In C++, a friend class is a class that is granted special access privileges to another class. When a class declares another class as its friend, it allows the friend class to access its private and protected members.

Here are some key points about friend classes in C++:

1. Friendship is not mutual: If class A declares class B as its friend, it means that class B can access the private and protected members of class A. However, this does not automatically make class A a friend of class B. Class A needs to explicitly declare class B as its friend as well if it wants access to the private and protected members of class B.
2. Friendship is not inherited: If class A declares class B as its friend, it does not mean that class C, derived from class A, will have access to the private and protected members of class B. Friendship is not inherited, so class C would need to declare class B as its friend separately.
3. Friend functions and friend classes: In addition to friend classes, C++ also allows individual functions to be declared as friends of a class. These functions are called friend functions. Friend classes and friend functions have similar access privileges to the private and protected members of the class they are friends with.

Friend classes are often used in situations where two or more classes need to access each other’s private or protected members in order to achieve a particular functionality or maintain data integrity. However, it is generally recommended to use friendship sparingly and only when necessary, as it can increase coupling between classes and potentially reduce encapsulation.