This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

2312. Structured bindings and mutable

[Accepted at the November, 2020 meeting.]

An example like the following is currently ill-formed:

  struct A { mutable int n; };
  void f() {
    const auto [a] = A();
    a = 0;
  }

According to 9.6 [dcl.struct.bind] paragraph 4, the type of a is const int, since the implicitly-declared variable is const. This seems obviously wrong: the member n is mutable, so the member access expression e.n has type int, which should also be the type of a. (mutable should presumably be taken into account when forming the referenced type too, so that decltype(a) is int as would presumably be expected, rather than const int.)

Proposed resolution, March, 2018: [SUPERSEDED]

Change 9.6 [dcl.struct.bind] paragraph 4 as follows:

...Designating the non-static data members of E as m₀, m₁, m₂, ... (in declaration order), each v_i is the name of an lvalue that refers to the member m_i of e and whose type is cv T_i, where T_i is the declared type of that member e.m_i; the referenced type is cv T_i the type of e.m_i. The lvalue is a bit-field if...

Notes from the June, 2018 meeting:

It was observed that this resolution does not handle members with reference type correctly. The main problem seems to be the statement in 7.6.1.5 [expr.ref] paragraph 4, which directly handles members with reference type rather than allowing the type of the member to be the result type and relying on the general rule that turns reference-typed expressions into lvalues.

Proposed resolution (April, 2020):

Change 9.6 [dcl.struct.bind] paragraph 5 as follows:

...Designating the non-static data members of E as m₀, m₁, m₂, ... (in declaration order), each v_i is the name of an lvalue that refers to the member m_i of e and whose type is cv T_i, where T_i is the declared type of that member that of e.m_i (7.6.1.5 [expr.ref]); the referenced type is cv T_i the declared type of m_i if that type is a reference type, or the type of e.m_i otherwise. The lvalue is a bit-field if that member is a bit-field.

[Example 2:

  struct S { mutable int x1 : 2; volatile double y1; };
  S f();
  const auto [ x, y ] = f();

The type of the id-expression x is “const int”, the type of the id-expression y is “const volatile double”. —end example]