr A a = 42;
  int ary[a];

According to 9.3.4.5 [dcl.array] paragraph 1, the array bound expression

shall be a converted constant expression of type std::size_t (7.7 [expr.const]).

The user-defined conversion to float would involve a floating-integral conversion (7.3.11 [conv.fpint]; however, such a conversion is not permitted by the list of acceptable conversions in 7.7 [expr.const] paragraph 10:

A converted constant expression of type T is an expression, implicitly converted to type T, where the converted expression is a constant expression and the implicit conversion sequence contains only

and where the reference binding (if any) binds directly.

It is not clear whether this list is intended to restrict the set of viable user-defined conversions, and there is implementation divergence on this point: clang accepts the example above, while g++ rejects it, presumably on the basis of an ambiguous conversion.

Notes from the August, 2020 teleconference:

No direction was established pending information about why the example is accepted by clang.

Additional note, December, 2020:

The clang behavior turns out to have been an oversight, corrected in the current version, so the example is now rejected by both compilers. However, it is unclear that this is desirable. In particular, given the example above, a can be used without error as a bit-field width, as an enumerator value, and as the operand of alignas. Presumably the difference between these integral constant expression contexts and an array bound is the fact that the target type is known to be size_t. However, both bit-field widths and alignas operands are also required to be non-negative. Furthermore, the definition of an “erroneous” array bound in 7.6.2.8 [expr.new] paragraph 9 goes to awkward lengths to check for negative values as the result of user-defined conversions, which might argue in favor of reconsidering the converted constant expression treatment of array bounds.

Notes from the February, 2021 teleconference:

CWG agreed with the considerations in the December, 2020 note, feeling that the difference in treatment between integral constant expressions and a converted constant expression to a specific integral type is somewhat gratuitous. However, it was felt that code like that of the example was unlikely to occur often in real-world code.




2123. Omitted constant initialization of local static variables

Section: 8.8  [stmt.dcl]     Status: open     Submitter: Hubert Tong     Date: 2015-02-02

According to 8.8 [stmt.dcl] paragraph 4,

The zero-initialization (9.4 [dcl.init]) of all block-scope variables with static storage duration (6.7.5.2 [basic.stc.static]) or thread storage duration (6.7.5.3 [basic.stc.thread]) is performed before any other initialization takes place. Constant initialization (6.9.3.2 [basic.start.static]) of a block-scope entity with static storage duration, if applicable, is performed before its block is first entered.

The fact that a variable need not be constant-initialized if its block is not entered appears to allow inspection of the variable after zero-initialization but before constant initialization:

  constexpr int x = 0;

  auto foo() {
    constexpr static const int *p = &x;
    struct A {
      const int *const &getPtr() { return p; }
    } a;
    return a;
  }

  int xcpy = *decltype(foo()){ }.getPtr();

  int main(void) {
    return xcpy;
  }

For a related example, consider:

  // tu1.cpp
  extern const int a = 1;
  inline auto f() {
    static const int b = a;
    struct A { auto operator()() { return &b; } } a;
    return a;
  }

  // tu2.cpp
  extern const int a;
  inline auto f() {
    static const int b = a;
    struct A { auto operator()() { return &b; } } a;
    return a;
  }
  int main() {
    return *decltype(f())()();
  }

Here, b may or may not have constant initialization, but we don't have an ODR violation.

If we want to support such code, the nicest option would be to say that the ODR requires us to act as if we pick one of the definitions of the inline function, which requires us to make a consistent choice for all static storage duration variables within a given function. Alternatively, we could say that if multiple definitions of a variable disagree over whether it has constant initialization, then it does not, allowing more implementation simplicity and no functional change outside of pathological cases.

Notes from the February, 2016 meeting:

The second example will be dealt with separately under issue 2242. For the first example, the Standard should require that local types can be used outside their function only via a returned object. It was still to be decided whether this should be undefined behavior or an error on use of such a type. It was also noted that the same issue can arise with static member functions.




498. Storage class specifiers in definitions of class members

Section: 9.2.2  [dcl.stc]     Status: open     Submitter: Matt Austern     Date: 13 Jan 2005

Suppose we've got this class definition:

    struct X {
       void f();
       static int n;
    };

I think I can deduce from the existing standard that the following member definitions are ill-formed:

    static void X::f() { }
    static int X::n;

To come to that conclusion, however, I have to put together several things in different parts of the standard. I would have expected to find an explicit statement of this somewhere; in particular, I would have expected to find it in 9.2.2 [dcl.stc]. I don't see it there, or anywhere.

Gabriel Dos Reis: Or in 6.6 [basic.link] which is about linkage. I would have expected that paragraph to say that that members of class types have external linkage when the enclosing class has an external linkage. Otherwise 6.6 [basic.link] paragraph 8:

Names not covered by these rules have no linkage.

might imply that such members do not have linkage.

Notes from the April, 2005 meeting:

The question about the linkage of class members is already covered by 6.6 [basic.link] paragraph 5.




2232. thread_local anonymous unions

Section: 9.2.2  [dcl.stc]     Status: open     Submitter: Mike Herrick     Date: 2016-02-23

It is not clear from the current wording whether the thread_local specifier can be applied to anonymous unions or not. According to 9.2.2 [dcl.stc] paragraph 3,

The thread_local specifier indicates that the named entity has thread storage duration (6.7.5.3 [basic.stc.thread]). It shall be applied only to the names of variables of namespace or block scope and to the names of static data members.

One might think that an anonymous union object would be a “variable,” but the next paragraph seems to treat variables and anonymous unions as distinct:

The static specifier can be applied only to names of variables and functions and to anonymous unions (11.5.2 [class.union.anon]).



2212. Typedef changing linkage after use

Section: 9.2.4  [dcl.typedef]     Status: open     Submitter: Richard Smith     Date: 2015-12-09

[Detailed description pending.]




2195. Unsolicited reading of trailing volatile members

Section: 9.2.9.2  [dcl.type.cv]     Status: open     Submitter: Hubert Tong     Date: 2015-11-06

[Detailed description pending.]

Additional notes from the November, 2016 meeting:

See also national body comment CH2.




144. Position of friend specifier

Section: 9.2.9.4  [dcl.type.elab]     Status: open     Submitter: Daveed Vandevoorde     Date: 22 Jul 1999

9.2.9.4 [dcl.type.elab] paragraph 1 seems to impose an ordering constraint on the elements of friend class declarations. However, the general rule is that declaration specifiers can appear in any order. Should

    class C friend;
be well-formed?


2389. Agreement of deduced and explicitly-specified variable types

Section: 9.2.9.6  [dcl.spec.auto]     Status: open     Submitter: Nina Ranns     Date: 2018-10-24

The Standard does not explicitly address whether an example like the following is well-formed or not:

  struct S {
    static int i;
  };
  auto S::i = 23;

There is implementation divergence on the handling of this example.




2412. SFINAE vs undeduced placeholder type

Section: 9.2.9.6  [dcl.spec.auto]     Status: open     Submitter: Mike Miller     Date: 2019-05-03

The status of the following example is not clear:

  template <typename T> auto foo(T);  // Not defined

  template <typename T> struct FooCallable {
    template<class U>
    static constexpr bool check_foo_callable(...) { return false; }

    template<class U, class = decltype(foo(U{})) >
    static constexpr bool check_foo_callable(int) { return true; }

    static constexpr bool value = check_foo_callable<T>(0);
  };
  static_assert(FooCallable<int>::value == false, "");

The static_assert causes the evaluation of the default template argument decltype(foo<int>(int{})). However, foo is not defined, leaving it with an undeduced placeholder return type. This situation could conceivably be handled in two different ways. According to 9.2.9.6 [dcl.spec.auto] paragraph 9,

If the name of an entity with an undeduced placeholder type appears in an expression, the program is ill-formed.

This would thus appear to be an invalid expression resulting from substitution in the immediate context of the declaration and thus a substitution failure.

The other alternative would be to treat the presence of an undeduced placeholder type for a function template as satisfying the requirements of 13.9.2 [temp.inst] paragraph 4,

Unless a function template specialization has been explicitly instantiated or explicitly specialized, the function template specialization is implicitly instantiated when the specialization is referenced in a context that requires a function definition to exist or if the existence of the definition affects the semantics of the program.

and attempt to instantiate foo<int>. That instantiation fails because the definition is not provided, which would then be an error outside the immediate context of the declaration and thus a hard error instead of substitution failure.

There is implementation divergence on the handling of this example.




2493. auto as a conversion-type-id

Section: 9.2.9.6.1  [dcl.spec.auto.general]     Status: open     Submitter: Jim X     Date: 2021-03-10

Given the example,

  struct A{
   operator auto(){
     return 0;
   }
  };
  int main(){
    A a;
    a.operator auto(); // #1
    a.operator int();  // #2
  }

there is implementation divergence regarding which, if either, of the calls is well-formed. MSVC and clang reject #2, g++ rejects #1, and EDG rejects both.

According to 9.2.9.6.1 [dcl.spec.auto.general] paragraph 6:

A program that uses a placeholder type in a context not explicitly allowed in 9.2.9.6 [dcl.spec.auto] is ill-formed.

The use of auto as a conversion-type-id in a function call is not mentioned in that section; however, the section is dealing with declarative contexts rather than expressions, so it's not clear how much weight that observation should carry.




504. Should use of a variable in its own initializer require a diagnostic?

Section: 9.3.4.3  [dcl.ref]     Status: open     Submitter: Bjarne Stroustrup     Date: 14 Apr 2005

Split off from issue 453.

It is in general not possible to determine at compile time whether a reference is used before it is initialized. Nevertheless, there is some sentiment to require a diagnostic in the obvious cases that can be detected at compile time, such as the name of a reference appearing in its own initializer. The resolution of issue 453 originally made such uses ill-formed, but the CWG decided that this question should be a separate issue.

Rationale (October, 2005):

The CWG felt that this error was not likely to arise very often in practice. Implementations can warn about such constructs, and the resolution for issue 453 makes executing such code undefined behavior; that seemed to address the situation adequately.

Note (February, 2006):

Recent discussions have suggested that undefined behavior be reduced. One possibility (broadening the scope of this issue to include object declarations as well as references) was to require a diagnostic if the initializer uses the value, but not just the address, of the object or reference being declared:

    int i = i;        // Ill-formed, diagnostic required
    void* p = &p;     // Okay



361. Forward reference to default argument

Section: 9.3.4.7  [dcl.fct.default]     Status: open     Submitter: Steve Clamage     Date: 17 June 2002

Is this program well-formed?

  struct S {
    static int f2(int = f1()); // OK?
    static int f1(int = 2);
  };
  int main()
  {
    return S::f2();
  }

A class member function can in general refer to class members that are declared lexically later. But what about referring to default arguments of member functions that haven't yet been declared?

It seems to me that if f2 can refer to f1, it can also refer to the default argument of f1, but at least one compiler disagrees.

Notes from the February, 2012 meeting:

Implementations seem to have come to agreement that this example is ill-formed.

Additional note (March, 2013):

Additional discussion has occurred suggesting the following examples as illustrations of this issue:

  struct B {
   struct A { int a = 0; };
   B(A = A());    // Not permitted?
  };

as well as

  struct C {
   struct A { int a = C().n; }; // can we use the default argument here?
   C(int k = 0);
   int n;
  };

  bool f();
  struct D {
   struct A { bool a = noexcept(B()); }; // can we use the default initializer here?
   struct B { int b = f() ? throw 0 : 0; };
  };

(See also issue 325.)

Additional note (October, 2013):

Issue 1330 treats exception-specifications like default arguments, evaluated in the completed class type. That raises the same questions regarding self-referential noexcept clauses that apply to default arguments.

Additional note (November, 2020):

Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.




1609. Default arguments and function parameter packs

Section: 9.3.4.7  [dcl.fct.default]     Status: open     Submitter: Jonathan Caves     Date: 2013-01-25

It is not clear from 9.3.4.7 [dcl.fct.default] whether the following is well-formed or not:

  template<typename... T>
  void f2(int a = 0, T... b, int c = 1);

  f2<>(); // parameter a has the value 0 and parameter c has the value 1

(T... b is a non-deduced context per 13.10.3.6 [temp.deduct.type] paragraph 5, so the template arguments must be specified explicitly.)

Notes from the April, 2013 meeting:

CWG agreed that the example should be ill-formed.

Additional note (August, 2013):

9.3.4.7 [dcl.fct.default] paragraph 4 explicitly allows for a function parameter pack to follow a parameter with a default argument:

In a given function declaration, each parameter subsequent to a parameter with a default argument shall have a default argument supplied in this or a previous declaration or shall be a function parameter pack.

However, any instantiation of such a function template with a non-empty pack expansion would result in a function declaration in which one or more parameters without default arguments (from the pack expansion) would follow a parameter with a default argument and thus would be ill-formed. Such a function template declaration thus violates 13.8 [temp.res] paragraph 8:

If every valid specialization of a variadic template requires an empty template parameter pack, the template is ill-formed, no diagnostic required.

Although the drafting review teleconference of 2013-08-26 suggested closing the issue as NAD, it is being kept open to discuss and resolve this apparent contradiction.

Notes from the September, 2013 meeting:

CWG agreed that this example should be accepted; the restriction on default arguments applies to the template declaration itself, not to its specializations.




2408. Temporaries and previously-initialized elements in aggregate initialization

Section: 9.4.2  [dcl.init.aggr]     Status: open     Submitter: Daveed Vandevoorde     Date: 2019-03-13

There is implementation divergence with respect to an example like:

  constexpr int f(int &r) { r *= 9; return r - 12; }
  struct A { int &&temporary; int x; int y; };

  constexpr A a1 = { 6, f(a1.temporary), a1.temporary }; // #1 

Some implementations accept this code and others say that a1.temporary is not a constant expression in the initializer at #1.




233. References vs pointers in UDC overload resolution

Section: 9.4.4  [dcl.init.ref]     Status: open     Submitter: Matthias Meixner     Date: 9 Jun 2000

There is an inconsistency in the handling of references vs pointers in user defined conversions and overloading. The reason for that is that the combination of 9.4.4 [dcl.init.ref] and 7.3.6 [conv.qual] circumvents the standard way of ranking conversion functions, which was probably not the intention of the designers of the standard.

Let's start with some examples, to show what it is about:

    struct Z { Z(){} };

    struct A {
       Z x;

       operator Z *() { return &x; }
       operator const Z *() { return &x; }
    };

    struct B {
       Z x;

       operator Z &() { return x; }
       operator const Z &() { return x; }
    };

    int main()
    {
       A a;
       Z *a1=a;
       const Z *a2=a; // not ambiguous

       B b;
       Z &b1=b;
       const Z &b2=b; // ambiguous
    }

So while both classes A and B are structurally equivalent, there is a difference in operator overloading. I want to start with the discussion of the pointer case (const Z *a2=a;): 12.2.4 [over.match.best] is used to select the best viable function. Rule 4 selects A::operator const Z*() as best viable function using 12.2.4.3 [over.ics.rank] since the implicit conversion sequence const Z* -> const Z* is a better conversion sequence than Z* -> const Z*.

So what is the difference to the reference case? Cv-qualification conversion is only applicable for pointers according to 7.3.6 [conv.qual]. According to 9.4.4 [dcl.init.ref] paragraphs 4-7 references are initialized by binding using the concept of reference-compatibility. The problem with this is, that in this context of binding, there is no conversion, and therefore there is also no comparing of conversion sequences. More exactly all conversions can be considered identity conversions according to 12.2.4.2.5 [over.ics.ref] paragraph 1, which compare equal and which has the same effect. So binding const Z* to const Z* is as good as binding const Z* to Z* in terms of overloading. Therefore const Z &b2=b; is ambiguous. [12.2.4.2.5 [over.ics.ref] paragraph 5 and 12.2.4.3 [over.ics.rank] paragraph 3 rule 3 (S1 and S2 are reference bindings ...) do not seem to apply to this case]

There are other ambiguities, that result in the special treatment of references: Example:

    struct A {int a;};
    struct B: public A { B() {}; int b;};

    struct X {
       B x;
       operator A &() { return x; }
       operator B &() { return x; }
    };

    main()
    {
       X x;
       A &g=x; // ambiguous
    }

Since both references of class A and B are reference compatible with references of class A and since from the point of ranking of implicit conversion sequences they are both identity conversions, the initialization is ambiguous.

So why should this be a defect?

So overall I think this was not the intention of the authors of the standard.

So how could this be fixed? For comparing conversion sequences (and only for comparing) reference binding should be treated as if it was a normal assignment/initialization and cv-qualification would have to be defined for references. This would affect 9.4.4 [dcl.init.ref] paragraph 6, 7.3.6 [conv.qual] and probably 12.2.4.3 [over.ics.rank] paragraph 3.

Another fix could be to add a special case in 12.2.4 [over.match.best] paragraph 1.




2168. Narrowing conversions and +/- infinity

Section: 9.4.5  [dcl.init.list]     Status: open     Submitter: Hubert Tong     Date: 2015-08-19

The intended treatment of a floating point infinity with respect to narrowing conversions is not clear. Is std::numeric_limits<double>::infinity() usable in a constant expression, for example, and should that be different from a calculation that results in an infinity?

Notes from the October, 2015 meeting:

CWG requests the assistance of SG6 in resolving this issue.

Notes from the November, 2016 meeting:

SG6 said that arithmetic operations (not conversions) that produce infinity are not allowed in a constant expression. However, using std::numeric_limits<T>::infinity() is okay, but it can't be used as a subexpression. Conversions that produce infinity from non-infinity values are considered to be narrowing conversions.




2340. Reference collapsing and structured bindings

Section: 9.6  [dcl.struct.bind]     Status: open     Submitter: Daveed Vandevoorde     Date: 2017-03-29

According to 9.6 [dcl.struct.bind] paragraph 3,

Given the type Ti designated by std::tuple_element<i, E>::type, each vi is a variable of type “reference to Ti” initialized with the initializer, where the reference is an lvalue reference if the initializer is an lvalue and an rvalue reference otherwise; the referenced type is Ti.

If Ti is already a reference type, should this do reference collapsing? Presumably yes, but reference collapsing is specified in terms of a typedef-name or decltype-specifier, which are not used in this description.

See also issue 2313.




2505. Nested unnamed namespace of inline unnamed namespace

Section: 9.8.2.2  [namespace.unnamed]     Status: open     Submitter: Nathan Sidwell     Date: 2021-11-22

According to 9.8.2.2 [namespace.unnamed] paragraph 1,

An unnamed-namespace-definition behaves as if it were replaced by

where inline appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the translation unit.

The use of a single identifier for all occurrences of unique within a translation unit leads to problems when an inline unnamed namespace contains a nested unnamed namespace, e.g.,

    inline namespace {
      namespace { }
    }

In this case, the unnamed namespace cannot be reopened because the lookup for unique finds both the outer and inner namespaces and is thus ambiguous.

Suggested resolution:

Change 9.8.2.2 [namespace.unnamed] paragraph 1 as follows:

...where inline appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in each scope in a translation unit are replaced by the same scope-specific identifier, and this identifier differs from all other identifiers in the translation unit.



813. typename in a using-declaration with a non-dependent name

Section: 9.9  [namespace.udecl]     Status: open     Submitter: UK     Date: 3 March, 2009

N2800 comment UK 101

9.9 [namespace.udecl] paragraph 20 says,

If a using-declaration uses the keyword typename and specifies a dependent name (13.8.3 [temp.dep]), the name introduced by the using-declaration is treated as a typedef-name (9.2.4 [dcl.typedef]).

This wording does not address use of typename in a using-declaration with a non-dependent name; the primary specification of the typename keyword in 13.8 [temp.res] does not appear to describe this case, either.




1742. using-declarations and scoped enumerators

Section: 9.9  [namespace.udecl]     Status: open     Submitter: Richard Smith     Date: 2013-08-28

A using-declaration cannot name a scoped enumerator, according to 9.9 [namespace.udecl] paragraph 7. This is presumably because a scoped enumerator belongs to an enumeration scope and thus logically cannot belong to the non-enumeration scope in which the using-declaration appears. It seems inconsistent, however, to permit using-declarations to name unscoped enumerators but not scoped enumerators.

Also, 9.9 [namespace.udecl] paragraph 3 says,

In a using-declaration used as a member-declaration, the nested-name-specifier shall name a base class of the class being defined.

The consequence of this is that

  enum E { e0 };
  void f() {
    using E::e0;
  }

is well-formed, but

  struct B {
    enum E { e0 };
  };
  struct D : B {
    using B::E::e0;
  };

is not. Again, this seems inconsistent. Should these rules be relaxed?




2483. Language linkage of static member functions

Section: 9.11  [dcl.link]     Status: open     Submitter: Davis Herring     Date: 2021-03-11

According to 9.11 [dcl.link] paragraph 5,

A C language linkage is ignored in determining the language linkage of class members, friend functions with a trailing requires-clause, and the function type of class member functions.

It doesn't make sense that static member functions should behave like non-static member functions in this regard:

   extern "C" {
     struct A {
       static void f();
       constexpr static void (*p)()=f; // error: must point to a function whose type has C language linkage
     };
   }

Suggested resolution:

Change 9.11 [dcl.link] paragraph 5 as follows:

A C language linkage is ignored in determining the language linkage of class members, friend functions with a trailing requires-clause, and the function type of non-static class member functions.

Notes from the August, 2021 teleconference:

There was some question as to whether a linkage specification should affect the language linkage of any function declarators within class scope. The question was also raised as to whether some non-typedef syntax should be available for affecting language linkage, which would be a question for EWG.




1617. alignas and non-defining declarations

Section: 9.12.2  [dcl.align]     Status: open     Submitter: Richard Smith     Date: 2012-02-02

According to 9.12.2 [dcl.align] paragraph 6,

If the defining declaration of an entity has an alignment-specifier, any non-defining declaration of that entity shall either specify equivalent alignment or have no alignment-specifier. Conversely, if any declaration of an entity has an alignment-specifier, every defining declaration of that entity shall specify an equivalent alignment. No diagnostic is required if declarations of an entity have different alignment-specifiers in different translation units.

Because this is phrased in terms of the definition of an entity, an example like the following is presumably well-formed (even though there can be no definition of n):

   alignas(8) extern int n;
   alignas(16) extern int n;

Is this intentional?




2463. Trivial copyability and unions with non-trivial members

Section: 11.2  [class.prop]     Status: open     Submitter: Daveed Vandevoorde     Date: 2020-11-30

According to 11.2 [class.prop] paragraph 1,

A trivially copyable class is a class:

This definition has surprising effects in a union whose members are not trivial. For example:

  struct S {
    S& operator=(const S&);
  };
  union U {
    S s;
  }; 

In this case, S is not trivially copyable because its assignment operator is non-trivial, although its copy constructor is trivial. U, however, is trivially copyable because its assignment operator is not eligible (11.4.4 [special] paragraph 6) because it is deleted, but its copy constructor is trivial, thus satisfying the second bullet.

It is unclear why, for example, a complete object of type S cannot be memcpyed but such an object can be memcpyed when embedded in a union.

There is implementation divergence in the handling of this example.




1969. Missing exclusion of ~S as an ordinary function name

Section: 11.4.7  [class.dtor]     Status: open     Submitter: Richard Smith     Date: 2014-07-14

There does not appear to be wording to exclude use of a name like ~S for entities other than destructors.




57. Empty unions

Section: 11.5  [class.union]     Status: open     Submitter: Steve Adamczyk     Date: 13 Oct 1998

There doesn't seem to be a prohibition in 11.5 [class.union] against a declaration like

    union { int : 0; } x;
Should that be valid? If so, 9.4 [dcl.init] paragraph 5 third bullet, which deals with default-initialization of unions, should say that no initialization is done if there are no data members.

What about:

    union { } x;
    static union { };
If the first example is well-formed, should either or both of these cases be well-formed as well?

(See also the resolution for issue 151.)

Notes from 10/00 meeting: The resolution to issue 178, which was accepted as a DR, addresses the first point above (default initialization). The other questions have not yet been decided, however.




718. Non-class, non-function friend declarations

Section: 11.8.4  [class.friend]     Status: open     Submitter: John Spicer     Date: 18 September, 2008

With the change from a scope-based to an entity-based definition of friendship (see issues 372 and 580), it could well make sense to grant friendship to enumerations and variables, for example:

    enum E: int;
    class C {
      static const int i = 5;  // Private
      friend E;
      friend int x;
    };
    enum E { e = C::i; };      // OK: E is a friend
    int x = C::i;              // OK: x is a friend

According to the current wording of 11.8.4 [class.friend] paragraph 3, the friend declaration of E is well-formed but ignored, while the friend declaration of x is ill-formed.




2244. Base class access in aggregate initialization

Section: 11.8.5  [class.protected]     Status: open     Submitter: Richard Smith     Date: 2016-03-08

The rules in 11.8.5 [class.protected] assume an object expression, perhaps implicit, that can be used to determine whether access to protected members is permitted or not. It is not clear how that applies to aggregates and constructors. For example:

  struct A { 
  protected: 
    A(); 
  }; 
  struct B : A { 
    friend B f(); 
    friend B g(); 
    friend B h(); 
  }; 
  B f() { return {}; }     // ok? 
  B g() { return {{}}; }   // ok? 
  B h() { return {A{}}; }  // ok?

Notes from the December, 2016 teleconference:

The consensus favored accepting f and g while rejecting h.

Notes from the March, 2018 meeting:

CWG affirmed the earlier direction and felt that there should be an implicit object expression assumed for these cases.




2403. Temporary materialization and base/member initialization

Section: 11.9.3  [class.base.init]     Status: open     Submitter: Daveed Vandevoorde     Date: 2018-12-11

Given the following example,

  struct Noncopyable {
    Noncopyable();
    Noncopyable(const Noncopyable &) = delete;
  };

  Noncopyable make(int kind = 0);

  struct AsBase : Noncopyable {
    AsBase() : Noncopyable(make()) {} // #1
  };

  struct AsMember {
    Noncopyable nc;
    AsMember() : nc(make()) { }  // #2?
  };

All implementations treat #1 as an error, invoking the deleted copy constructor, while #2 is accepted. It's not clear from the current wording why they should be treated differently.




2504. Inheriting constructors from virtual base classes

Section: 11.9.4  [class.inhctor.init]     Status: open     Submitter: Hubert Tong     Date: 2021-11-03

According to 11.9.4 [class.inhctor.init] paragraph 1,

When a constructor for type B is invoked to initialize an object of a different type D (that is, when the constructor was inherited (9.9 [namespace.udecl])), initialization proceeds as if a defaulted default constructor were used to initialize the D object and each base class subobject from which the constructor was inherited, except that the B subobject is initialized by the invocation of the inherited constructor. The complete initialization is considered to be a single function call; in particular, the initialization of the inherited constructor's parameters is sequenced before the initialization of any part of the Dobject.

First, this assumes that the base class constructor will be invoked from the derived class constructor, which will not be true if the base is virtual and initialized by a more-derived constructor.

If the call to the virtual base constructor is omitted, the last sentence is unclear whether the initialization of the base class constructor's parameters by the inheriting constructor occurs or not. There is implementation divergence in the initialization of V's parameter in the following example:

  struct NonTriv {
    NonTriv(int);
    ~NonTriv();
  };
  struct V { V() = default; V(NonTriv); };
  struct Q { Q(); };
  struct A : virtual V, Q {
    using V::V;
    A() : A(42) { }
  };
  struct B : A { };
  void foo() { B b; }



2189. Surrogate call template

Section: 12.2.2.2.3  [over.call.object]     Status: open     Submitter: Jason Merrill     Date: 2015-10-22

[Detailed description pending.]




545. User-defined conversions and built-in operator overload resolution

Section: 12.2.2.3  [over.match.oper]     Status: open     Submitter: Steve Clamage     Date: 31 October 2005

Consider the following example:

    class B1 {};
    typedef void (B1::*PB1) (); // memptr to B1

    class B2 {};
    typedef void (B2::*PB2) (); // memptr to B2

    class D1 : public B1, public B2 {};
    typedef void (D1::*PD) (); // memptr to D1

    struct S {
         operator PB1(); // can be converted to PD
    } s;
    struct T {
         operator PB2(); // can be converted to PD
    } t;

    void foo() {
         s == t; // Is this an error?
    }

According to 12.5 [over.built] paragraph 16, there is an operator== for PD (“For every pointer to member type...”), so why wouldn't it be used for this comparison?

Mike Miller: The problem, as I understand it, is that 12.2.2.3 [over.match.oper] paragraph 3, bullet 3, sub-bullet 3 is broader than it was intended to be. It says that candidate built-in operators must “accept operand types to which the given operand or operands can be converted according to 12.2.4.2 [over.best.ics].” 12.2.4.2.3 [over.ics.user] describes user-defined conversions as having a second standard conversion sequence, and there is nothing to restrict that second standard conversion sequence.

My initial thought on addressing this would be to say that user-defined conversion sequences whose second standard conversion sequence contains a pointer conversion or a pointer-to-member conversion are not considered when selecting built-in candidate operator functions. They would still be applicable after the hand-off to Clause 5 (e.g., in bringing the operands to their common type, 7.6.10 [expr.eq], or composite pointer type, 7.6.9 [expr.rel]), just not in constructing the list of built-in candidate operator functions.

I started to suggest restricting the second standard conversion sequence to conversions having Promotion or Exact Match rank, but that would exclude the Boolean conversions, which are needed for !, &&, and ||. (It would have also restricted the floating-integral conversions, though, which might be a good idea. They can't be used implicitly, I think, because there would be an ambiguity among all the promoted integral types; however, none of the compilers I tested even tried those conversions because the errors I got were not ambiguities but things like “floating point operands not allowed for %”.)

Bill Gibbons: I recall seeing this problem before, though possibly not in committee discussions. As written this rule makes the set of candidate functions dependent on what classes have been defined, including classes not otherwise required to have been defined in order for "==" to be meaningful. For templates this implies that the set is dependent on what templates have been instantiated, e.g.

  template<class T> class U : public T { };
  U<B1> u;  // changes the set of candidate functions to include
            // operator==(U<B1>,U<B1>)?

There may be other places where the existence of a class definition, or worse, a template instantiation, changes the semantics of an otherwise valid program (e.g. pointer conversions?) but it seems like something to be avoided.

(See also issue 954.)




1919. Overload resolution for ! with explicit conversion operator

Section: 12.2.2.3  [over.match.oper]     Status: open     Submitter: Johannes Schaub     Date: 2014-04-30

Although the intent is that the ! operator should be usable with an operand that is a class object having an explicit conversion to bool (i.e., its operand is “contextually converted to bool”), the selection of the conversion operator is done via 12.2.2.3 [over.match.oper], 12.2.3 [over.match.viable], and 12.2.4 [over.match.best], which do not make specific allowance for this special characteristic of the ! operator and thus will not select the explicit conversion function.

Notes from the June, 2014 meeting:

CWG noted that this same issue affects && and ||.




2311. Missed case for guaranteed copy elision

Section: 12.2.2.8  [over.match.list]     Status: open     Submitter: Richard Smith     Date: 2016-08-09

[Detailed description pending.]




2425. Confusing wording for deduction from a type

Section: 12.2.2.9  [over.match.class.deduct]     Status: open     Submitter: Dawn Perchik     Date: 2019-08-06

In 12.2.2.9 [over.match.class.deduct] paragraph 3 we read:

The arguments of a template A are said to be deducible from a type T if, given a class template

  template <typename> class AA;

with a single partial specialization whose template parameter list is that of A and whose template argument list is a specialization of A with the template argument list of A (13.8.3.2 [temp.dep.type]), AA<T> matches the partial specialization.

The relationship between A, AA and its partial specialization, and the argument list of A is not clear. An example would be very helpful here. Also, using a different name than A would help, since A is used in close proximity to this wording to denote an alias template, while this wording applies to both class and alias templates. Finally, there should be a cross-reference to _N4868_.13.7.6.2 [temp.class.spec.match] for matching the partial specialization.




2467. CTAD for alias templates and the deducible check

Section: 12.2.2.9  [over.match.class.deduct]     Status: open     Submitter: Richard Smith     Date: 2019-08-12

Given the declarations

  template<typename T = int> using X = vector<int>;
  X x = {1, 2, 3};

  template<typename...> using Y = vector<int>;
  Y y = {1, 2, 3};

CTAD deduces vector<int>. Then we are asked to perform a check that the arguments of X and Y are deducible from vector<int>.

I think this check should succeed, deducing T = int in the first case and <pack> = <empty> in the second case, so both declarations should be valid. That seems consistent with what would happen for a non-alias with template parameters that CTAD can't deduce, where there is either a default template argument or the parameter is a pack. But what actually happens is that we're asked to form

  template<typename T> struct AA;
  template<typename T = int> struct AA<X<T>>;

and

  template<typename T> struct AA;
  template<typename ...Ts> struct AA<Y<Ts...>>;

However, both of those partial specializations are ill-formed: a partial specialization can't have default template arguments, and neither of these is more specialized than the primary template, because T / Ts are not used in deducible contexts.

I think we have the wrong model here, and should instead be considering (effectively) whether function template argument deduction would succeed for

  template<typename T> struct AA {};
  template<typename T = int> void f(AA<X<T>>);

and

  template<typename T> struct AA {};
  template<typename ...Ts> void f(AA<Y<Ts...>>);

respectively, when given an argument of type AA<deduced return type>. That is, get rid of the weird class template partial specialization restrictions, and instead add in the rules from function templates to use default template arguments and to default non-deduced packs to empty packs.




1459. Reference-binding tiebreakers in overload resolution

Section: 12.2.4.3  [over.ics.rank]     Status: open     Submitter: Jason Merrill     Date: 2012-02-07

Both paragraph 3 and paragraph 4 of 12.2.4.3 [over.ics.rank] have overload resolution tiebreakers for reference binding. It might be possible to merge those into a single treatment.




2337. Incorrect implication of logic ladder for conversion sequence tiebreakers

Section: 12.2.4.3  [over.ics.rank]     Status: open     Submitter: Richard Smith     Date: 2017-03-02

The bulleted list of 12.2.4.3 [over.ics.rank] paragraph 3 consists of a logic ladder of the form “A is better than B if [some predicate relating A to B], or, if not that, ...” For example, bullet 3.1 says,

The intent is not to fall into the array case if L2 converts to std::initializer_list<X> and L1 does not — i.e., the inverse predicate holds — but that intent is not well reflected in the actual wording.




1038. Overload resolution of &x.static_func

Section: 12.3  [over.over]     Status: open     Submitter: Mike Miller     Date: 2010-03-02

The Standard is not clear whether the following example is well-formed or not:

    struct S {
        static void f(int);
        static void f(double);
    };
    S s;
    void (*pf)(int) = &s.f;

According to 7.6.1.5 [expr.ref] paragraph 4 bullet 3, you do function overload resolution to determine whether x.f is a static or non-static member function. 7.6.2.2 [expr.unary.op] paragraph 6 says that you can only take the address of an overloaded function in a context that determines the overload to be chosen, and the initialization of a function pointer is such a context (12.3 [over.over] paragraph 1). The problem is that 12.3 [over.over] is phrased in terms of “an overloaded function name,” and this is a member access expression, not a name.

There is variability among implementations as to whether this example is accepted; some accept it as written, some only if the & is omitted, and some reject it in both forms.

Additional note (October, 2010):

A related question concerns an example like

    struct S {
        static void g(int*) {}
        static void g(long) {}
    } s;

    void foo() {
        (&s.g)(0L);
    }

Because the address occurs in a call context and not in one of the contexts mentioned in 12.3 [over.over] paragraph 1, the call expression in foo is presumably ill-formed. Contrast this with the similar example

    void g1(int*) {}
    void g1(long) {}

    void foo1() {
        (&g1)(0L);
    }

This call presumably is well-formed because 12.2.2.2 [over.match.call] applies to “the address of a set of overloaded functions.” (This was clearer in the wording prior to the resolution of issue 704: “...in this context using &F behaves the same as using the name F by itself.”) It's not clear that there's any reason to treat these two cases differently.

This question also bears on the original question of this issue, since the original wording of 12.2.2.2 [over.match.call] also described the case of an ordinary member function call like s.g(0L) as involving the “name” of the function, even though the postfix-expression is a member access expression and not a “name.” Perhaps the reference to “name” in 12.3 [over.over] should be similarly understood as applying to member access expressions?




1549. Overloaded comma operator with void operand

Section: 12.4.3  [over.binary]     Status: open     Submitter: Nikolay Ivchenkov     Date: 2012-09-04

Even though a function cannot take a parameter of type void, the current rules for overload resolution require consideration of overloaded operators when one operand has a user-defined or enumeration type and the other has type void. This can result in side effects and possibly errors, for example:

  template <class T> struct A {
    T t;
    typedef T type;
  };

  struct X {
    typedef A<void> type;
  };

  template <class T> void operator ,(typename T::type::type, T) {}

  int main() {
    X(), void(); // OK
    void(), X(); // error: A<void> is instantiated with a field of
                 // type void
  }



260. User-defined conversions and built-in operator=

Section: 12.5  [over.built]     Status: open     Submitter: Scott Douglas     Date: 4 Nov 2000

According to the Standard (although not implemented this way in most implementations), the following code exhibits non-intuitive behavior:

  struct T {
    operator short() const;
    operator int() const;
  };

  short s;

  void f(const T& t) {
    s = t;  // surprisingly calls T::operator int() const
  }

The reason for this choice is 12.5 [over.built] paragraph 18:

For every triple (L, VQ, R), where L is an arithmetic type, VQ is either volatile or empty, and R is a promoted arithmetic type, there exist candidate operator functions of the form

Because R is a "promoted arithmetic type," the second argument to the built-in assignment operator is int, causing the unexpected choice of conversion function.

Suggested resolution: Provide built-in assignment operators for the unpromoted arithmetic types.

Related to the preceding, but not resolved by the suggested resolution, is the following problem. Given:

    struct T {
	 operator int() const;
	 operator double() const;
    };

I believe the standard requires the following assignment to be ambiguous (even though I expect that would surprise the user):

    double x;
    void f(const T& t) { x = t; }

The problem is that both of these built-in operator=()s exist (12.5 [over.built] paragraph 18):

    double& operator=(double&, int);
    double& operator=(double&, double);

Both are an exact match on the first argument and a user conversion on the second. There is no rule that says one is a better match than the other.

The compilers that I have tried (even in their strictest setting) do not give a peep. I think they are not following the standard. They pick double& operator=(double&, double) and use T::operator double() const.

I hesitate to suggest changes to overload resolution, but a possible resolution might be to introduce a rule that, for built-in operator= only, also considers the conversion sequence from the second to the first type. This would also resolve the earlier question.

It would still leave x += t etc. ambiguous -- which might be the desired behavior and is the current behavior of some compilers.

Notes from the 04/01 meeting:

The difference between initialization and assignment is disturbing. On the other hand, promotion is ubiquitous in the language, and this is the beginning of a very slippery slope (as the second report above demonstrates).

Additional note (August, 2010):

See issue 507 for a similar example involving comparison operators.




954. Overload resolution of conversion operator templates with built-in types

Section: 12.5  [over.built]     Status: open     Submitter: Steve Clamage     Date: 19 August, 2009

Consider the following example:

    struct NullClass {
        template<typename T> operator T () { return 0 ; }
    };

    int main() {
        NullClass n;
        n==5;        // #1
        return 0;
    }

The comparison at #1 is, according to the current Standard, ambiguous. According to 12.5 [over.built] paragraph 12, the candidates for operator==(L, R) include functions “for every pair of promoted arithmetic types,” so L could be either int or long, and the conversion operator template will provide an exact match for either.

Some implementations unambiguously choose the int candidate. Perhaps the overload resolution rules could be tweaked to prefer candidates in which L and R are the same type?

(See also issue 545.)




1620. User-defined literals and extended integer types

Section: 12.6  [over.literal]     Status: open     Submitter: Jason Merrill     Date: 2013-02-12

Although numeric literals can have extended integer types, user-defined literal operators cannot have a parameter of an extended integer type. This seems like an oversight.




2395. Parameters following a pack expansion

Section: 13.2  [temp.param]     Status: open     Submitter: Richard Smith     Date: 2018-12-03

The Standard is not clear, and there is implementation divergence, for an example like the following:

  template<class ...Types> struct Tuple_ { // _VARIADIC_TEMPLATE 
    template<Types ...T, int I> int f() { 
      return sizeof...(Types); 
    } 
  }; 
  int main() { 
    Tuple_<char,int> a; 
    int b = a.f<1, 2, 3>(); 
  } 

The question is whether the 3 is accepted as the argument for I or an error, exceeding the number of arguments for T, which is set as 2 by the template arguments for Tuple_. See also issue 2383 for a related example.




579. What is a “nested” > or >>?

Section: 13.3  [temp.names]     Status: open     Submitter: Daveed Vandevoorde     Date: 11 May 2006

The Standard does not normatively define which > and >> tokens are to be taken as closing a template-argument-list; instead, 13.3 [temp.names] paragraph 3 uses the undefined and imprecise term “non-nested:”

When parsing a template-id, the first non-nested > is taken as the end of the template-argument-list rather than a greater-than operator. Similarly, the first non-nested >> is treated as two consecutive but distinct > tokens, the first of which is taken as the end of the template-argument-list and completes the template-id.

The (non-normative) footnote clarifies that

A > that encloses the type-id of a dynamic_cast, static_cast, reinterpret_cast or const_cast, or which encloses the template-arguments of a subsequent template-id, is considered nested for the purpose of this description.

Aside from the questionable wording of this footnote (e.g., in what sense does a single terminating character “enclose” anything, and is a nested template-id “subsequent?”) and the fact that it is non-normative, it does not provide a complete definition of what “nesting” is intended to mean. For example, is the first > in this putative template-id “nested” or not?

    X<a ? b > c : d>

Additional note (January, 2014):

A similar problem exists for an operator> template:

  struct S;
  template<void (*)(S, S)> struct X {};
  void operator>(S, S);
  X<operator> > x;

Somehow the specification must be written to avoid taking the > token in the operator name as the end of the template argument list for X.




440. Allow implicit pointer-to-member conversion on nontype template argument

Section: 13.4  [temp.arg]     Status: open     Submitter: David Abrahams     Date: 13 Nov 2003

None of my compilers accept this, which surprised me a little. Is the base-to-derived member function conversion considered to be a runtime-only thing?

  template <class D>
  struct B
  {
      template <class X> void f(X) {}
      template <class X, void (D::*)(X) = &B<D>::f<X> >
      struct row {};
  };
  struct D : B<D>
  {
      void g(int);
      row<int,&D::g> r1;
      row<char*> r2;
  };

John Spicer: This is not among the permitted conversions listed in 14.3.

I'm not sure there is a terribly good reason for that. Some of the template argument rules for external entities were made conservatively because of concerns about issues of mangling template argument names.

David Abrahams: I'd really like to see that restriction loosened. It is a serious inconvenience because there appears to be no way to supply a usable default in this case. Zero would be an OK default if I could use the function pointer's equality to zero as a compile-time switch to choose an empty function implementation:

  template <bool x> struct tag {};

  template <class D>
  struct B
  {
      template <class X> void f(X) {}

      template <class X, void (D::*pmf)(X) = 0 >
      struct row {
          void h() { h(tag<(pmf == 0)>(), pmf); }
          void h(tag<1>, ...) {}
          void h(tag<0>, void (D::*q)(X)) { /*something*/}
      };
  };

  struct D : B<D>
  {
      void g(int);
      row<int,&D::g> r1;
      row<char*> r2;
  };

But there appears to be no way to get that effect either. The result is that you end up doing something like:

      template <class X, void (D::*pmf)(X) = 0 >
      struct row {
          void h() { if (pmf) /*something*/ }
      };

which invariably makes compilers warn that you're switching on a constant expression.




2105. When do the arguments for a parameter pack end?

Section: 13.4  [temp.arg]     Status: open     Submitter: Hubert Tong     Date: 2015-03-17

There does not appear to be a clear statement in the Standard that the first template parameter pack in a template parameter list corresponds to all remaining arguments in the template argument list. For example:

  template <int> struct A;

  template <int ...N, typename T> void foo(A<N> *..., T);
  void bar() {
   foo<0>(0, 0);      // okay: N consists of one template parameter, 0. T is deduced to int
   foo<0, int>(0, 0); // error: int does not match the form of the corresponding parameter N
  }

See also issue 2055.

Notes from the February, 2016 meeting:

The comments in the example reflect the intent.




2401. Array decay vs prohibition of subobject non-type arguments

Section: 13.4.3  [temp.arg.nontype]     Status: open     Submitter: John Spicer     Date: 2019-02-06

Consider an example like:

  template <const char *N> struct A { static const int val; };

  template <const char *N> const int A<N>::val = 0;

  static const char c[2] = "";

  int main() {
    A<c> a;
    return A<c>::val;
  } 

Formally, this appears to violate the prohibition of using the address of a subobject as a non-type template argument, since the array reference c in the argument decays to a pointer to the first element of the array. However, at least some implementations accept this example, and at least conceptually the template argument designates the complete object. Should an exception be made for the result of array decay?




2398. Template template parameter matching and deduction

Section: 13.4.4  [temp.arg.template]     Status: open     Submitter: Jason Merrill     Date: 2016-12-03

Do the changes from P0522R0 regarding template template parameter matching apply to deduction? For example:

  template<class T, class U = T> class B { /* ... */ };
  template<template<class> class P, class T> void f(P<T>);

  int main()  {
    f(B<int>());       // OK?
    f(B<int,float>()); // ill-formed, T deduced to int and float
  }

In deduction we can determine that P is more specialized than B, then substitute B into P<T>, and then compare B<T,T> to B<int,int>. This will allow deduction to succeed, whereas comparing <T> to <int,int> without this substitution would fail. I suppose this is similar to deducing a type parameter, substituting it into the type of a non-type parameter, then deducing the value of the non-type parameter

Does this make sense? Do we need more wording?

Consider also this example;

  template<typename> struct match;

  template<template<typename> class t,typename T>
  struct match<t<T> > { typedef int type; };      // #1

  template<template<typename,typename> class t,typename T0,typename T1>
  struct match<t<T0,T1> > { typedef int type; };  // #2

  template<typename,typename = void> struct other { };
  typedef match<other<void,void> >::type type;

Before this change, partial specialization #1 was not a candidate; now it is, and neither partial specialization is at least as specialized as the other, so we get an ambiguity. It seems that the consistent way to address this would be to use other during partial ordering, so we'd be comparing

  template<typename T>
  void fn (match<other<T>>); // i.e. other<T,void>
  template<typename T0, typename T1>
  void fn (match<other<T0,T1>>);

So #1 is more specialized, whereas before this change we chose #2.




1918. friend templates with dependent scopes

Section: 13.7.5  [temp.friend]     Status: open     Submitter: Richard Smith     Date: 2014-04-27

It is not clear what should happen for an example like:

  template<typename T> struct A {
    class B {
      class C {};
    };
  };
  class X {
    static int x;
    template <typename T> friend class A<T>::B::C;
  };
  template<> struct A<int> {
    typedef struct Q B;
  };
  struct Q {
    class C {
      int f() { return X::x; }
    };
  };

It appears that the friend template matches Q::C, because that class is also A<int>::B::C, but neither GCC nor EDG allow this code (saying X::x is inaccessible). (Clang doesn't support friend template declarations with a dependent scope.)

A strict reading of 13.7.5 [temp.friend] paragraph 5 might suggest that the friend declaration itself is ill-formed, because it does not declare a member of a class template, but I can't find any compiler that implements template friends that way.




1945. Friend declarations naming members of class templates in non-templates

Section: 13.7.5  [temp.friend]     Status: open     Submitter: Richard Smith     Date: 2014-06-19

During the discussion of issue 1918, it was decided that the last part of the issue should be split off into a separate issue. According to 13.7.5 [temp.friend] paragraph 5,

A member of a class template may be declared to be a friend of a non-template class.

Does this make the example from issue 1918,

  template<typename T> struct A {
    class B {
      class C {};
    };
  };
  class X {
    static int x;
    template <typename T> friend class A<T>::B::C;
  };
  template<> struct A<int> {
    typedef struct Q B;
  };
  struct Q {
    class C {
      int f() { return X::x; }
    };
  };

ill-formed because the friend declaration does not refer to a member of a class template? This does not appear to be the interpretation chosen by most implementations.




310. Can function templates differing only in parameter cv-qualifiers be overloaded?

Section: 13.7.7.2  [temp.over.link]     Status: open     Submitter: Andrei Iltchenko     Date: 29 Aug 2001

I get the following error diagnostic [from the EDG front end]:

line 8: error: function template "example<T>::foo<R,A>(A)" has
          already been declared
     R  foo(const A);
        ^
when compiling this piece of code:
struct  example  {
   template<class R, class A>   // 1-st member template
   R  foo(A);
   template<class R, class A>   // 2-nd member template
   const R  foo(A&);
   template<class R, class A>   // 3-d  member template
   R  foo(const A);
};

/*template<> template<>
int  example<char>::foo(int&);*/


int  main()
{
   int  (example<char>::* pf)(int&) =
      &example<char>::foo;
}

The implementation complains that

   template<class R, class A>   // 1-st member template
   R  foo(A);
   template<class R, class A>   // 3-d  member template
   R  foo(const A);
cannot be overloaded and I don't see any reason for it since it is function template specializations that are treated like ordinary non-template functions, meaning that the transformation of a parameter-declaration-clause into the corresponding parameter-type-list is applied to specializations (when determining its type) and not to function templates.

What makes me think so is the contents of 13.7.7.2 [temp.over.link] and the following sentence from 13.10.3.2 [temp.deduct.call] "If P is a cv-qualified type, the top level cv-qualifiers of P are ignored for type deduction". If the transformation was to be applied to function templates, then there would be no reason for having that sentence in 13.10.3.2 [temp.deduct.call].

13.10.3.3 [temp.deduct.funcaddr], which my example is based upon, says nothing about ignoring the top level cv-qualifiers of the function parameters of the function template whose address is being taken.

As a result, I expect that template argument deduction will fail for the 2-nd and 3-d member templates and the 1-st one will be used for the instantiation of the specialization.




402. More on partial ordering of function templates

Section: 13.7.7.3  [temp.func.order]     Status: open     Submitter: Nathan Sidwell     Date: 7 Apr 2003

This was split off from issue 214 at the April 2003 meeting.

Nathan Sidwell: John Spicer's proposed resolution does not make the following well-formed.

  template <typename T> int Foo (T const *) {return 1;} //#1
  template <unsigned I> int Foo (char const (&)[I]) {return 2;} //#2

  int main ()
  {
    return Foo ("a") != 2;
  }

Both #1 and #2 can deduce the "a" argument, #1 deduces T as char and #2 deduces I as 2. However, neither is more specialized because the proposed rules do not have any array to pointer decay.

#1 is only deduceable because of the rules in 13.10.3.2 [temp.deduct.call] paragraph 2 that decay array and function type arguments when the template parameter is not a reference. Given that such behaviour happens in deduction, I believe there should be equivalent behaviour during partial ordering. #2 should be resolved as more specialized as #1. The following alteration to the proposed resolution of DR214 will do that.

Insert before,

the following

For the example above, this change results in deducing 'T const *' against 'char const *' in one direction (which succeeds), and 'char [I]' against 'T const *' in the other (which fails).

John Spicer: I don't consider this a shortcoming of my proposed wording, as I don't think this is part of the current rules. In other words, the resolution of 214 might make it clearer how this case is handled (i.e., clearer that it is not allowed), but I don't believe it represents a change in the language.

I'm not necessarily opposed to such a change, but I think it should be reviewed by the core group as a related change and not a defect in the proposed resolution to 214.

Notes from the October 2003 meeting:

There was some sentiment that it would be desirable to have this case ordered, but we don't think it's worth spending the time to work on it now. If we look at some larger partial ordering changes at some point, we will consider this again.




1157. Partial ordering of function templates is still underspecified

Section: 13.7.7.3  [temp.func.order]     Status: open     Submitter: CA     Date: 2010-08-03

N3092 comment CA 7

13.7.7.3 [temp.func.order] paragraph 3 says,

To produce the transformed template, for each type, non-type, or template template parameter (including template parameter packs (13.7.4 [temp.variadic]) thereof) synthesize a unique type, value, or class template respectively and substitute it for each occurrence of that parameter in the function type of the template.

The characteristics of the synthesized entities and how they are determined is not specified. For example, members of a dependent type referred to in non-deduced contexts are not specified to exist, even though the transformed function type would be invalid in their absence.

Example 1:

  template<typename T, typename U> struct A;
  template<typename T> void foo(A<T, typename T::u> *) { } // #1
    // synthetic T1 has member T1::u
  template <typename T> void foo(A<T, typename T::u::v> *) { } // #2
    // synthetic T2 has member T2::u and member T2::u::v
    // T in #1 deduces to synthetic T2 in partial ordering;
    // deduced A for the parameter is A<T2, T2::u> * --this is not necessarily compatible
    // with A<T2, T2::u::v> * and it does not need to be. See Note 1. The effect is that
    // (in the call below) the compatibility of B::u and B::u::v is respected.
    // T in #2 cannot be successfully deduced in partial ordering from A<T1, T1::u> *;
    // invalid type T1::u::v will be formed when T1 is substituted into non-deduced contexts.
  struct B {
    struct u { typedef u v; };
  };
  int main() {
    foo((A<B, B::u> *)0); // calls #2
  }

Note 1: Template argument deduction is an attempt to match a P and a deduced A; however, template argument deduction is not specified to fail if the P and the deduced A are incompatible. This may occur in the presence of non-deduced contexts. Notwithstanding the parenthetical statement in 13.10.3.5 [temp.deduct.partial] paragraph 9, template argument deduction may succeed in determining a template argument for every template parameter while producing a deduced A that is not compatible with the corresponding P.

Example 2:

  template <typename T, typename U, typename V> struct A;
  template <typename T>
    void foo(A<T, struct T::u, struct T::u::u> *); // #2.1
      // synthetic T1 has member non-union class T1::u
  template <typename T, typename U>
    void foo(A<T, U , U> *); // #2.2
      // synthetic T2 and U2 has no required properties
      // T in #2.1 cannot be deduced in partial ordering from A<T2, U2, U2> *;
      // invalid types T2::u and T2::u::u will be formed when T2 is substituted in nondeduced contexts.
      // T and U in #2.2 deduces to, respectively, T1 and T1::u from A<T1, T1::u, struct
T1::u::u> * unless
      // struct T1::u::u does not refer to the injected-class-name of the class T1::u (if that is possible).
  struct B {
    struct u { };
  };
  int main() {
    foo((A<B, B::u, struct B::u::u> *)0); // calls #2.1
  }

It is, however, unclear to what extent an implementation will have to go to determine these minimal properties.




2160. Issues with partial ordering

Section: 13.7.7.3  [temp.func.order]     Status: open     Submitter: Richard Smith     Date: 2015-07-16

(From this editorial issue.)

Consistency of deduced values

  template <typename T> void foo(T, T); // (1)
  template <typename T, typename U> void foo(T, U); // (2)

13.10.3.6 [temp.deduct.type] paragraph 2 makes it clear that there must be exactly one set of deduced values for the Ps. But there is no such statement in the partial ordering rule. The algorithm described only does pairwise P/A matching, so a synthesized call from (2) to (1) via foo(U{}, V{}) could succeed in deduction. Both gcc and clang agree that (1) is more specialized.

Type Synthesis Template Instantiation

  template <typename T>
  struct identity { using type = T; };

  template<typename T> void bar(T, T ); // (1) 
  template<typename T> void bar(T, typename identity<T>::type ); // (2)

Here, if synthesized for (2) Unique2 and typename identity<Unique2>::type == Unique2 , then type deduction would succeed in both directions and the call bar(0,0) would be ambiguous. However, it seems that both compilers instead simply treat typename identity<Unique2>::type as Unique2_b, thus making template deduction from (2) to (1) fail (based on the implied missing Consistency rule).

Non-deduced Context Omission

This is the same as the previous example, except now define

  template <typename T> struct identity;
  template <> struct identity<int> { using type = int; };

With no template instantiation during synthesis and consistency, the (2) ==> (1) deduction fails. But if we consider the (1) ==> (2) call, we'd match T against Unique1 and then have the non-deduced context typename identity<Unique1>::type to match against Unique1, but that would be a substitution failure. It seems that the approach taken by gcc and clang (both of which prefer (1) here) is to ignore the non-deduced context argument, as long as that parameter type is deduced from a different template parameter type that did get matched.

Notes from the February, 2016 meeting:

None of these examples appears to reflect a defect in the current wording; in particular, the second and third examples involve a dependent type and there could be a later specialization of identity, so it's impossible to reason about those cases in the template definition context. The issue will be left open to allow for possible clarification of the intent of the wording.




1257. Instantiation via non-dependent references in uninstantiated templates

Section: 13.8  [temp.res]     Status: open     Submitter: Johannes Schaub     Date: 2011-03-09

The Standard does not appear to specify whether a non-dependent reference to a template specialization in a template definition that is never instantiated causes the implicit instantiation of the referenced specialization.




2067. Generated variadic templates requiring empty pack

Section: 13.8  [temp.res]     Status: open     Submitter: Richard Smith     Date: 2015-01-09

According to 13.8 [temp.res] paragraph 8,

If every valid specialization of a variadic template requires an empty template parameter pack, the template is ill-formed, no diagnostic required.

I'm inclined to think that this rule should only apply to code the user wrote. That is, if every valid instantiation of an entity (that was not itself instantiated) requires at least one of the enclosing template argument lists to include an empty template argument pack, then the program is ill-formed (no diagnostic required).




186. Name hiding and template template-parameters

Section: 13.8.2  [temp.local]     Status: open     Submitter: John Spicer     Date: 11 Nov 1999

The standard prohibits a class template from having the same name as one of its template parameters (13.8.2 [temp.local] paragraph 4). This prohibits

    template <class X> class X;
for the reason that the template name would hide the parameter, and such hiding is in general prohibited.

Presumably, we should also prohibit

    template <template <class T> class T> struct A;
for the same reason.


459. Hiding of template parameters by base class members

Section: 13.8.2  [temp.local]     Status: open     Submitter: Daveed Vandevoorde     Date: 2 Feb 2004

Currently, member of nondependent base classes hide references to template parameters in the definition of a derived class template.

Consider the following example:

   class B {
      typedef void *It;    // (1)
      // ...
    };

    class M: B {};

    template<typename> X {};

    template<typename It> struct S   // (2)
        : M, X<It> {   // (3)
      S(It, It);   // (4)
      // ...
    };

As the C++ language currently stands, the name "It" in line (3) refers to the template parameter declared in line (2), but the name "It" in line (4) refers to the typedef in the private base class (declared in line (1)).

This situation is both unintuitive and a hindrance to sound software engineering. (See also the Usenet discussion at http://tinyurl.com/32q8d .) Among other things, it implies that the private section of a base class may change the meaning of the derived class, and (unlike other cases where such things happen) there is no way for the writer of the derived class to defend the code against such intrusion (e.g., by using a qualified name).

Changing this can break code that is valid today. However, such code would have to:

  1. name a template parameter and not use it after the opening brace, and
  2. use that same name to access a base-class name within the braces.
I personally have no qualms breaking such a program.

It has been suggested to make situations like these ill-formed. That solution is unattractive however because it still leaves the writer of a derived class template without defense against accidental name conflicts with base members. (Although at least the problem would be guaranteed to be caught at compile time.) Instead, since just about everyone's intuition agrees, I would like to see the rules changed to make class template parameters hide members of the same name in a base class.

See also issue 458.

Notes from the March 2004 meeting:

We have some sympathy for a change, but the current rules fall straightforwardly out of the lookup rules, so they're not “wrong.” Making private members invisible also would solve this problem. We'd be willing to look at a paper proposing that.

Additional discussion (April, 2005):

John Spicer: Base class members are more-or-less treated as members of the class, [so] it is only natural that the base [member] would hide the template parameter.

Daveed Vandevoorde: Are base class members really “more or less” members of the class from a lookup perspective? After all, derived class members can hide base class members of the same name. So there is some pretty definite boundary between those two sets of names. IMO, the template parameters should either sit between those two sets, or they should (for lookup purposes) be treated as members of the class they parameterize (I cannot think of a practical difference between those two formulations).

John Spicer: How is [hiding template parameters] different from the fact that namespace members can be hidden by private parts of a base class? The addition of int C to N::A breaks the code in namespace M in this example:

    namespace N {
       class A {
    private:
         int C;
       };
    }

    namespace M {
       typedef int C;
       class B : public N::A {
         void f() {
             C c;
         }
       };
    }

Daveed Vandevoorde: C++ has a mechanism in place to handle such situations: qualified names. There is no such mechanism in place for template parameters.

Nathan Myers: What I see as obviously incorrect ... is simply that a name defined right where I can see it, and directly attached to the textual scope of B's class body, is ignored in favor of something found in some other file. I don't care that C1 is defined in A, I have a C1 right here that I have chosen to use. If I want A::C1, I can say so.

I doubt you'll find any regular C++ coder who doesn't find the standard behavior bizarre. If the meaning of any code is changed by fixing this behavior, the overwhelming majority of cases will be mysterious bugs magically fixed.

John Spicer: I have not heard complaints that this is actually a cause of problems in real user code. Where is the evidence that the status quo is actually causing problems?

In this example, the T2 that is found is the one from the base class. I would argue that this is natural because base class members are found as part of the lookup in class B:

    struct A {
             typedef int T2;
    };
    template <class T2> struct B : public A {
             typedef int T1;
             T1 t1;
             T2 t2;
    };

This rule that base class members hide template parameters was formalized about a dozen years ago because it fell out of the principle that base class members should be found at the same stage of lookup as derived class members, and that to do otherwise would be surprising.

Gabriel Dos Reis: The bottom line is that:

  1. the proposed change is a silent change of meaning;
  2. the proposed change does not make the language any more regular; the current behavior is consistent with everything else, however “surprising” that might be;
  3. the proposed change does have its own downsides.

Unless presented with real major programming problems the current rules exhibit, I do not think the simple rule “scopes nest” needs a change that silently mutates program meaning.

Mike Miller: The rationale for the current specification is really very simple:

  1. “Unless redeclared in the derived class, members of a base class are also considered to be members of the derived class.” (11.7 [class.derived] paragraph 2)
  2. In class scope, members hide nonmembers.

That's it. Because template parameters are not members, they are hidden by member names (whether inherited or not). I don't find that “bizarre,” or even particularly surprising.

I believe these rules are straightforward and consistent, so I would be opposed to changing them. However, I am not unsympathetic toward Daveed's concern about name hijacking from base classes. How about a rule that would make a program ill-formed if a direct or inherited member hides a template parameter?

Unless this problem is a lot more prevalent than I've heard so far, I would not want to change the lookup rules; making this kind of collision a diagnosable error, however, would prevent hijacking without changing the lookup rules.

Erwin Unruh: I have a different approach that is consistent and changes the interpretation of the questionable code. At present lookup is done in this sequence:

If we change this order to

it is still consistent in that no lookup is placed between the base class and the derived class. However, it introduces another inconsistency: now scopes do not nest the same way as curly braces nest — but base classes are already inconsistent this way.

Nathan Myers: This looks entirely satisfactory. If even this seems like too big a change, it would suffice to say that finding a different name by this search order makes the program ill-formed. Of course, a compiler might issue only a portability warning in that case and use the name found Erwin's way, anyhow.

Gabriel Dos Reis: It is a simple fact, even without templates, that a writer of a derived class cannot protect himself against declaration changes in the base class.

Richard Corden: If a change is to be made, then making it ill-formed is better than just changing the lookup rules.

    struct B
    {
      typedef int T;
      virtual void bar (T const & );
    };

    template <typename T>
    struct D : public B
    {
      virtual void bar (T const & );
    };

    template class D<float>;

I think changing the semantics of the above code silently would result in very difficult-to-find problems.

Mike Miller: Another case that may need to be considered in deciding on Erwin's suggestion or the “ill-formed” alternative is the treatment of friend declarations described in 6.5.3 [basic.lookup.unqual] paragraph 10:

    struct A {
        typedef int T;
        void f(T);
    };
    template<typename T> struct B {
        friend void A::f(T);  // Currently T is A::T
    };

Notes from the October, 2005 meeting:

The CWG decided not to consider a change to the existing rules at this time without a paper exploring the issue in more detail.




1619. Definition of current instantiation

Section: 13.8.3.2  [temp.dep.type]     Status: open     Submitter: Johannes Schaub     Date: 2013-02-04

The definition of the current instantiation, given in 13.8.3.2 [temp.dep.type] paragraph 1, is phrased in terms of the meaning of a name (“A name refers to the current instantiation if it is...”); it does not define when a type is the current instantiation. Thus the interpretation of *this and of phrases like “member of a class that is the current instantiation” is not formally specified.




2250. Implicit instantiation, destruction, and TUs

Section: 13.8.4.1  [temp.point]     Status: open     Submitter: Dawn Perchik     Date: 2016-03-21

[Detailed description pending.]

Notes from the December, 2016 teleconference:

The problem is that the current wording only connects name lookup with point of instantiation; other semantic checks, such as the requirement for completeness of a class, should also be performed at that point.




2435. Alias template specializations

Section: 13.9  [temp.spec]     Status: open     Submitter: Krystian Stasiowski     Date: 2019-09-28

According to 13.9 [temp.spec] paragraph 4,

An instantiated template specialization can be either implicitly instantiated (13.9.2 [temp.inst]) for a given argument list or be explicitly instantiated (13.9.3 [temp.explicit]). A specialization is a class, variable, function, or class member that is either instantiated or explicitly specialized (13.9.4 [temp.expl.spec]).

The definition of “specialization” does not cover alias templates, although the terms “specialization of an alias template” and ”alias template specialization” are used in 13.7.8 [temp.alias]. (Note that there are differences between alias specializations and the specializations mentioned here; in particular, an alias template cannot be explicitly specialized, and it is not the result of instantiation (paragraph 1) but simply of substitution (13.7.8 [temp.alias] paragraph 2).)




1378. When is an instantiation required?

Section: 13.9.2  [temp.inst]     Status: open     Submitter: Jason Merrill     Date: 2011-08-18

A template instantiation can be “required” without there being a need for it at link time if it can appear in a constant expression:

    template <class T> struct A {
       static const T t;
    };
    template <class T> const T A<T>::t = 0;
    template <int I> struct B { };
    int a = sizeof(B<A<int>::t>);

    template <class T> constexpr T f(T t) { return t; }
    int b = sizeof(B<f(42)>);

It seems like it might be useful to define a term other than odr-used for this sort of use, which is like odr-used but doesn't depend on potentially evaluated context or lvalue-rvalue conversions.

Nikolay Ivchenkov:

Another possibility would be to introduce the extension described in the closed issue 1272 and then change 6.3 [basic.def.odr] paragraph 2 as follows:

An expression E is potentially evaluated unless it is an unevaluated operand (Clause Clause 7 [expr]) or a subexpression thereof. if and only if

An expression S is a direct subexpression of an expression E if and only if S and E are different expressions, S is a subexpression of E, and there is no expression X such that X differs from both S and E, S is a subexpression of X, and X is a subexpression of E. A variable whose name appears as a potentially-evaluated expression is odr-used unless it is an object that satisfies the requirements for appearing in a constant expression (7.7 [expr.const]) and the lvalue-to-rvalue conversion (4.1) is immediately applied...

[Example:

    template <class T> struct X {
        static int const m = 1;
        static int const n;
    };
    template <class T> int const X<T>::n = 2;

    int main() {
        // X<void>::m is odr-used,
        // X<void>::m is defined implicitly
        std::cout << X<void>::m << std::endl;

        // X<void>::n is odr-used,
        // X<void>::n is defined explicitly
        std::cout << X<void>::n << std::endl;

        // OK (issue 712 is not relevant here)
        std::cout << (1 ? X<void>::m : X<void>::n) << std::endl;
    }

(See also issues 712 and 1254.)




1602. Linkage of specialization vs linkage of template arguments

Section: 13.9.2  [temp.inst]     Status: open     Submitter: Richard Smith     Date: 2013-01-09

The Standard does not appear to specify the linkage of a template specialization. 13.9.2 [temp.inst] paragraph 11 does say,

Implicitly instantiated class and function template specializations are placed in the namespace where the template is defined.

which could be read as implying that the specialization has the same linkage as the template itself. Implementation practice seems to be that the weakst linkage of the template and the arguments is used for the specialization.




1856. Indirect nested classes of class templates

Section: 13.9.2  [temp.inst]     Status: open     Submitter: Richard Smith     Date: 2014-02-11

During the discussion of issue 1484, it was observed that the current rules do not adequately address indirect nested classes of class templates (i.e., member classes of member classes of class templates) in regard to their potential separate instantiation.




293. Syntax of explicit instantiation/specialization too permissive

Section: 13.9.3  [temp.explicit]     Status: open     Submitter: Mark Mitchell     Date: 27 Jun 2001

13.9.3 [temp.explicit] defines an explicit instantiation as

Syntactically, that allows things like:

    template int S<int>::i = 5, S<int>::j = 7;

which isn't what anyone actually expects. As far as I can tell, nothing in the standard explicitly forbids this, as written. Syntactically, this also allows:

    template namespace N { void f(); }

although perhaps the surrounding context is enough to suggest that this is invalid.

Suggested resolution:

I think we should say:

[Steve Adamczyk: presumably, this should have template at the beginning.]

and then say that:

There are similar problems in 13.9.4 [temp.expl.spec]:

Here, I think we want:

with similar restrictions as above.

[Steve Adamczyk: This also needs to have template <> at the beginning, possibly repeated.]




1046. What is a “use” of a class specialization?

Section: 13.9.3  [temp.explicit]     Status: open     Submitter: Michael Wong     Date: 2010-03-08

According to 13.9.3 [temp.explicit] paragraph 10,

An entity that is the subject of an explicit instantiation declaration and that is also used in the translation unit shall be the subject of an explicit instantiation definition somewhere in the program; otherwise the program is ill-formed, no diagnostic required.

The term “used” is too vague and needs to be defined. In particular, “use” of a class template specialization as an incomplete type — to form a pointer, for instance — should not require the presence of an explicit instantiation definition elsewhere in the program.




2501. Explicit instantiation and trailing requires-clauses

Section: 13.9.3  [temp.explicit]     Status: open     Submitter: Davis Herring     Date: 2021-08-09

CWG determined that issue 2488 was not a defect. However, the discussion uncovered an issue regarding the handling of an explicit instantiation of a class template containing such members. According to 13.9.3 [temp.explicit] paragraph 10,

An explicit instantiation that names a class template specialization is also an explicit instantiation of the same kind (declaration or definition) of each of its direct non-template members that has not been previously explicitly specialized in the translation unit containing the explicit instantiation, provided that the associated constraints, if any, of that member are satisfied by the template arguments of the explicit instantiation (13.5.3 [temp.constr.decl], 13.5.2 [temp.constr.constr]), except as described below.

Paragraph 12 says,

An explicit instantiation of a prospective destructor (11.4.7 [class.dtor]) shall correspond to the selected destructor of the class.

Perhaps the virtual and constrained members could be handled in an analogous fashion.

Notes from the November, 2021 teleconference:

Issue 2488 is being reopened due to subsequent comments.




2478. Properties of explicit specializations of implicitly-instantiated class templates

Section: 13.9.4  [temp.expl.spec]     Status: open     Submitter: Mark Hall     Date: 2021-02-02

According to 13.9.4 [temp.expl.spec] paragraph 16,

A member or a member template of a class template may be explicitly specialized for a given implicit instantiation of the class template, even if the member or member template is defined in the class template definition. An explicit specialization of a member or member template is specified using the syntax for explicit specialization.

The relationship between this construct and paragraph 14 is not clear:

Whether an explicit specialization of a function or variable template is inline, constexpr, or an immediate function is determined by the explicit specialization and is independent of those properties of the template.

(See also 9.2.6 [dcl.constexpr] paragraph 1, note 1.) Is this intended to apply to explicit specializations of members of implicitly-instantiated class templates? For example:

  template<typename T> struct S {
    int f();
    constexpr int g();
  };
  template<> constexpr int S<int>::f() {  // OK, constexpr?
    return 0;
  }
  template<> int S<int>::g() {            // OK, not constexpr?
    return 0;
  }

There is implementation divergence on the treatment of this example. This divergence may relate to interpretation of the requirement in 9.2.6 [dcl.constexpr] paragraph 1,

If any declaration of a function or function template has a constexpr or consteval specifier, then all its declarations shall contain the same specifier.

Is an explicit specialization of a member of an implicitly-instantiated class template a declaration of that member? A similar question also applies to the constinit specifier as specified in 9.2.7 [dcl.constinit] paragraph 1:

If the specifier is applied to any declaration of a variable, it shall be applied to the initializing declaration.

(Note that constinit is not mentioned in 13.9.4 [temp.expl.spec] paragraph 14.) For example:

  template<typename T> struct S {
    static constinit T x;
  };
  template<> int S<int>::x = 10;    // constinit required?
  extern char c;
  template<> short S<char>::x = c;  // error, c not constant?

(Possibly relevant is the fact that default arguments are prohibited in explicit specializations of member functions of implicitly-instantiated class templates, per 13.9.4 [temp.expl.spec] bullet 21.3.)




264. Unusable template constructors and conversion functions

Section: 13.10.2  [temp.arg.explicit]     Status: open     Submitter: John Spicer     Date: 17 Nov 2000

The note in paragraph 5 of 13.10.2 [temp.arg.explicit] makes clear that explicit template arguments cannot be supplied in invocations of constructors and conversion functions because they are called without using a name. However, there is nothing in the current wording of the Standard that makes declaring a constructor or conversion operator that is unusable because of nondeduced parameters (i.e., that would need to be specified explicitly) ill-formed. It would be a service to the programmer to diagnose this useless construct as early as possible.




697. Deduction rules apply to more than functions

Section: 13.10.3  [temp.deduct]     Status: open     Submitter: Doug Gregor     Date: 6 June, 2008

13.10.3 [temp.deduct] is all about function types, but these rules also apply, e.g., when matching a class template partial specialization. We should add a note stating that we could be doing substitution into the template-id for a class template partial specialization.

Additional note (August 2008):

According to _N4868_.13.7.6.2 [temp.class.spec.match] paragraph 2, argument deduction is used to determine whether a given partial specialization matches a given argument list. However, there is nothing in _N4868_.13.7.6.2 [temp.class.spec.match] nor in 13.10.3 [temp.deduct] and its subsections that describes exactly how argument deduction is to be performed in this case. It would seem that more than just a note is required to clarify this processing.




2054. Missing description of class SFINAE

Section: 13.10.3  [temp.deduct]     Status: open     Submitter: Ville Voutilainen     Date: 2014-12-07

Presumably something like the following should be well-formed, where a deduction failure in a partial specialization is handled as a SFINAE case as it is with function templates and not a hard error:

  template <class T, class U> struct X   {
    typedef char member;
  };

  template<class T> struct X<T,
   typename enable_if<(sizeof(T)>sizeof(
     float)), float>::type>
  {
    typedef long long member;
  };

  int main() {
    cout << sizeof(X<double, float>::member);
  }

However, this does not appear to be described anywhere in the Standard.




2498. Partial specialization failure and the immediate context

Section: 13.10.3.1  [temp.deduct.general]     Status: open     Submitter: Daveed Vandevoorde     Date: 2021-06-15

Consider the following example:

  template<typename T, typename U> struct S {};
  template<typename T> struct S<T, T> {};
  template<typename T, typename U> struct S<T*, U*> {};
  template<typename... Ts> using V = void;
  template<typename T, typename U = void> struct X {};
  template<typename T> struct X<T, V<typename S<T, T>::type>>;
  X<int*> xpi;

Determining whether the partial specialization of X matches X<int*> requires determining whether one of the partial specializations of S matches S<int*,int*>. The partial specializations of S are ambiguous for this case. The question is whether that ambiguity should be considered in the “immediate context” of the type (SFINAE) or whether it should result in a hard error. There is implementation divergence on the handling of this example.

Notes from the November, 2021 teleconference:

A similar example can be constructed involving overload resolution instead of partial specialization:

  template<typename T, typename U> struct S {};
  template<typename T> struct S<T, T> {};
  template<typename T, typename U> struct S<T*, U*> {};

  template<class T>
  bool f(T, typename S<T, T>::type = 0);
  bool f(...);

  int x;
  bool b = f(&x);  // hard error with gcc, ok with clang



503. Cv-qualified function types in template argument deduction

Section: 13.10.3.2  [temp.deduct.call]     Status: open     Submitter: Gabriel Dos Reis     Date: 22 Feb 2005

Consider the following program:

    template <typename T> int ref (T&)                { return 0; }
    template <typename T> int ref (const T&)          { return 1; }
    template <typename T> int ref (const volatile T&) { return 2; }
    template <typename T> int ref (volatile T&)       { return 4; }

    template <typename T> int ptr (T*)                { return 0; }
    template <typename T> int ptr (const T*)          { return 8; }
    template <typename T> int ptr (const volatile T*) { return 16; }
    template <typename T> int ptr (volatile T*)       { return 32; }

    void foo() {}

    int main()
    {
        return ref(foo) + ptr(&foo);
    }

The Standard appears to specify that the value returned from main is 2. The reason for this result is that references and pointers are handled differently in template argument deduction.

For the reference case, 13.10.3.2 [temp.deduct.call] paragraph 3 says that “If P is a reference type, the type referred to by P is used for type deduction.” Because of issue 295, all four of the types for the ref function parameters are the same, with no cv-qualification; overload resolution does not find a best match among the parameters and thus the most-specialized function is selected.

For the pointer type, argument deduction does not get as far as forming a cv-qualified function type; instead, argument deduction fails in the cv-qualified cases because of the cv-qualification mismatch, and only the cv-unqualified version of ptr survives as a viable function.

I think the choice of ignoring cv-qualifiers in the reference case but not the pointer case is very troublesome. The reason is that when one considers function objects as function parameters, it introduces a semantic difference whether the function parameter is declared a reference or a pointer. In all other contexts, it does not matter: a function name decays to a pointer and the resulting semantics are the same.

(See also issue 1584.)




1221. Partial ordering and reference collapsing

Section: 13.10.3.5  [temp.deduct.partial]     Status: open     Submitter: Michael Wong     Date: 2010-11-08

The current partial ordering rules produce surprising results in the presence of reference collapsing.

Since partial ordering is currently based solely on the signature of the function templates, the lack of difference following substitution of the template type parameter in the following is not taken into account.

Especially unsettling is that the allegedly "more specialized" template (#2) is not a candidate in the first call where template argument deduction fails for it despite a lack of non-deduced contexts.

    template <typename T>
    void foo(T&&);  // #1

    template <typename T>
    void foo(volatile T&&);  // #2

    int main(void) {
      const int x = 0;
      foo(x);  // calls #1 with T='const int &'
      foo<const int &>(x);  // calls #2
    }



1763. Length mismatch in template type deduction

Section: 13.10.3.6  [temp.deduct.type]     Status: open     Submitter: Canada     Date: 2013-09-23

N3690 comment CA 4

It is not clear how an example like the following is to be handled:

  template <typename U> 
  struct A { 
    template <typename V> operator A<V>(); 
  }; 

  template <typename T> 
  void foo(A<void (T)>); 

  void foo(); 

  int main() { 
    A<void (int, char)> a; 
    foo<int>(a); 
    foo(a); // deduces T to be int
  } 

In subclause 13.10.3.6 [temp.deduct.type] paragraph 10, deduction from a function type considers P/A pairs from the parameter-type-list only where the "P" function type has a parameter. Deduction is not specified to fail if there are additional parameters in the corresponding "A" function type.

Notes from the September, 2013 meeting:

CWG agreed that this example should not be accepted. The existing rules seem to cover this case (deduction is not specified to “succeed,” so it's a reasonable conclusion that it fails), but it might be helpful to be clearer.




2417. Explicit instantiation and exception specifications

Section: 14.5  [except.spec]     Status: open     Submitter: John Spicer     Date: 2019-06-19

Consider the following example:

  template<class T>struct Y {
    typedef typename T::value_type blah;  // #1
    void swap(Y<T> &);
  };
  template<class T>
  void swap(Y<T>& Left, Y<T>& Right) noexcept(noexcept(Left.swap(Right))) { }

  template <class T> struct Z {
    void swap(Z<T> &);
  };
  template<class T>
  void swap(Z<T>& Left, Z<T>& Right) noexcept(noexcept(Left.swap(Right))) { }

  Z<int> x00, y00;
  constexpr bool b00 = noexcept(x00.swap(y00));
  template void swap<int>(Z<int>&, Z<int>&) noexcept(b00);  // #2

The question here is whether the explicit instantiation of

  swap<int>(Z<int>&, Z<int>&)

at #2 instantiates the exception specification of

  swap<int>(Y<int>&, Y<int>&)

which would instantiate Y<int>, resulting in an error on the declaration of

  typedef typename T::value_type blah;

at #1.

According to 13.9.2 [temp.inst] paragraph 14,

The noexcept-specifier of a function template specialization is not instantiated along with the function declaration; it is instantiated when needed (14.5 [except.spec]).

According to 14.5 [except.spec] bullet 13.3, one of the reasons an exception specification is needed is:

the exception specification is compared to that of another declaration (e.g., an explicit specialization or an overriding virtual function);

Such a comparison is presumably needed when determining which function template the explicit instantiation is referring to, making the program ill-formed. However, there is implementation variance on this point.




2420. Exception specifications in explicit instantiation

Section: 14.5  [except.spec]     Status: open     Submitter: John Spicer     Date: 2019-06-19

The expected behavior of the following example is not clear:

  template<class T> struct Y {
    typedef typename T::value_type blah;
    void swap(Y<T> &);
  };
  template<class T>
  void swap(Y<T>& Left, Y<T>& Right) noexcept(noexcept(Left.swap(Right))) {
  }

  template <class T> struct Z {
     void swap(Z<T> &);
  };
  template<class T>
  void swap(Z<T>& Left, Z<T>& Right) noexcept(noexcept(Left.swap(Right))) {
  }
  Z<int> x00, y00;
  constexpr bool b00 = noexcept(x00.swap(y00));
  // Instantiates the Z<int> overload:
  template void swap<int>(Z<int>&, Z<int>&) noexcept(b00); 

The question is whether the explicit instantiation directive also instantiates the Y<int> overload and thus Y<int> (because of the exception specification), which will fail because of the reference to T::value_type with T=int.

According to 14.5 [except.spec] bullet 13.3, one of the contexts in which an exception specification is needed (thus triggering its instantiation) is when:

the exception specification is compared to that of another declaration (e.g., an explicit specialization or an overriding virtual function);

In this example, the declarations of swap must be compared in order to determine which function template is being instantiated, resulting in the instantiation of Y<int>. There is implementation divergence, however, with some accepting the example and some issuing an error for the instantiation of Y<int>.




925. Type of character literals in preprocessor expressions

Section: 15.2  [cpp.cond]     Status: open     Submitter: Michael Wong     Date: 29 June, 2009

According to 15.2 [cpp.cond] paragraph 4,

The resulting tokens comprise the controlling constant expression which is evaluated according to the rules of 7.7 [expr.const] using arithmetic that has at least the ranges specified in 17.3 [support.limits], except that all signed and unsigned integer types act as if they have the same representation as, respectively, intmax_t or uintmax_t (_N3035_.18.4.2 [stdinth]). This includes interpreting character literals, which may involve converting escape sequences into execution character set members.

Ordinary character literals with a single c-char have the type char, which is neither a signed nor an unsigned integer type. Although 7.3.7 [conv.prom] paragraph 1 is clear that char values promote to int, regardless of whether the implementation treats char as having the values of signed char or unsigned char, 15.2 [cpp.cond] paragraph 4 isn't clear on whether character literals should be treated as signed or unsigned values. In C99, such literals have type int, so the question does not arise. If an implementation in which plain char has the values of unsigned char were to treat character literals as unsigned, an expression like '0'-'1' would thus have different values in C and C++, namely -1 in C and some large unsigned value in C++.




2190. Insufficient specification of __has_include

Section: 15.2  [cpp.cond]     Status: open     Submitter: Hubert Tong     Date: 2015-10-24

[Detailed description pending.]




1625. Adding spaces between tokens in stringizing

Section: 15.6.3  [cpp.stringize]     Status: open     Submitter: Chandler Carruth     Date: 2013-02-18

Given the following input,

  #define F(A, B, C) A ## x.B ## y.C ## z
  #define STRINGIFY(x) #x
  #define EXPAND_AND_STRINGIFY(x) STRINGIFY(x)
  char v[] = EXPAND_AND_STRINGIFY(F(a, b, c))

there is implementation variance in the value of v: some produce the string "ax.by.cz" and others produce the string "ax. by. cz". Although 15.6.3 [cpp.stringize] paragraph 2 is explicit in its treatment of leading and trailing white space, it is not clear whether there is latitude for inserting spaces between tokens, as some implementations do, since the description otherwise is written solely in terms of preprocessing tokens. There may be cases in which such spaces would be needed to preserve the original tokenization, but it is not clear whether the result of stringization needs to produce something that would lex to the same tokens.

Notes from the April, 2013 meeting:

Because the preprocessor specification is primarily copied directly from the C Standard, this issue has been referred to the C liaison for consultation with WG14.




268. Macro name suppression in rescanned replacement text

Section: 15.6.5  [cpp.rescan]     Status: open     Submitter: Bjarne Stroustrup     Date: 18 Jan 2001

It is not clear from the Standard what the result of the following example should be:

#define NIL(xxx) xxx
#define G_0(arg) NIL(G_1)(arg)
#define G_1(arg) NIL(arg)
G_0(42)

The relevant text from the Standard is found in 15.6.5 [cpp.rescan] paragraph 2:

If the name of the macro being replaced is found during this scan of the replacement list (not including the rest of the source file's preprocessing tokens), it is not replaced. Further, if any nested replacements encounter the name of the macro being replaced, it is not replaced. These nonreplaced macro name preprocessing tokens are no longer available for further replacement even if they are later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.

The sequence of expansion of G0(42) is as follows:

G0(42)
NIL(G_1)(42)
G_1(42)
NIL(42)

The question is whether the use of NIL in the last line of this sequence qualifies for non-replacement under the cited text. If it does, the result will be NIL(42). If it does not, the result will be simply 42.

The original intent of the J11 committee in this text was that the result should be 42, as demonstrated by the original pseudo-code description of the replacement algorithm provided by Dave Prosser, its author. The English description, however, omits some of the subtleties of the pseudo-code and thus arguably gives an incorrect answer for this case.

Suggested resolution (Mike Miller): Replace the cited paragraph with the following:

As long as the scan involves only preprocessing tokens from a given macro's replacement list, or tokens resulting from a replacement of those tokens, an occurrence of the macro's name will not result in further replacement, even if it is later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.

Once the scan reaches the preprocessing token following a macro's replacement list — including as part of the argument list for that or another macro — the macro's name is once again available for replacement. [Example:

    #define NIL(xxx) xxx
    #define G_0(arg) NIL(G_1)(arg)
    #define G_1(arg) NIL(arg)
    G_0(42)                         // result is 42, not NIL(42)

The reason that NIL(42) is replaced is that (42) comes from outside the replacement list of NIL(G_1), hence the occurrence of NIL within the replacement list for NIL(G_1) (via the replacement of G_1(42)) is not marked as nonreplaceable. —end example]

(Note: The resolution of this issue must be coordinated with J11/WG14.)

Notes (via Tom Plum) from April, 2004 WG14 Meeting:

Back in the 1980's it was understood by several WG14 people that there were tiny differences between the "non-replacement" verbiage and the attempts to produce pseudo-code. The committee's decision was that no realistic programs "in the wild" would venture into this area, and trying to reduce the uncertainties is not worth the risk of changing conformance status of implementations or programs.




745. Effect of ill-formedness resulting from #error

Section: 15.8  [cpp.error]     Status: open     Submitter: Clark Nelson     Date: 13 November, 2008

C99 is very clear that a #error directive causes a translation to fail: Clause 4 paragraph 4 says,

The implementation shall not successfully translate a preprocessing translation unit containing a #error preprocessing directive unless it is part of a group skipped by conditional inclusion.

C++, on the other hand, simply says that a #error directive “renders the program ill-formed” (15.8 [cpp.error]), and the only requirement for an ill-formed program is that a diagnostic be issued; the translation may continue and succeed. (Noted in passing: if this difference between C99 and C++ is addressed, it would be helpful for synchronization purposes in other contexts as well to introduce the term “preprocessing translation unit.”)




897. _Pragma and extended string-literals

Section: 15.12  [cpp.pragma.op]     Status: open     Submitter: Daniel Krügler     Date: 9 May, 2009

The specification of how the string-literal in a _Pragma operator is handled does not deal with the new kinds of string literals. 15.12 [cpp.pragma.op] says,

The string literal is destringized by deleting the L prefix, if present, deleting the leading and trailing double-quotes, replacing each escape sequence...

The various other prefixes should either be handled or prohibited.

Additional note (October, 2013):

If raw string literals are supported, the question of how to handle line splicing is relevant. The wording says that “the characters are processed through translation phase 3,” which is a bit ambiguous as to whether that includes phases 1 and 2 or not. It would be better to be explicit and say that the processing of phase 3 or of phases 1 through 3 is applied.




2464. Constexpr launder and unions

Section: 17.6.5  [ptr.launder]     Status: open     Submitter: Hubert Tong     Date: 2020-11-07

According to 17.6.5 [ptr.launder], referring to std::launder,

An invocation of this function may be used in a core constant expression whenever the value of its argument may be used in a core constant expression.

It is not clear whether this wording is intended to permit an example like

  #include <new>

  struct A { char x; };

  union U {
    A a;
    A a2;
  };

  constexpr A foo() {
    U u = {{42}};
    A *ap = &u.a2;
    return *std::launder(ap);
  }

  extern constexpr A globA = foo();

In particular, is the wording intended to restrict use of std::launder in a constant expression to cases in which the function returns its argument unchanged? As a further example, consider

  #include <new>

  struct A { char x; };
  struct B { A a; };
  struct BytesAndMore {
    unsigned char bytes[sizeof(A)];
    unsigned char more;
  };

  union U {
    BytesAndMore bytes;
    A a;
    B b;
  };

  constexpr B foo() {
    U u;
    A *ap = &u.a;
    B *bp = &u.b;
    u.bytes.more = 0;
    std::launder(ap)->x = 42;
    return *std::launder(bp);
  }

  extern constexpr B globB = foo();

Notes from the December, 2020 teleconference:

See also LWG issue 3495.




2361. Unclear description of longjmp undefined behavior

Section: 17.13.3  [csetjmp.syn]     Status: open     Submitter: Zhihao Yuan     Date: 2017-10-20

According to 17.13.3 [csetjmp.syn] paragraph 2,

A setjmp/longjmp call pair has undefined behavior if replacing the setjmp and longjmp by catch and throw would invoke any non-trivial destructors for any automatic objects.

The intent is clear, that transferring control from point A to point B via longjmp has undefined behavior if throwing an exception at point A and catching it at point B would invoke non-trivial destructors. The wording could be more precise.

See also the corresponding editorial issue for additional discussion.

Notes from the October, 2018 teleconference:

There are a number of unanswered questions in the current wording, including the impact on the current exception (whether it still exists) after a longjmp out of a handler, the impact on the initialization locks if jumping from the initialization of a local static data member, etc. One thought was to restrict use of longjmp to “plain C functions”. Another was to say if the program would have different behavior via the use of exceptions the behavior is undefined. There was no consensus on how to proceed.




1944. New C incompatibilities

Section: Annex C  [diff]     Status: open     Submitter: Mike Miller     Date: 2014-06-18

Some new features of C++ not only introduce incompatibilities with previous versions of C++ but also with C; however, the organization of Annex Annex C [diff] makes it difficult to specify that a given feature is incompatible with both languages, and the practice has been only to document the C++ incompatibilities. Some means of specifying both sets of incompatibilities should be found, hopefully without excessive duplication between the C and C++ sections.




1248. Updating Annex C to C99

Section: C.5  [diff.iso]     Status: open     Submitter: Jonathan Wakely     Date: 2011-02-28

The description of incompatibilities with C in Annex C.5 [diff.iso] refers to C89, but there are a number of new features in C99 that should be covered.




511. POD-structs with template assignment operators

Section: Clause 11  [class]     Status: open     Submitter: Alisdair Meredith     Date: 19 Mar 2005

A POD-struct is not permitted to have a user-declared copy assignment operator (Clause 11 [class] paragraph 4). However, a template assignment operator is not considered a copy assignment operator, even though its specializations can be selected by overload resolution for performing copy operations (_N4750_.15.8 [class.copy] paragraph 9 and especially footnote 114). Consequently, X in the following code is a POD, notwithstanding the fact that copy assignment (for a non-const operand) is a member function call rather than a bitwise copy:

    struct X {
      template<typename T> const X& operator=(T&);
    };
    void f() {
      X x1, x2;
      x1 = x2;  // calls X::operator=<X>(X&)
    }

Is this intentional?




2428. Deprecating a concept

Section: Clause 13  [temp]     Status: open     Submitter: Eric Niebler     Date: 2018-12-10

The grammar for a concept-definition does not include an attribute-specifier-seqopt, making it impossible to deprecate an attribute. This seems like an oversight.




2002. White space within preprocessing directives

Section: Clause 15  [cpp]     Status: open     Submitter: Richard Smith     Date: 2014-09-10

According to Clause 15 [cpp] paragraphg 4,

The only white-space characters that shall appear between preprocessing tokens within a preprocessing directive (from just after the introducing # preprocessing token through just before the terminating new-line character) are space and horizontal-tab (including spaces that have replaced comments or possibly other white-space characters in translation phase 3).

The effect of this restriction is unclear, however, since translation phase 3 is permitted to transform all white space characters and comments into spaces. The relationship between these two rules should be clarified.




783. Definition of “argument”

Section: Clause 3  [intro.defs]     Status: open     Submitter: UK     Date: 3 March, 2009

N2800 comment UK 3

The definition of an argument does not seem to cover many assumed use cases, and we believe that is not intentional. There should be answers to questions such as: Are lambda-captures arguments? Are type names in a throw-spec arguments? “Argument” to casts, typeid, alignof, alignas, decltype and sizeof? why in x[arg] arg is not an argument, but the value forwarded to operator[]() is? Does not apply to operators as call-points not bounded by parentheses? Similar for copy initialization and conversion? What are deduced template “arguments?” what are “default arguments?” can attributes have arguments? What about concepts, requires clauses and concept_map instantiations? What about user-defined literals where parens are not used?




1642. Missing requirements for prvalue operands

Section: Clause 7  [expr]     Status: open     Submitter: Joseph Mansfield     Date: 2013-03-15

Although the note in 7.2.1 [basic.lval] paragraph 1 states that

The discussion of each built-in operator in Clause Clause 7 [expr] indicates the category of the value it yields and the value categories of the operands it expects

in fact, many of the operators that take prvalue operands do not make that requirement explicit. Possible approaches to address this failure could be a blanket statement that an operand whose value category is not stated is assumed to be a prvalue; adding prvalue requirements to each operand description for which it is missing; or changing the description of the usual arithmetic conversions to state that they imply the lvalue-to-rvalue conversion, which would cover the majority of the omissions.

(See also issue 1685, which deals with an inaccurately-specified value category.)




157. Omitted typedef declarator

Section: Clause 9  [dcl.dcl]     Status: open     Submitter: Daveed Vandevoorde     Date: 19 Aug 1999

Clause 9 [dcl.dcl] paragraph 3 reads,

In a simple-declaration, the optional init-declarator-list can be omitted only when... the decl-specifier-seq contains either a class-specifier, an elaborated-type-specifier with a class-key (11.3 [class.name] ), or an enum-specifier. In these cases and whenever a class-specifier or enum-specifier is present in the decl-specifier-seq, the identifiers in those specifiers are among the names being declared by the declaration... In such cases, and except for the declaration of an unnamed bit-field (11.4.10 [class.bit] ), the decl-specifier-seq shall introduce one or more names into the program, or shall redeclare a name introduced by a previous declaration. [Example:
    enum { };           // ill-formed
    typedef class { };  // ill-formed
—end example]
In the absence of any explicit restrictions in 9.2.4 [dcl.typedef] , this paragraph appears to allow declarations like the following:
    typedef struct S { };    // no declarator
    typedef enum { e1 };     // no declarator
In fact, the final example in Clause 9 [dcl.dcl] paragraph 3 would seem to indicate that this is intentional: since it is illustrating the requirement that the decl-specifier-seq must introduce a name in declarations in which the init-declarator-list is omitted, presumably the addition of a class name would have made the example well-formed.

On the other hand, there is no good reason to allow such declarations; the only reasonable scenario in which they might occur is a mistake on the programmer's part, and it would be a service to the programmer to require that such errors be diagnosed.




2188. empty-declaration ambiguity

Section: Clause 9  [dcl.dcl]     Status: open     Submitter: Jens Maurer     Date: 2015-10-21

[Detailed description pending.]






Issues with "Concurrency" Status


1842. Unevaluated operands and “carries a dependency”

Section: 6.9.2  [intro.multithread]     Status: concurrency     Submitter: Hans Boehm     Date: 2014-01-23

According to 6.9.2 [intro.multithread] paragraph 9,

An evaluation A carries a dependency to an evaluation B if

The intent is that this does not apply to the second operands of such operators if the first operand is such that they are not evaluated, but the wording is not clear to that effect. (A similar question applies to the non-selected operand of the conditional operator ?:.)

Notes from the October, 2015 meeting:

It appears likely that the text involved will be removed by a revision to the memory_order_consume specification.

Notes from the February, 2016 meeting:

Action on this issue will be deferred until the specification for memory_order_consume is complete; it should not currently be used.




2298. Actions and expression evaluation

Section: 6.9.2.2  [intro.races]     Status: concurrency     Submitter: Kazutoshi Satoda     Date: 2016-01-21

Section 6.9.2.2 [intro.races] uses the terms “action” and “expression evaluation” interchangeably. “Sequenced before” is defined on expression evaluations. Probably none of those is correct.

We should really be talking about individual accesses to “memory locations”. Talking about larger “expression evaluations” is incorrect, since they may include internal synchronization. Thus concurrent evaluation of large conflicting expression evaluations may not actually correspond to a data race. I'm not sure what term we should be using instead of “expression evaluation” to denote such individual accesses. Call it X for now.

There is also an issue with the fact that “sequenced before” is defined on expression evaluation. “Sequenced before” should also be defined on Xs. It doesn't make any sense to talk about “sequenced before” ordering on two evaluations when one includes the other. Whenever we say “A is sequenced before B”, we probably really mean that all Xs in A are sequenced before all Xs in B. We could probably just include a blanket statement to that effect.