This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 118b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-09-28

3063. Lifetime extension of temporaries past function return

(From submission #760.)

Consider:

  struct B { ~B(); };
  struct A { const B &b; };
  A foo() { return {{}}; }   // #1
  void bar();
  int main() {
    A a = foo();
    bar();
  }

At #1, a temporary of type B is created and, due to guaranteed copy elision, is bound to a.b in main. The current rules, as amended by P2748R5 (Disallow Binding a Returned Glvalue to a Temporary), prescribe lifetime extension of that temporary, which is a novel requirement accidentally imposed by P2748R5 that is hard to implement.

Suggested resolution (Option 1):

Add in 6.8.7 [class.temporary] bullet 6.11 as follows:

• ...
• A temporary object bound to a reference element of an aggregate of class type initialized from a parenthesized expression-list (9.5 [dcl.init]) persists until the completion of the full-expression containing the expression-list.
• The lifetime of a temporary bound to a constituent reference (7.7 [expr.const]) of the returned object in a function return statement (8.8.4 [stmt.return]) is not extended; the temporary is destroyed at the end of the full-expression in the return statement.
• A temporary bound to a reference in a new-initializer (7.6.2.8 [expr.new]) persists until ...

Suggested resolution (Option 2):

1. Move to before 6.8.2 [intro.object] paragraph 11 from 7.7 [expr.const] paragraph 2:

   The constituent values of an object o are

   • if o has scalar type, the value of o;
   • otherwise, the constituent values of any direct subobjects of o other than inactive union members.

   The constituent references of an object o are

   • any direct members of o that have reference type, and
   • the constituent references of any direct subobjects of o other than inactive union members.

   Some operations are described as implicitly creating objects ...

2. Remove 7.7 [expr.const] paragraph 2 and change paragraph 3 as follows:

   The constituent values of an object o are

   • if o has scalar type, the value of o;
   • otherwise, the constituent values of any direct subobjects of o other than inactive union members.

   The constituent references of an object o are

   • any direct members of o that have reference type, and
   • the constituent references of any direct subobjects of o other than inactive union members.

   The constituent values and constituent references of a variable x are defined as follows:

   • If x declares an object, the constituent values and references of that object (6.8.2 [intro.object]) are constituent values and references of x.
   • If x declares a reference, that reference is a constituent reference of x.

   ...

3. Add in 8.8.4 [stmt.return] paragraph 6 as follows:

   In a function whose return type is a reference, other than an invented function for std::is_convertible (21.3.8 [meta.rel]), a return statement that binds

   • the a returned reference or
   • a constituent reference (6.8.2 [intro.object]) of a returned object

   to a temporary expression (6.8.7 [class.temporary]) is ill-formed. [ Example: ... ]