This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-11-11

45. Access to nested classes

[Moved to DR at 4/01 meeting.]

Example:

    #include <iostream.h>

    class C {  // entire body is private
        struct Parent {
            Parent() { cout << "C::Parent::Parent()\n"; }
        };

        struct Derived : Parent {
            Derived() { cout << "C::Derived::Derived()\n"; }
        };

        Derived d;
    };


    int main() {
        C c;      //  Prints message from both nested classes
        return 0;
    }

11.8 [class.access] paragraph 1:

A member of a class can be

• private; that is, its name can be used only by members and friends of the class in which it is declared. [...]

The members of a nested class have no special access to members of an enclosing class, nor to classes or functions that have granted friendship to an enclosing class; the usual access rules (11.8 [class.access] ) shall be obeyed. [...]

From Mike Miller:

I think it is completely legal, by the reasoning given in the (non-normative) 11.8.8 [class.access.nest] paragraph 2. The use of a private nested class as a base of another nested class is explicitly declared to be acceptable there. I think the rationale in the comments in the example ("// OK because of injection of name A in A") presupposes that public members of the base class will be public members in a (publicly-derived) derived class, regardless of the access of the base class, so the constructor invocation should be okay as well.

I can't find anything normative that explicitly says that, though.

(See also papers J16/99-0009 = WG21 N1186, J16/00-0031 = WG21 N1254, and J16/00-0045 = WG21 N1268.)

Proposed Resolution (04/01):

1. Insert the following as a new paragraph following 11.8 [class.access] paragraph 1:

   A member of a class can also access all names as the class of which it is a member. A local class of a member function may access the same names that the member function itself may access. [Footnote: Access permissions are thus transitive and cumulative to nested and local classes.]

2. Delete 11.8 [class.access] paragraph 6.

3. In 11.8.8 [class.access.nest] paragraph 1, change

   The members of a nested class have no special access to members of an enclosing class, nor to classes or functions that have granted friendship to an enclosing class; the usual access rules (11.8 [class.access]) shall be obeyed.

   to

   A nested class is a member and as such has the same access rights as any other member.

   Change

       B b;       // error: E::B is private
   

   to

       B b;       // Okay, E::I can access E::B
   

   Change

       p->x = i;      // error: E::x is private
   

   to

       p->x = i;      // Okay, E::I can access E::x
   

4. Delete 11.8.8 [class.access.nest] paragraph 2.

(This resolution also resolves issues 8 and 10.