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

2024-09-25

1696. Temporary lifetime and non-static data member initializers

[Moved to DR at the November, 2014 meeting.]

Presumably a temporary bound to a reference in a non-static data member initializer should be treated analogously with what happens in a ctor-initializer, but the current wording of 6.7.7 [class.temporary] paragraph 5 is not clear on this point.

See also issue 1815 for similar questions regarding aggregate initialization.

Proposed resolution (June, 2014):

1. Add the following after 9.4.2 [dcl.init.aggr] paragraph 7:

If a reference member is initialized from its brace-or-equal-initializer and a potentially-evaluated subexpression thereof is an aggregate initialization that would use that brace-or-equal-initializer, the program is ill-formed. [Example:

  struct A;
  extern A a;
  struct A {
    const A& a1 { A{a,a} };   // OK
    const A& a2 { A{} };      // error
  };
  A a{a,a};                   // OK

If an aggregate class C contains a subaggregate...

2. Delete the first bullet of 6.7.7 [class.temporary] paragraph 5:

The second context is when a reference is bound to a temporary.¹¹⁷ The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference except:

• A temporary bound to a reference member in a constructor's ctor-initializer (11.9.3 [class.base.init]) persists until the constructor exits.

• ...

3. Insert the following as a new paragraph after 11.9.3 [class.base.init] paragraph 7:

A temporary expression bound to a reference member in a mem-initializer is ill-formed. [Example:

  struct A {
    A() : v(42) { }  // error
    const int& v;
  };

—end example]

In a non-delegating constructor, if a given potentially constructed subobject...

4. Insert the following as a new paragraph after 11.9.3 [class.base.init] paragraph 9:

A temporary expression bound to a reference member from a brace-or-equal-initializer is ill-formed. [Example:

  struct A {
    A() = default;          // OK
    A(int v) : v(v) { }     // OK
    const int& v = 42;      // OK
  };
  A a1;                     // error: ill-formed binding of temporary to reference
  A a2(1);                  // OK, unfortunately

—end example]

In a non-delegating constructor, the destructor for each potentially constructed subobject...

This resolution also resolves issue 1815.