This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 113d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-03-20


9. Clarification of access to base class members

Section: 11.8.3  [class.access.base]     Status: CD1     Submitter: unknown     Date: unknown

[Moved to DR at 4/01 meeting.]

11.8.3 [class.access.base] paragraph 4 says:

A base class is said to be accessible if an invented public member of the base class is accessible. If a base class is accessible, one can implicitly convert a pointer to a derived class to a pointer to that base class.
Given the above, is the following well-formed?
    class D;

    class B
    {
     protected:
       int b1;

       friend void foo( D* pd );
    };

    class D : protected B { };

    void foo( D* pd )
    {
       if ( pd->b1 > 0 ); // Is 'b1' accessible?
    }
Can you access the protected member b1 of B in foo? Can you convert a D* to a B* in foo?

1st interpretation:

A public member of B is accessible within foo (since foo is a friend), therefore foo can refer to b1 and convert a D* to a B*.

2nd interpretation:

B is a protected base class of D, and a public member of B is a protected member of D and can only be accessed within members of D and friends of D. Therefore foo cannot refer to b1 and cannot convert a D* to a B*.

(See J16/99-0042 = WG21 N1218.)

Proposed Resolution (04/01):

  1. Add preceding 11.8.3 [class.access.base] paragraph 4:
    A base class B of N is accessible at R, if
    • an invented public member of B would be a public member of N, or
    • R occurs in a member or friend of class N, and an invented public member of B would be a private or protected member of N, or
    • R occurs in a member or friend of a class P derived from N, and an invented public member of B would be a private or protected member of P, or
    • there exists a class S such that B is a base class of S accessible at R and S is a base class of N accessible at R. [Example:
          class B {
          public:
              int m;
          };
      
          class S: private B {
              friend class N;
          };
      
          class N: private S {
              void f() {
      	    B* p = this;  // OK because class S satisfies the
      			// fourth condition above: B is a base
      			// class of N accessible in f() because
      			// B is an accessible base class of S
      			// and S is an accessible base class of N.
              }
          };
      
      end example]
  2. Delete the first sentence of 11.8.3 [class.access.base] paragraph 4:
    A base class is said to be accessible if an invented public member of the base class is accessible.
  3. Replace the last sentence ("A member m is accessible...") by the following:
    A member m is accessible at the point R when named in class N if
    • m as a member of N is public, or
    • m as a member of N is private, and R occurs in a member or friend of class N, or
    • m as a member of N is protected, and R occurs in a member or friend of class N, or in a member or friend of a class P derived from N, where m as a member of P is private or protected, or
    • there exists a base class B of N that is accessible at R, and m is accessible at R when named in class B. [Example:...

The resolution for issue 207 modifies this wording slightly.