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

2273. Inheriting constructors vs implicit default constructor

[Voted into the WP at the July, 2017 meeting.]

In an example like

   struct A { A(int = 0); };
   struct B : A { using A::A; };
   B b0(0); // #1 
   B b;     // #2 

Is #2 valid (presumably calling the constructor inherited from A, or ill-formed due to ambiguity with 's implicit default constructor?

Proposed resolution (May, 2017):

1. Change 9.9 [namespace.udecl] paragraph 16 as follows:

For the purpose of forming a set of candidates during overload resolution, the functions that are introduced by a using-declaration into a derived class are treated as though they were members of the derived class. In particular, the implicit this parameter shall be treated as if it were a pointer to the derived class rather than to the base class. This has no effect on the type of the function, and in all other respects the function remains a member of the base class. Likewise, constructors that are introduced by a using-declaration are treated as though they were constructors of the derived class when looking up the constructors of the derived class (6.5.5.2 [class.qual]) or forming a set of overload candidates (12.2.2.4 [over.match.ctor], 12.2.2.5 [over.match.copy], 12.2.2.8 [over.match.list]). If such a constructor is selected to perform the initialization of an object of class type, all subobjects other than the base class from which the constructor originated are implicitly initialized (11.9.4 [class.inhctor.init]). [Note: A member of a derived class is sometimes preferred to a member of a base class if they would otherwise be ambiguous (12.2.4 [over.match.best]). —end note]

2. Insert the following as a new bullet following 12.2.4 [over.match.best] bullet 1.7:

• ...

• F1 and F2 are function template specializations, and the function template for F1 is more specialized than the template for F2 according to the partial ordering rules described in 13.7.7.3 [temp.func.order], or, if not that,

• F1 is a constructor for a class D, F2 is a constructor for a base class B of D, and for all arguments the corresponding parameters of F1 and F2 have the same type. [Example:

    struct A {
      A(int = 0);
    };
  
    struct B: A {
      using A::A;
      B();
    };
  
    int main() {
      B b;  // OK, B::B()
    }
  

  —end example], or, if not that,

• F1 is generated from a deduction-guide (12.2.2.9 [over.match.class.deduct])...

This resolution also resolves issue 2277.