The Waite Group's C++ Primer Plus, Third Edition

Friends

Several examples in this book have used friend functions as part of the extended interface for a class. Such functions are not the only kinds of friends a class can have. A class also can be a friend. In that case, any method of the friend class can access private and protected members of the original class. Also, you can be more restrictive and designate just particular member functions of a class to be friends to another class. A class defines which functions, member functions, or classes are friends; friendship cannot be imposed from the outside. Thus, although friends do grant outside access to a class's private portion, they don't really violate the spirit of object-oriented programming. Instead, they provide more flexibility to ...