What is the concept of a friend class in C++?

In C++, a friend class refers to a class that can access the private and protected members of another class. By declaring one class as a friend class of another, the member functions in the friend class are able to directly access the private and protected members of the accessed class without needing to go through the public interface.

The declaration of a friend class is typically done within the definition of a class, using the “friend class” keyword at the beginning or end of the class. The member functions of a friend class can directly access the private and protected members of the class being accessed. However, the friend class itself is not considered a member of the class being accessed, so it cannot inherit the members of the class being accessed.

Using friend classes can increase the flexibility and readability of code between classes, but it may also lead to increased coupling. Therefore, when designing relationships between classes, it is important to carefully use friend classes to balance flexibility and maintainability of the code.