CWG Issue 1249 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 118e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-11-05

1249. Cv-qualification of nested lambda capture

Section: 7.5.6.3 [expr.prim.lambda.capture]Status: CD6Submitter: James WidmanDate: 2011-03-02

[Accepted as a DR at the October, 2021 meeting.]

Consider the following example:

void f(int i) {
  auto l1 = [i] {
    auto l2 = [&i] {
      ++i;    // Well-formed?
    };
  };
}

Because the l1 lambda is not marked as mutable, its operator() is const; however, it is not clear from the wording of 7.5.6 [expr.prim.lambda] paragraph 16 whether the captured member of the enclosing lambda is considered const or not.

Proposed resolution (August, 2021):

Change 7.5.6.3 [expr.prim.lambda.capture] paragraph 14 as follows:

If a lambda-expression m2 captures an entity and that entity is captured by an immediately enclosing lambda-expression m1, then m2's capture is transformed as follows:

• if If m1 captures the entity by copy, m2 captures the corresponding non-static data member of m1's closure type; if m1 is not mutable, the non-static data member is considered to be const-qualified.
• if If m1 captures the entity by reference, m2 captures the same entity captured by m1.