Protected members (C++ only)

A protected nonstatic base class member can be accessed by members and friends of any classes derived from that base class by using one of the following:

• A pointer to a directly or indirectly derived class
• A reference to a directly or indirectly derived class
• An object of a directly or indirectly derived class

If a class is derived privately from a base class, all protected base class members become private members of the derived class.

If you reference a protected nonstatic member x of a base class A in a friend or a member function of a derived class B, you must access x through a pointer to, reference to, or object of a class derived from A. However, if you are accessing x to create a pointer to member, you must qualify x with a nested name specifier that names the derived class B. The following example demonstrates this:

class A {
public:
protected:
  int i;
};


class B : public A {
  friend void f(A*, B*);
  void g(A*);
};

void f(A* pa, B* pb) {
//  pa->i = 1;
  pb->i = 2;

//  int A::* point_i = &A::i;
  int A::* point_i2 = &B::i;
}

void B::g(A* pa) {
//  pa->i = 1;
  i = 2;

//  int A::* point_i = &A::i;
  int A::* point_i2 = &B::i;
}

void h(A* pa, B* pb) {
//  pa->i = 1;
//  pb->i = 2;
}

int main() { }

Class A contains one protected data member, an integer i. Because B derives from A, the members of B have access to the protected member of A. Function f() is a friend of class B:

• The compiler would not allow pa->i = 1 because pa is not a pointer to the derived class B.
• The compiler would not allow int A::* point_i = &A::i because i has not been qualified with the name of the derived class B.

Function g() is a member function of class B. The previous list of remarks about which statements the compiler would and would not allow apply for g() except for the following:

• The compiler allows i = 2 because it is equivalent to this->i = 2.

Function h() cannot access any of the protected members of A because h() is neither a friend or a member of a derived class of A.