An example of friend function and friend class declaration

Note:
A friend is a normal function or class that is defined outside a class, but it needs to be described in the class, and in order to distinguish it from the member functions of that class, the keyword friend is preceded by the description. A friend is not a member function, but it can access a private member in a class. Therefore, a friend declaration can only appear in the class definition. Because the friend is not a member of the authorization class, it is not affected by the public private and protected of the declaration area of the class in which it resides. A declaration of a friend function can appear anywhere in the class

Class A{friend class a_friend_class;friend void a_friend_function (void);}; Class A_friend_class{};void a_friend_function (void) {}

An example of friend function and friend class declaration