Title
Lifetime-extended temporaries in constant expressions
Status
c++20
Section
7.7 [expr.const]
Submitter
Richard Smith

Created on 2015-05-20.00:00:00 last changed 7 months ago

Messages

Date: 2019-10-15.00:00:00

Proposed resolution (October, 2019):

Change 7.7 [expr.const] paragraph 3 as follows:

A variable is usable in constant expressions after its initializing declaration is encountered if it is a constexpr variable, or it is a constant-initialized variable of reference type or of const-qualified integral or enumeration type. An object or reference is usable in constant expressions if it is

  • a variable that is usable in constant expressions, or

  • a template parameter object (13.2 [temp.param]), or

  • a string literal object (5.13.5 [lex.string]), or

  • a temporary object of non-volatile const-qualified literal type whose lifetime is extended (6.7.7 [class.temporary]) to that of a variable that is usable in constant expressions, or

  • a non-mutable subobject or reference member of any of the above, or.

  • a complete temporary object of non-volatile const-qualified integral or enumeration type that is initialized with a constant expression.

This resolution also resolves issue 2439.

Date: 2019-11-15.00:00:00

[Adopted as a DR at the November, 2019 meeting.]

Consider an example like the following:

  typedef const int CI[3];
  constexpr CI &ci = CI{11, 22, 33};
  static_assert(ci[1] == 22, "");

This is ill-formed because the lifetime of the array temporary did not start within the current evaluation. Perhaps we should treat all lifetime-extended temporaries of const-qualified literal type that are initialized by constant expressions as if they are constexpr objects?

History
Date User Action Args
2020-12-15 00:00:00adminsetmessages: + msg6446
2020-12-15 00:00:00adminsetstatus: drafting -> c++20
2015-05-20 00:00:00admincreate