This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-11-11

2887. Missing compatibility entries for xvalues

[Accepted as a DR at the June, 2024 meeting.]

(From submission #528.)

Paper N3055 (A Taxonomy of Expression Value Categories) introduced xvalues. This changed the behavior of well-formed code for typeid and the conditional operator, but corresponding entries in Annex C are missing.

Proposed resolution (approved by CWG 2024-05-31):

1. Add a new paragraph to C.6.3 [diff.cpp03.expr] as follows:

   Affected subclause: 7.6.1.8 [expr.typeid]
   Change: Evaluation of operands in typeid.
   Rationale: Introduce additional expression value categories.
   Effect on original feature: Valid C++ 2003 code that uses xvalues as operands for typeid may change behavior. For example,
   

     void f() {
       struct B {
         B() {}
         virtual ~B() { }
       };
   
       struct C { B b; };
       typeid(C().b); // unevaluated in C++03, evaluated in C++11
     }
   

2. Add a new paragraph to C.6.3 [diff.cpp03.expr] as follows:

   Affected subclause: 7.6.16 [expr.cond]
   Change: Fewer copies in the conditional operator.
   Rationale: Introduce additional expression value categories.
   Effect on original feature: Valid C++ 2003 code that uses xvalues as operands for the conditional operator may change behavior. For example,
   

     void f() {
       struct B {
         B() {}
         B(const B&) { }
       };
       struct D : B {};
   
       struct BB { B b; };
       struct DD { D d; };
   
       true ? BB().b : DD().d; // additional copy in C++03, no copy or move in C++11
     }