🌐 [class.access.nest] - timsong-cpp.github.io

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

The members of an enclosing class have no special access to members of a nested class; the usual access rules ([class.access]) shall be obeyed.

[ Example

class E {
  int x;
  class B { };

  class I {
    B b;                        // OK: E::I can access E::B
    int y;
    void f(E* p, int i) {
      p->x = i;                 // OK: E::I can access E::x
    }
  };

  int g(I* p) {
    return p->y;                // error: I::y is private
  }
};

— end example

]

11 Classes [class]

11.9 Member access control [class.access]

11.9.7 Nested classes [class.access.nest]