Title
Trivial destructor does not imply constant destruction
Status
review
Section
7.7 [expr.const]
Submitter
Jiang An

Created on 2023-07-27.00:00:00 last changed 4 months ago

Messages

Date: 2024-03-04.22:36:01

Proposed resolution (reviewed by CWG 2024-03-01):

Change in 7.7 [expr.const] paragraph 10 as follows:

An object a is said to have constant destruction if:
  • it is not of class type nor (possibly multidimensional) array thereof, or
  • it is of class type or (possibly multidimensional) array thereof, that class type has a constexpr destructor, and for a hypothetical expression E whose only effect is to destroy a, E would be a core constant expression if any invocation of a trivial destructor for a mutable subobject is ignored and if the lifetime of a and its non-mutable subobjects (but not its mutable subobjects) were considered to start within E.
Date: 2024-03-04.22:36:01

Consider:

  struct Foo {};
  struct Bar {
    mutable Foo f;
  };
  constexpr Bar b{};     // #1

In C++20, Foo and Bar are literal types, and #1 is well-formed. In C++23, the requirement for the variable b changed to "constant destruction". However, Bar does not have constant destruction, because its mutable member violates 7.7 [expr.const] bullet 5.17:

  • ...
  • an invocation of a destructor (11.4.7 [class.dtor]) or a function call whose postfix-expression names a pseudo-destructor (7.6.1.3 [expr.call]), in either case for an object whose lifetime did not begin within the evaluation of E;
  • ...

This is because the mutable member is considered to have its lifetime begin outside of E per 7.7 [expr.const] paragraph 9:

An object a is said to have constant destruction if:
  • it is not of class type nor (possibly multidimensional) array thereof, or
  • it is of class type or (possibly multidimensional) array thereof, that class type has a constexpr destructor, and for a hypothetical expression E whose only effect is to destroy a, E would be a core constant expression if the lifetime of a and its non-mutable subobjects (but not its mutable subobjects) were considered to start within E.
History
Date User Action Args
2024-03-04 22:36:01adminsetmessages: + msg7631
2024-03-04 22:36:01adminsetstatus: open -> review
2023-07-27 00:00:00admincreate