Title
Conversion to function pointer for lambda with explicit object parameter
Status
open
Section
7.5.5.2 [expr.prim.lambda.closure]
Submitter
Barry Revzin

Created on 2022-02-14.00:00:00 last changed 1 month ago

Messages

Date: 2022-04-02.13:19:03

Suggested resolution:

  1. Change in 7.5.5.2 [expr.prim.lambda.closure] paragraph 8 as follows:

    ... The value returned by this conversion function is
    • for a lambda-expression whose parameter-declaration-clause has an explicit object parameter, the address of the function call operator (7.6.2.2 [expr.unary.op];
    • otherwise, the address of a function F that, when invoked, has the same effect as invoking the closure type's function call operator on a default-constructed instance of the closure type.
    F is a constexpr function if... is an immediate function.

    [ Example:

      struct C {
        C(auto) { }
      };
    
      void foo() {
        auto a = [](C) { return 0; };
        int (*fp)(C) = a;   // OK
        fp(1);              // same effect as decltype(a){}(1)
        auto b = [](this C) { return 1; };
        fp = b;             // OK
        fp(1);              // same effect as (&decltype(b)::operator())(1)
      }
    

    -- end example ]

  2. Change in 7.5.5.2 [expr.prim.lambda.closure] paragraph 11 as follows:

    The value returned by any given specialization of this conversion function template is
    • for a lambda-expression whose parameter-declaration-clause has an explicit object parameter, the address of the corresponding function call operator template specialization (7.6.2.2 [expr.unary.op]);
    • otherwise, the address of a function F that, when invoked, has the same effect as invoking the generic lambda's corresponding function call operator template specialization on a default-constructed instance of the closure type.
    F is a constexpr function if...
Date: 2021-10-15.00:00:00

P0847R7 (Deducing this) (approved October, 2021) added explicit-object member functions. Consider:

  struct C {
    C(auto) { }
  };

  void foo() {
    auto l = [](this C) { return 1; };
    void (*fp)(C) = l;
    fp(1); // same effect as decltype(l){}() or decltype(l){}(1) ?
  }

Subclause 7.5.5.2 [expr.prim.lambda.closure] paragraph 8 does not address explicit object member functions:

The closure type for a non-generic lambda-expression with no lambda-capture whose constraints (if any) are satisfied has a conversion function to pointer to function with C++ language linkage (9.11 [dcl.link]) having the same parameter and return types as the closure type's function call operator. The conversion is to “pointer to noexcept function” if the function call operator has a non-throwing exception specification. The value returned by this conversion function is the address of a function F that, when invoked, has the same effect as invoking the closure type's function call operator on a default-constructed instance of the closure type. F is a constexpr function if...
History
Date User Action Args
2022-04-02 13:19:03adminsetmessages: + msg6786
2022-02-14 00:00:00admincreate