|Author:||John Spicer, Edison Design Group|
externkeyword. This document proposes specific semantics for this feature.
This feature was first proposed to the committee in N1448 by Mat Marcus and Gabriel Dos Reis. While there was widespread agreement on the need to support "extern template", there was not support for having "extern template" suppress the instantiation of inline functions. In addition, N1448 proposed "extern template" as a mechanism to provide greater control over the point of instantiation of a template. It should be noted that the current standard provides no such guarantees, even in the presence of explicit instantiations, and this proposal does not attempt to provide such a feature.
As a result of reflector discussions and discussions in the Core and Evolution groups, the following rules are proposed:
extern templatedirective appears in a translation unit, an explicit instantiation of the entity must appear in another translation unit, or later in the same translation unit.
extern templatedirective may not name a static function (but may name a static class member).
extern templatedirective that names a class applies to the members of the class, not the class itself. It results in the implicit instantiation of the class as well as that of any nested classes.
extern templatehas no normative affect on inline functions. A note should be included indicating that implementations are encouraged to suppress out-of-line copies of inline functions that were declared with
extern template. Such inline functions are instantiated though (so that they can be inlined).
An issue I ran into when drafting this is that something that used to say "If X is explicitly instantiated" is transformed to "If X is the subject of an explicit instantiation declaration". A more pithy phrase would be helpful. One possibility is "If X is explicitly instantiation-declared".
Revise 7.1.1 paragraph 5 as follows:
The extern specifier can be applied only to the names of objects and functions, or in an explicit instantiation ([temp.explicit]).
Revise 18.104.22.168 paragraph 5 as follows:
An explicit instantiation is an instantiation point for the specialization ...
Revise 14.7 paragraph 5 as follows:
- an explicit instantiation shall appear at most once in a program.
Revise 14.7.2, paragraph 2 as follows:
explicit-instantiation: template declaration
Revise 14.7.2 paragraph 4 as follows:
- Otherwise, the definition of a non-exported ...
Revise 14.7.2 paragraph 7 as follows:
Theexplicit instantiation ofa class template specialization alsoexplicit instantiat eseach of its members (not including members inherited from base classes) whose definition is visible at the point of instantiation andthat has not been previously explicitly specialized in the translation unit containing the explicit specialization, except as described below.
At the end of 14.7.2 paragraph 7, add the following:
An explicit instantiation definition that names a class template specialization is also an explicit instantiation definition of the class template specialization and is only an explicit instantiation definition of members whose definition is visible at the point of instantiation.
An explicit instantiation declaration that names a class template specialization has no effect on the class template specialization itself (except for perhaps resulting in its implicit instantiation). Other explicit instantiation declarations have the effect of suppressing the implicit instantiation of the entity to which they refer, except that an explicit instantiation declaration of an inline function has no effect. [Note: The intent is that an inline function that is the subject of an explicit instantiation declaration will still be implicitly instantiated when used so that the body can be considered for inlining, but that no out-of-line copy of the inline function would be generated in the translation unit. -- end note]
If an entity is the subject of both an explicit instantiation declaration and an explicit instantiation definition in the same translation unit, the definition shall follow the declaration. 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. [Note: this rule does apply to inline functions even though an explicit instantiation declaration of such an entity has no other normative effect. This is needed to ensure that if the address of an inline function is taken in a translation unit in which the implementation chose to suppress the out-of-line body, that another translation unit will supply the body. --end note] An explicit instantiation declaration shall not name a specialization of a template with internal linkage.