FAQ 19.03 What are some advantages of using friend classes?

Friend classes are useful when a class wants to hide features from users that are needed only by another, tightly coupled class. Compared to making a member public:, it is sometimes better to make the member private:, which eliminates potential misuse by unknown users, and grant friendship status to the tightly cohesive class, thereby keeping implementation details hidden from the rest of the world.

Friend classes also arise when a member function on a class needs to maintain state between calls and when multiple copies of this state must exist. Under these circumstances, the member function becomes a friend class, and the multiple copies of the state become multiple objects of the friend class.