Movatterモバイル変換

[Moved to DR at the April, 2013 meeting.]

Proposed resolution (February, 2012):

This issue is resolved by the resolution ofissue 616.

6.8.6.5.3 [basic.stc.dynamic.deallocation] paragraph 4 mentions that the effectof using an invalid pointer value is undefined. However, the standardnever says what it means to 'use' a value.

There are a number of possible interpretations, but it appears thateach of them leads to undesired conclusions:

1. A value is 'used' in a program if a variable holding this valueappears in an expression that is evaluated.This interpretation would render the sequence

      int *x = new int(0);   delete x;   x = 0;

   into undefined behaviour. As this is a common idiom, this isclearly undesirable.
2. A value is 'used' if an expression evaluates to that value.This would render the sequence

      int *x = new int(0);   delete x;   x->~int();

   into undefined behaviour; according to _N4778_.7.6.1.4 [expr.pseudo],the variable x is'evaluated' as part of evaluating the pseudo destructor call. This,in turn, would mean that all containers (Clause 23 [containers])of pointers showundefined behaviour, e.g. 23.3.11.4 [list.modifiers]requires to invoke thedestructor as part of theclear() method of the container.

If any other meaning was intended for 'using an expression', thatmeaning should be stated explicitly.

(See alsoissue 623.)