Friendship and Inheritance

Just as friendship is not transitive (§7.3.4, p. 279), friendship is also not inherited. Friends of the base have no special access to members of its derived classes, and friends of a derived class have no special access to the base class:

class Base {
    // added friend declaration; other members as before
    friend class Pal; // Pal has no access to classes derived from Base
};
class Pal {
public:
    int f(Base b) { return b.prot_mem; } // ok: Pal is a friend of Base
    int f2(Sneaky s) { return s.j; } // error: Pal not friend of Sneaky
    // access to a base class is controlled by the base class, even inside a derived object
    int f3(Sneaky s) { return s.prot_mem; } // ok: Pal is a friend
};

The fact that f3 is legal may seem surprising, but it follows directly from the notion that each class controls access to its own members. Pal is a friend of Base, so Pal can access the members of Base objects. That access includes access to Base objects that are embedded in an object of a type derived from Base.

When a class makes another class a friend, it is only that class to which friendship is granted. The base classes of, and classes derived from, the friend have no special access to the befriending class:

// D2 has no access to protected or private members in Base
class D2 : public Pal {
public:
   int mem(Base b)
       { return b.prot_mem; } // error: friendship doesn't inherit
};