C++: Friend Functions

Friend functions are functions that a class permits to use the class's private members. This is commonly done to overload binary operators that have another class as the left operand, eg <<. Another use is to make some function calls more attractive. For example,

   Time t1, t2;
   . . .
   if (t1.compareTo(t2) == 0) . . .

Might be more attractively written as

   if (compareTo(t1, t2) == 0) . . .

Because compareTo needs to examine the private values in t1 and t2, this second form is only possible if the class declares that compareTo is a friend. Another use of friend functions is to permit operators to be commutative. For example x+i normally will mean the same thing as i+x, but if x is an object and i is an int, the first can be written as a member function and the second only as a friend function. A final reason for member functions is efficiency since they can access the private members directly they can avoid some of the costs of going thru functions.

Put friend declarations before public and private

A class doesn't control the scope of friend functions so friend function declarations are usually written at the beginning of a .h file. Public and private don't apply to them.

Example - redefine << to allow Points to use cout

//=== Point.h file =============================
class Point {
    friend ostream& operator<<(ostream& output, const Point p);
    public:
        Point();
        Point(int x1, int y1)
    . . .
 
//=== Point.cpp file ===========================
. . .
ostream& operator<<(ostream& output, const Point p) {
    output << "(" <<  p.x << ", " << p.y <<")";
    return output;  // make sure it can be used for more output
}
. . .