friend (C++) 

In some circumstances, it is more convenient to grant member-level access to functions that are not members of a class or to all functions in a separate class. The friend keyword allows a function or class to gain access to the private and protected members of a class. You can declare friend functions or friend classes to access not only public members but also protected and private class members.

friend class-name;
friend function-declarator;

See Also

Reference

C++ Keywords
Friend Declarations
Defining Friend Functions in Class Declarations