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


2721. When exactly is storage reused?

Section: 6.7.3  [basic.life]     Status: DRWP     Submitter: Richard Smith     Date: 2023-03-23

[Accepted as a DR at the June, 2023 meeting.]

Subclause 6.7.3 [basic.life] bullet 1.5 specifies:

The lifetime of an object o of type T ends when:

Consider the expression new (p) T(x). Does the lifetime of *p end when p is returned from the allocation function, before x is evaluated? Or does the lifetime end when the constructor body starts executing, after x is evaluated?

The second option is conceivable for initialization by constructor and non-class types; for aggregate initialization, the first option must be used, because evaluation of x directly initializes a part of the resulting object. The first option is simpler to implement for constant evaluation.

Proposed resolution (approved by CWG 2023-05-12):

Change in 6.7.3 [basic.life] paragraph 1 as follows:

The lifetime of an object o of type T ends when: When evaluating a new-expression, storage is considered reused after it is returned from the allocation function, but before the evaluation of the new-initializer (7.6.2.8 [expr.new]). [ Example:
  struct S {
    int m;
  };

  void f() {
    S x{1};
    new(&x) S(x.m);  // undefined behavior
  }
-- end example ]