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1294. Difference between callable wrapper and forwarding call wrapper unclear

Section: 22.10.4 [func.require] Status: C++11 Submitter: Jens Maurer Opened: 2009-12-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current wording in the standard makes it hard to discriminate the difference between a "call wrapper" as defined in 22.10.3 [func.def]/5+6:

5 A call wrapper type is a type that holds a callable object and supports a call operation that forwards to that object.

6 A call wrapper is an object of a call wrapper type.

and a "forwarding call wrapper" as defined in 22.10.4 [func.require]/4:

4 [..] A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

template<class... ArgTypes>
R operator()(ArgTypes&&... args) cv-qual;

— end note]

Reason for this lack of clear difference seems to be that the wording adaption to variadics and rvalues that were applied after it's original proposal in N1673:

[..] A forwarding call wrapper is a call wrapper that can be called with an argument list t1, t2, ..., tN where each ti is an lvalue. The effect of calling a forwarding call wrapper with one or more arguments that are rvalues is implementation defined. [Note: in a typical implementation forwarding call wrappers have overloaded function call operators of the form

template<class T1, class T2, ..., class TN>
R operator()(T1& t1, T2& t2, ..., TN& tN) cv-qual;

— end note]

combined with the fact that the word "forward" has two different meanings in this context. This issue attempts to clarify the difference better.

[ 2010-09-14 Daniel provides improved wording and verified that it is correct against N3126. Previous resolution is shown here: ]

4 [..] A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

template<class... ArgTypes>
R operator()(ArgTypes&&... args) cv-qual;

— end note]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 22.10.4 [func.require]/4 as indicated:

[..] A forwarding call wrapper is a call wrapper that can be called with an argument list . [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

template<class... UnBoundArgs>
R operator()(UnBoundArgs&&... unbound_args) cv-qual;

— end note ]