Title
Deducing rvalue references in declarative contexts
Status
c++11
Section
13.10.3.6 [temp.deduct.type]
Submitter
John Spicer

Created on 2009-04-16.00:00:00 last changed 96 months ago

Messages

Date: 2010-08-23.00:00:00

[Voted into WP at August, 2010 meeting.]

Date: 2010-02-15.00:00:00

Proposed resolution (February, 2010):

  1. Change 13.10.3.6 [temp.deduct.type] paragraph 10 as follows:

  2. Similarly, if P has a form that contains (T), then each parameter type Pi of the respective parameter-type-list of P is compared with the corresponding parameter type Ai of the corresponding parameter-type-list of A. If P and A are function types that originated from deduction when taking the address of a function template (13.10.3.3 [temp.deduct.funcaddr]) or when deducing template arguments from a function declaration ([temp.deduct.decl]) and Pi and Ai are parameters of the top-level parameter-type-list of P and A, respectively, Pi is adjusted if it is an rvalue reference to a cv-unqualified template parameter and Ai is an lvalue reference, in which case the type of Pi is changed to be the template parameter type (i.e., T&& is changed to simply T). [Note: As a result, when Pi is T&& and Ai is X&, the adjusted Pi will be T, causing T to be deduced as X&. —end note] [Example:

      template<typename T> void f(T&&);
      template<> void f(int&) { }      // #1
      template<> void f(int&&) { }     // #2
      void g(int i) {
        f(i);   // calls f<int&>(int&), i.e., #1
        f(0);   // calls f<int>(int&&), i.e., #2
      }
    

    end example]

    If the parameter-declaration corresponding to Pi is a function parameter pack...

  3. Add a new section under 13.10.3 [temp.deduct], either before or after 13.10.3.6 [temp.deduct.type], as follows:

  4. 14.8.2.x Deducing template arguments from a function declaration [temp.deduct.decl]

    In a declaration whose declarator-id refers to a specialization of a function template, template argument deduction is performed to identify the specialization to which the declaration refers. Specifically, this is done for explicit instantiations (13.9.3 [temp.explicit]), explicit specializations (13.9.4 [temp.expl.spec]), and certain friend declarations (13.7.5 [temp.friend]). This is also done to determine whether a function template specialization matches a placement operator new (6.7.5.5.3 [basic.stc.dynamic.deallocation], 7.6.2.8 [expr.new]). In all these cases, P is the type of the function template being considered as a potential match and A is the function type from the declaration. The deduction is done as described in 13.10.3.6 [temp.deduct.type].

    If, for the set of function templates so considered, there is either no match or more than one match after partial ordering has been considered (13.7.7.3 [temp.func.order]), deduction fails and the declaration is ill-formed.

(Note that the resolution of issue 674 depends on this resolution.)

Date: 2010-08-23.00:00:00

13.10.3.2 [temp.deduct.call] paragraph 3 gives the deduction of rvalue references special treatment in the context of a function call:

If P is of the form T&&, where T is a template parameter, and the argument is an lvalue, the type A& is used in place of A for type deduction.

A similar provision is needed, but is not present, in declarative contexts. For example:

    template<typename T> void f(T&&);
    template<> void f(int&) { }    // #1
    template<> void f(int&&) { }   // #2
    void g(int i) {
        f(i);    // calls f<int&>(int&), i.e., #1
        f(0);    // calls f<int>(int&&), i.e., #2
    }

There need to be rules that deduce the template arguments for the specializations in the same way that the arguments are deduced in the calls.

History
Date User Action Args
2014-03-03 00:00:00adminsetstatus: fdis -> c++11
2011-04-10 00:00:00adminsetstatus: wp -> fdis
2010-11-29 00:00:00adminsetstatus: dr -> wp
2010-08-23 00:00:00adminsetmessages: + msg2935
2010-08-23 00:00:00adminsetmessages: + msg2934
2010-08-23 00:00:00adminsetstatus: ready -> dr
2010-03-29 00:00:00adminsetstatus: review -> ready
2010-02-16 00:00:00adminsetstatus: drafting -> review
2009-08-03 00:00:00adminsetstatus: open -> drafting
2009-04-16 00:00:00admincreate