10.1. Introduction

This chapter shows how to enable C++’s operators to work with class objects—a process called operator overloading. One example of an overloaded operator built into C++ is <<, which is used both as the stream insertion operator and as the bitwise left-shift operator (which is discussed in Chapter 20). Similarly, >> also is overloaded; it’s used both as the stream extraction operator and the bitwise right-shift operator. Both of these operators are overloaded in the C++ Standard Library. You’ve been using overloaded operators since early in the book. The overloads are built into the base C++ language itself. For example, C++ overloads the addition operator (+) and the subtraction operator (-) to perform differently, depending on their context in integer, floating-point and pointer arithmetic with data of fundamental types.

You can overload most operators to be used with class objects—the compiler generates the appropriate code based on the types of the operands. The jobs performed by overloaded operators also can be performed by explicit function calls, but operator notation is often more natural.

Our examples start by demonstrating the C++ Standard Library’s class string, which has lots of overloaded operators. This enables you to see overloaded operators in use before implementing your own overloaded operators. Next, we create a PhoneNumber class that enables us to use overloaded operators << and >> to conveniently output and input fully formatted, 10-digit phone numbers. We then present a Date class that overloads the prefix and postfix increment (++) operators to add one day to the value of a Date. The class also overloads the += operator to allow a program to increment a Date by the number of days specified on the right side of the operator.

Next, we present a capstone case study—an Array class that uses overloaded operators and other capabilities to solve various problems with pointer-based arrays. This is one of the most important case studies in the book. Many of our readers have indicated that the Array case study is their “light bulb moment” in truly understanding what classes and object technology are all about. As part of this class, we’ll overload stream insertion, stream extraction, assignment, equality, relational and subscript operators. Once you master this Array class, you’ll indeed understand the essence of object technology—crafting, using and reusing valuable classes.

The chapter concludes with discussions of how you can convert between types (incuding class types), problems with certain implicit conversions and how to prevent those problems.