Changing the access of a class member (C++ only)

Suppose class B is a direct base class of class A. To restrict access of class B to the members of class A, derive B from A using either the access specifiers protected or private.

To increase the access of a member x of class A inherited from class B, use a using declaration. You cannot restrict the access to x with a using declaration. You may increase the access of the following members:

A member inherited as private. (You cannot increase the access of a member declared as private because a using declaration must have access to the member's name.)
A member either inherited or declared as protected

The following example demonstrates this:

struct A {
protected:
  int y;
public:
  int z;
};

struct B : private A { };

struct C : private A {
public:
  using A::y;
  using A::z;
};

struct D : private A {
protected:
  using A::y;
  using A::z;
};

struct E : D {
  void f() {
    y = 1;
    z = 2;
  }
};

struct F : A {
public:
  using A::y;
private:
  using A::z;
};

int main() {
  B obj_B;
//  obj_B.y = 3;
//  obj_B.z = 4;

  C obj_C;
  obj_C.y = 5;
  obj_C.z = 6;

  D obj_D;
//  obj_D.y = 7;
//  obj_D.z = 8;

  F obj_F;
  obj_F.y = 9;
  obj_F.z = 10;
}

The compiler would not allow the following assignments from the above example:

obj_B.y = 3 and obj_B.z = 4: Members y and z have been inherited as private.
obj_D.y = 7 and obj_D.z = 8: Members y and z have been inherited as private, but their access have been changed to protected.

The compiler allows the following statements from the above example:

y = 1 and z = 2 in D::f(): Members y and z have been inherited as private, but their access have been changed to protected.
obj_C.y = 5 and obj_C.z = 6: Members y and z have been inherited as private, but their access have been changed to public.
obj_F.y = 9: The access of member y has been changed from protected to public.
obj_F.z = 10: The access of member z is still public. The private using declaration using A::z has no effect on the access of z.

Related information

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