friend function « Class « C++ Tutorial

9.22.friend function
9.22.1.A friend function
9.22.2.Friend functions can be shared by two or more classes
9.22.3.A function can be a member of one class and a friend of another
9.22.4.Share friend function between classes
9.22.5.friend square() function for Distance
9.22.6.Friend functions and operator overloading
9.22.7.Friend functions can access private members of a class