Friend functions
Programming in C++ language - Concepts

In C++, the unit of protection is the class, not the object. This means that a member function of a class can access all the private members of any object in its class. On the other hand, these private members remain inaccessible to any member function of another class or to any independent function.

The notion of "friendly function", or more precisely "friendship declaration", allows you to declare in a class the functions that you authorize to access its private members (data or functions). There are several friendship situations.