How can a friend function common to two different classes be declared in a C++ program?

How can a friend function common to two different classes be declared in a C++ program?

In Object Oriented Programming (OOP's), friend functions are not considered class members; they are considered as normal external functions that are given special access privileges for class data members and member functions. Friends are not in the class scope and are not called using the member-selection operators (. and ->) unless they are members of another class. 

Any data members which is declared private inside a class is not accessible from outside the class. So a function which is not a member or an external class can never access such private data members. But there may be some cases in real life, where a programmer will need access to the private data from non-member functions or from external classes. C++ offers some exceptions in such cases. 

A class in C++ can allow non-member functions and other classes to access its own private data, by making them as friends. This part of C++ tutorial essentially gives two important points. Once a non-member function is declared as a friend, it can access the private data of the class similarly when a class is declared as a friend, the friend class can have access to the private data of the class which made this a friend.

Thanks

Mukesh Rajput