When a class declares an entire class as its friend, the friendship status is reciprocal. That is, each class's member functions have free access to the other's private members.

Question

Quizplus · Accepted Answer

Friendship in C++ is not reciprocal by default. If class A declares class B as a friend, B has access to A's private members, but A does not automatically get access to B's private members unless B also declares A as a friend.

When a Class Declares an Entire Class as Its Friend