Document #:  P2481R0 
Date:  20211015 
Project:  Programming Language C++ 
Audience: 
EWG 
Replyto: 
Barry Revzin <barry.revzin@gmail.com> 
There are many situations where the goal of a function template is deduce an arbitrary type  an arbitrary range, an arbitrary value, an arbitrary predicate, and so forth. But sometimes, we need something more specific. While we still want to allow for deducing const
vs mutable and lvalue vs rvalue, we know either what concrete type or concrete class template we need  and simply want to deduce just that. With the adoption of [P0847R7], the incidence of this will only go up.
It may help if I provide a few examples.
std::tuple
converting constructorstd::tuple<T...>
is constructible from std::tuple<U...> cv ref
when T...
and U...
are the same size and each T
is constructible from U cv ref
(plus another constraint to avoid clashing with other constructors that I’m just going to ignore for the purposes of this paper). The way this would be written today is:
template <typename... Ts> struct tuple { template <typename... Us> requires sizeof...(Ts) == sizeof...(Us) && (is_constructible_v<Ts, Us&> && ...) tuple(tuple<Us...>&); template <typename... Us> requires sizeof...(Ts) == sizeof...(Us) && (is_constructible_v<Ts, Us const&> && ...) tuple(tuple<Us...> const&); template <typename... Us> requires sizeof...(Ts) == sizeof...(Us) && (is_constructible_v<Ts, Us&&> && ...) tuple(tuple<Us...>&&); template <typename... Us> requires sizeof...(Ts) == sizeof...(Us) && (is_constructible_v<Ts, Us const&&> && ...) tuple(tuple<Us...> const&&); };
This is pretty tedious to say the least. But it also has a subtle problem: these constructors are all overloads  which means that if the one you’d think would be called is valid, it is still possible for a different one to be invoked. For instance:
Here, we’re trying to construct a tuple<int&>
from an rvalue tuple<int&&>
. The desired behavior is that the tuple<Us...>&&
is considered and then rejected (because int&
is not constructible from Us&&
 int&&
). That part indeed happens. But the tuple<Us...> const&
constructor still exists and that one ends up being fine (because int&
is constructible from Us const&
 which is int&
here). That’s surprising and undesirable.
But in order to avoid this, we’d need to only have a single constructor template. What we want is to have some kind of tuple<Us...>
and just deduce the cv ref
part. But our only choice today is either the code above (tedious, yet mostly functional, despite this problem) or to go full template:
How do we write the rest of the constraint? We don’t really have a good way of doing so. Besides, for types which inherit from std::tuple
, this is now actually wrong  that derived type will not be a specialization of tuple, but rather instead inherit from one. We have to do extra work to get that part right, as we currently do in the std::visit
specification  see 20.7.7 [variant.visit]/1.
std::get
for std::tuple
We run into the same thing for nonmember functions, where we want to have std::get<I>
be invocable on every kind of tuple. Which today likewise has to be written:
template <size_t I, typename... Ts> auto get(tuple<Ts...>&) > tuple_element_t<I, tuple<Ts...>>&; template <size_t I, typename... Ts> auto get(tuple<Ts...> const&) > tuple_element_t<I, tuple<Ts...>> const&; template <size_t I, typename... Ts> auto get(tuple<Ts...>&&) > tuple_element_t<I, tuple<Ts...>>&&; template <size_t I, typename... Ts> auto get(tuple<Ts...> const&&) > tuple_element_t<I, tuple<Ts...>> const&&;
This one we could try to rewrite as a single function template, but in order to do that, we need to first coerce the type down to some kind of specialization of tuple
 which ends up requiring a lot of the same work anyway.
transform
for std::optional
The previous two examples want to deduce to some specialization of a class template, this example wants to deduce to a specific type. One of the motivation examples from “deducing this
” was to remove the quadruplication necessary when writing a set of overloads that want to preserve const
ness and value category. The adoption of [P0798R8] gives us several such examples.
In C++20, we’d write it as:
complete with quadruplicating the body. But with deducing this, we might consider writing it as:
But this deduces too much! We don’t want to deduce derived types (which in addition to unnecessary extra template instantiations, can also run into shadowing issues). We just want to know what the const
ness and value category of the optional
are.
view_interface
membersSimilar to the above, but even more specific, are the members for view_interface
(see 24.5.3 [view.interface]). Currently, we have a bunch of pairs of member functions:
template <typename D> class view_interface { constexpr D& derived() noexcept { return static_cast<D&>(*this); } constexpr D const& derived() const noexcept { return static_cast<D const&>(*this); } public: constexpr bool empty() requires forward_range<D> { return ranges::begin(derived()) == ranges::end(derived()); } constexpr bool empty() const requires forward_range<D const> { return ranges::begin(derived()) == ranges::end(derived()); } };
With deducing this, we could write this as a single function template  deducing the self parameter such that it ends up being the derived type. But that’s again deducing way too much, when all we want to do is know if we’re a D&
or a D const&
. But we can’t deduce just const
ness.
To be more concrete, the goal here is to be able to specify a particular type or a particular class template such that template deduction will just deduce the const
ness and value category, while also (where relevant) performing a derivedtobase conversion. That is, I want to be able to implement a single function template for optional<T>::transform
such that if I invoke it with an rvalue of type D
that inherits publicly and unambiguously from optional<int>
, the function template will be instantiated with a first parameter of optional<int>&&
(not D&&
).
If you don’t find the above examples and the need for more concrete deduction motivating, then I’m sorry for wasting your time.
If you do find the above examples and the need for more concrete deduction motivating, then I’m sorry that I don’t actually have a solution for you. What I have instead are several example syntaxes that I’ve thought about over the years that are all varying degrees of mediocre. My hope with this paper is that other, more creative, people are equally interesting in coming up with a solution to this problem and can come up with a better syntax for it.
Here are those syntax options. I will, for each option, demonstrate how to implement the tuple
converting constructor, optional::transform
, and view_interface::empty
. I will use the following tools:
#define FWD(e) static_cast<decltype(e)&&>(e) template <bool RV, typename T> using apply_ref = std::conditional_t<RV, T&&, T&>; template <bool C, typename T> using apply_const = std::conditional_t<C, T const, T>; template <bool C, bool RV, typename T> using apply_const_ref = apply_ref<RV, apply_const<C, T>>; template <typename T, typename U> using copy_cvref_t = apply_const_ref< is_const_v<remove_reference_t<T>>, !is_lvalue_reference_v<T>, U>;
T auto&&
The principle here is that in the same say that range auto&&
is some kind of range
, that int auto&&
is some kind of int
. It kind of makes sense, kind of doesn’t. Depends on how you think about it.
tuple



optional


view_interface

The advantage of this syntax is that it’s concise and lets you do what you need to do.
The disadvantage of this syntax is that the only way you can get the type is by writing decltype(param)
 and the only way to can pass through the const
ness and qualifiers is by grabbing them off of decltype(param)
. That’s fine if the type itself is all that is necessary (as it the case for view_interface
) but not so much when you actually need to apply them (as is the case for tuple
). This also means that the only place you can put the requires
clause is after the parameters. Another disadvantage is that the derivedtobase conversion aspect of this makes it inconsistent with what Concept auto
actually means  which is not actually doing any conversion.
T&&&
Rather than writing tuple<U...> auto&& rhs
we can instead introduce a new kind of reference and spell it tuple<U...>&&& rhs
. This syntactically looks nearly the same as the T auto&&
version, so I’m not going to copy it.
If we went this route, we would naturally have to also allow:
The advantage here is that it’s less arguably broken than the previous version, since it’s more reasonable that the tuple<U...>&&&
syntax would allow derivedtobase conversion.
The disadvantages are all the other disadvantages of the previous version, plus also a whole new reference token? Swell.
const(bool)
We have noexcept(true)
and explicit(true)
. What about const(true)
?
On some level, this seems to make perfect sense. At least for const
 since we want to deduce either T
or T const
, and so const
is either absent or present. But what about value category? How do you represent T&
vs T&&
? Surely, we wouldn’t do T &(LV) &&(RV)
for deducing two different bool
s  these two cases are mutually exclusive. Keeping one of the &
s around, as in T& &(RV)
(with a mandatory space) also seems pretty bad. So for the purposes of this section, let’s try T && (RV)
(where RV
is true
for rvalues and false
for lvalues, but still a forwarding reference).
This syntax is… pretty weird. Very weird.
The advantages are that it’s clearer that we’re only deducing const
ness and ref qualifiers. It also allows you to put requires
clauses after the templatehead rather than much later. When only deducing const
, it’s arguably pretty clear what’s going on.
The disadvantages are the obvious weirdness of the syntax, especially for figuring out the value category, and the mandatory metaprogramming around applying those boolean values that we deduce through the types. apply_const
and apply_const_ref
(as I’m arbitrarily calling them here, the former appears as an expositiononly trait in Ranges under the name maybeconst
) will be everywhere, and those aren’t exactly obvious to understand either. It may be tempting to allow writing int const(false) &&(true)
as a type directly to facilitate writing such code (this would be int&&
), but this seems facially terrible.
There’s further issues that int const(true)
isn’t quite valid grammar today, but it’s pretty close. const(true)
looks like a cast, and it’s not unreasonable that we may at some point consider const(x)
as a language cast version of std::as_const(x)
.
But there is one entirely unrelated benefit. Consider trying to write a function that accepts any function pointer:
That’s all you need (if we ignore varargs, assume the adoption of [CWG2355]  which was accepted but not fully processed yet). And note here the deduction of noexcept
ness (and the usage of it on a function type) very closely resembles the kind of deduction of const
and value category discussed in this section.
But today if we wanted to deduce a pointer to member function, we have to write a whole lot more  precisely because we can’t deduce all the other stuff at the end of the type:
// this only accepts nonconst pointers to member functions that have no refqualifier template <typename R, typename C, typename... Args, bool B> void accepts_member_function_ptr(R (C::*p)(Args...) noexcept(B)); // so we also need this one template <typename R, typename C, typename... Args, bool B> void accepts_member_function_ptr(R (C::*p)(Args...) const noexcept(B)); // ... and this one template <typename R, typename C, typename... Args, bool B> void accepts_member_function_ptr(R (C::*p)(Args...) & noexcept(B)); // ... and also this one template <typename R, typename C, typename... Args, bool B> void accepts_member_function_ptr(R (C::*p)(Args...) && noexcept(B)); // ... and then also these two template <typename R, typename C, typename... Args, bool B> void accepts_member_function_ptr(R (C::*p)(Args...) const& noexcept(B)); template <typename R, typename C, typename... Args, bool B> void accepts_member_function_ptr(R (C::*p)(Args...) const&& noexcept(B));
The direction where we can deduce const
in the same way that we can deduce noexcept
provides a much better solution for this. Although here, unlike the examples presented earlier, we’re not simply selecting between &
and &&
. Here, we have three options. Does that mean a different design then? And what would it look like to handle both cases? It’s a bit unclear.
qualifiers Q
This approach is quite different and involves introducing a new kind of template parameter, which I’m calling qualifiers
, which will deduce an alias template. It may be easier to look at the examples:
tuple



optional


view_interface

The idea here is that a parameter of the form Q<T> x
will deduce T
and Q
separately, but Q
will be deduced as one of the following four alias templates:
template <typename T> using Q = T&;
template <typename T> using Q = T const&;
template <typename T> using Q = T&&;
template <typename T> using Q = T const&&;
Whereas a parameter of the form Q<T>& x
or Q<T>&&
will deduce Q
either as:
template <typename T> using Q = T;
template <typename T> using Q = T const;
The significant advantage here is that applying the const
and reference qualifiers that we just deduced is trivial, since we already have exactly the tool we need to do that: Q
. This makes all the implementations simpler. It also gives you a way to name the parameter other than decltype(param)
, since there is a proper C++ spelling for the parameter itself in all cases.
The disadvantage is that this is quite novel for C++, and extremely weird. Even more dramatically weird than the other two solutions. And that’s even with using the nice name of qualifiers
, which is probably untenable (although cvrefquals
or refqual
might be available?). Also Q<T> x
does not look like a forwarding reference, but since Q<T>& x
is the only meaningful way to deduce just const
 this suggests that Q<T>&& x
also needs to deduce just const
(even though why would anyone write this), which leaves Q<T> x
alone.
There’s also the question of how we could provide an explicit template argument for Q
. Perhaps that’s spelled qualifiers::rvalue
(to add &&
) or qualifiers::const_lvalue
(to add const&
) and the like? There’d need to be some language magic way of spelling such a thing  since we probably wouldn’t want to just allow an arbitrary alias template. std::add_pointer_t
, for instance, would suddenly introduce a nondeduced context, and wouldn’t make any sense anyway.
If there’s a different approach someone has, I’d love to hear it. But this is what I’ve got so far.
[CWG2355] John Spicer. 20170906. Deducing noexceptspecifiers.
https://wg21.link/cwg2355
[P0798R8] Sy Brand. 2021. Monadic operations for std::optional.
https://wiki.edg.com/pub/Wg21virtual202110/StrawPolls/p0798r8.html
[P0847R7] Barry Revzin, Gašper Ažman, Sy Brand, Ben Deane. 20210714. Deducing this.
https://wg21.link/p0847r7