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

2024-10-26

1733. Return type and value for operator= with ref-qualifier

[Accepted as a DR at the October, 2021 meeting.]

9.5.2 [dcl.fct.def.default] paragraph 1 specifies that an explicitly-defaulted function shall

have the same declared function type (except for possibly differing ref-qualifiers and except that in the case of a copy constructor or copy assignment operator, the parameter type may be “reference to non-const T”, where T is the name of the member function's class) as if it had been implicitly declared...

This allows an example like

  struct A {
    A& operator=(A const&) && = default;
  };

but forbids

  struct B {
    B&& operator=(B const&) && = default;
  };

which seems backward.

In addition, 11.4.5.3 [class.copy.ctor] paragraph 22 only specifies the return value for implicitly-declared copy/move assignment operators, not for explicitly-defaulted ones.

Proposed resolution (August, 2021):

1. Change 11.4.6 [class.copy.assign] paragraph 6 as follows:

The implicitly-declared copy/move assignment operator for class X has the return type X&; it returns the object for which the assignment operator is invoked, that is, the object assigned to. An implicitly-declared copy/move assignment operator is an inline public member of its class.

2. Add the following as a new paragraph following 11.4.6 [class.copy.assign] paragraph 13:

The implicitly-defined copy assignment operator for a union X copies the object representation (6.8 [basic.types]) of X. If the source and destination of the assignment are not the same object, then for each object nested within (6.7.2 [intro.object]) the object that is the source of the copy, a corresponding object o nested within the destination is created, and the lifetime of o begins before the copy is performed.

The implicitly-defined copy/move assignment operator for a class returns the object for which the assignment operator is invoked, that is, the object assigned to.

[Note: The first point in the issue, that of the relationship between the ref-qualifier and the return type, will be referred to EWG for consideration. The draft resolution above addresses only the second point of the issue.