Specify a class member function as a friend of another class?

Question

According to the C++ Primer book, the author mentioned that We can specify a class member function as a friend of another class, instead of the entire class (page 634).

Answer 1

@juanchopanza @ipkiss Regarding the problem that you can't access data members of A inside fB(A& a) because A is not defined yet. Instead of defining it in a separate file and including it, you can just define the function fB(A& a) after the definition of class A so that fB(A& a) is able to see data members of A.