CIS 22 - Data Structures: Operator Overloading - Comments on Example

1. Prefix and Postfix Versions of Operator ++.

airtime operator++ ()            // prefix version
airtime operator++ (int)        // postfix version

    You do not actually supply a value for this dummy argument; the mere inclusion of the int is enough to let the compiler know it's postfix.

    The postfix function operator++(int) operates differently from the prefix function operator++(). The postfix version must remember the original value of its object, increment the object, and then return the original value, not the new value. This is accomplished by creating a temporary object, temp, that is initialized to the original value of the object. After the object is incremented, temp, is returned.

2. Overloading the << and >> operators for cout and cin.

    The operators take two arguments, both passed by reference. The first is an object of isteam (for >>; often this is cin) or of ostream (for <<; often this is cout). The second argument is the object to be displayed, an object of class airtime in this example. The fact that the stream and airtime object are passed by reference allows them to be modified by the function. The >> operator takes input from the stream specified in the first argument and copies it into the member data of the object specified by the second argument. The << operator copies the data from the object specified by the second argument and sends it into the stream specified by the first argument.
    The operator << () and operator >>() functions must be friends of the airtime class, because the istream and ostream objects appear on the left side of the operator.  They return, by reference, an object of istream
(for >>) or ostream (for <<). These return values permit changing so that more than one value can be input or output in a single statement.

Michael Gorenburg - Spring '98