14 Member access control [class.access]

14.7 Nested classes [class.access.nest]

1
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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 (Clause [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
 ]