C++ Primer Plus

If you want to overload an operator for a class and you want to use the operator with a nonclass term as the first operand, you can use a friend function to reverse the operand order.

A Common Kind of Friend: Overloading the << Operator

One very useful feature of classes is that you can overload the << operator so that you can use it with cout to display an object’s contents. In some ways, this overloading is a bit trickier than the earlier examples, so we’ll develop it in two steps instead of in one.

Suppose trip is a Time object. To display Time values, we’ve been using Show(). Wouldn’t it be nice, however, if you could do the following?

cout << trip;  // make cout recognize Time class?

You can do this because << is one of the C++ operators that can be overloaded. In fact, it already is heavily overloaded. In its most basic incarnation, the << operator is one of C and C++’s bit manipulation operators; it shifts bits left in a value (see Appendix E). But the ostream class overloads the operator, converting it into an output tool. Recall that cout is an ostream object and that it is smart enough to recognize all the basic C++ types. That’s because the ostream class declaration includes an overloaded operator<<() definition for each of the basic types. That is, one definition uses an int argument, one uses a double argument, and so on. So one way to teach cout to recognize a Time object is to add a new function operator definition to the ostream class declaration. But it’s a dangerous idea to alter the iostream file and mess around with a standard interface. Instead, use the Time class declaration to teach the Time class how to use cout.

The First Version of Overloading <<

To teach the Time class to use cout, you can use a friend function. Why? Because a statement like the following uses two objects, with the ostream class object (cout) first:

cout << trip;

If you use a Time member function to overload <<, the Time object would come first, as it did when you overloaded the * operator with a member function. That means you would have to use the << operator this way:

trip << cout;   // if operator<<() were a Time member function

This would be confusing. But by using a friend function, you can overload the operator this way:

void operator<<(ostream & os, const Time & t)

{

    os << t.hours << " hours, " << t.minutes << " minutes";

}

This lets you use

cout << trip;

to print data in the following format:

4 hours, 23 minutes

Friend or No Friend?

The new Time class declaration makes the operator<<() function a friend function to the Time class. But this function, although not inimical to the ostream class, is not a friend to that class. The operator<<() function takes an ostream argument and a Time argument, so it might seem that this function has to be a friend to both classes. If you look at the code for the function, however, you’ll notice that the function accesses individual members of the Time object but only uses the ostream object as a whole. Because operator<<() accesses private Time object members directly, it has to be a friend to the Time class. But because it does not directly access private ostream object members, the function does not have to be a friend to the ostream class. That’s nice because it means you don’t have to tinker with the ostream definition.

Note that the new operator<<() definition uses the ostream reference os as its first argument. Normally, os refers to the cout object, as it does in the expression cout << trip. But you could use the operator with other ostream objects, in which case os would refer to those objects.

What? You Don’t Know of Any Other ostream Objects?

Another ostream object is cerr, which routes output to the standard error stream, which, by default, is the display. But in Unix, Linux, and the Windows Command Line environment you can redirect the standard error stream to a file. Also recall that Chapter 6, “Branching Statements and Logical Operators,” introduces ofstream objects, which can be used to send output to a file. Through the magic of inheritance (see Chapter 13, “Class Inheritance”), ofstream objects can use ostream methods. Thus you can use the operator<<() definition to write Time data to files as well as to the screen. You just pass a suitably initialized ofstream object instead of cout as the first argument.

The call cout << trip should use the cout object itself, not a copy, so the function passes the object as a reference instead of by value. Thus, the expression cout << trip causes os to be an alias for cout, and the expression cerr << trip causes os to be an alias for cerr. The Time object can be passed by value or by reference because either form makes the object values available to the function. Again, passing by reference uses less memory and time than passing by value.