C++: OOP: Overloading << and >>

Perhaps the most common use of friend functions is overloading << for I/O. This example overloads << (ie, defines a operator<< function) so that Point objects can use cout and <<.

// example usage
Point p;
. . .
cout << p; // not legal without << friend function.

Declare before `public` and `private` in header file

Declare friend functions outside the public and private sections of the header file. Often they are placed first as in the example below.

//=== Point.h file =============================
class Point {
    friend ostream& operator<<(ostream& output, const Point& p);
    public:
        . . .
    private:
        . . .

Definition

The definition of the operator<< function can be in any file. It is not a member function, so it is defined with two explicit operands. The operator<< function must return the value of the left operand (the ostream) so that multiple << operators may be used in the same statement. Note that operator<< for your type can be defined in terms of << on other types, specifically the types of the data members of your class (eg, ints x and y in the Point class).

//=== Point.cpp file ===========================
. . .
ostream& operator<<(ostream& output, const Point& p) {
    output << "(" <<  p.x << ", " << p.y <<")";
    return output;  // for multiple << operators.
}

Notational practice

The following are identical. First using traditional operator notation.

Point p, q;
cout << p << q;

This can be written with parentheses to show the order of evaluation. This makes it clearer that the first operation has a result that is used by the second output operation.

((cout << p) << q);

An equivalent way is to write this using function for << is

operator<<(operator<<(cout, p), q);

Friend functions are not always needed

If there is no need to reference private data members, operator<< doesn't need to be a friend function; it can be a function that isn't associated with the class in any way.

Defining >> for input