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

2677. Replacing union subobjects

The resolution to NB comment US 041 (C++20 CD) does not seem to have fully addressed the original issue, allowing:

  union U { int i, j; };
  U u;
  new (&u) U{.i = 5};
  int k = u.j;  // OK! New U::i transparently replaces existing u.j!

The suggestion is to allow a newly-created complete object to transparently replace any object of the same type in the same storage, except for a potentially-overlapping subobject or a const complete object, and to allow a newly-created subobject to transparently replace only a corresponding subobject of an existing object.

Suggested resolution [SUPERSEDED]:

Change in 6.7.3 [basic.life] paragraph 8 as follows:

... An object o1 is transparently replaceable by an object o2 if:

• o2 is a complete object and:
  • the storage that o2 occupies exactly overlays the storage that o1 occupied, and
  • o1 and o2 are of the same type (ignoring the top-level cv-qualifiers), and
  • o1 is not a const, complete object, and
  • neither o1 nor o2 is not a potentially-overlapping subobject (6.7.2 [intro.object]), and or
• either o1 and o2 are both complete objects, or o1 and o2 are corresponding direct subobjects of objects p1 and p2, respectively, and p1 is transparently replaceable by p2.

Additional notes (February, 2023)

The suggested resolution above does not handle the additional example in issue 2676.

Suggested resolution:

1. Change in 6.7.2 [intro.object] paragraph 2 as follows:

   Objects can contain other objects, called subobjects. A subobject can be a member subobject (11.4 [class.mem]), a base class subobject (11.7 [class.derived]), or an array element. An object that is not a subobject of any other object is called a complete object. If an object is created in storage associated with a member subobject or array element subobject e (which may or may not be within its lifetime), the created object is a subobject of e's containing object if:

   • the lifetime of e's containing object has begun and not ended, and
   • the storage for the new object exactly overlays the storage location associated with e, and
   • e is not a potentially-overlapping subobject, and
   • the new object is of the same type as e (ignoring cv-qualification).

   In this case, e and the created object are corresponding direct subobjects.

2. Change in 6.7.3 [basic.life] paragraph 8 as follows:

   ... An object o1 is transparently replaceable by an object o2 if either

   • o1 and o2 are complete objects for which:
     • o1 is not const,
     • the storage that o2 occupies exactly overlays the storage that o1 occupied, and
     • o1 and o2 are of the same type (ignoring the top-level cv-qualifiers), and or
     • o1 is not a const, complete object, and
     • neither o1 nor o2 is a potentially-overlapping subobject (6.7.2 [intro.object]), and
   • either o1 and o2 are both complete objects, or o1 and o2 are corresponding direct subobjects of objects p1 and p2, respectively, and p1 is transparently replaceable by p2 (6.7.2 [intro.object]) for which:
     • the complete object of o1 is not const or
     • o1 is a mutable member subobject or a subobject thereof.