A class can declare external functions or other classes as friends. Friendship grants full access to all of the grantor’s members, even private and protected ones:
void encrypt (string & rep) {/*..*/} //global functionclass spy { public: static void transmit(const string& rep) { /*..*/} //... };class secret { friend class spy;//spy can access all members of 'secret' friend void encrypt(string & rep);//...and so can encrypt private: string report; public: void scramble() { ::encrypt(report); } void transmit() const { spy::transmit(report); }};Notes about friendship:
- A friend declaration exposes implementations details of its class, so it should be used wisely. However, friendship has the advantage of allowing code re-use in a simple manner; in fact, many of the standard functions and overloaded operators are used in standard containers (like string<>) by means of friendship.
- Friendship is not inherited, so non-public members of any class derived from secret are not accessible to spy and encrypt.
