C++ Primer Plus

Let’s look at a technique for giving object operations a prettier look. Operator overloading is an example of C++ polymorphism. In Chapter 8, “Adventures in Functions,” you saw how C++ enables you to define several functions that have the same name, provided that they have different signatures (argument lists). That is called function overloading, or functional polymorphism. Its purpose is to let you use the same function name for the same basic operation, even though you apply the operation to different data types. (Imagine how awkward English would be if you had to use a different verb form for each different type of object—for example, lift_lft your left foot, but lift_sp your spoon.) Operator overloading extends the overloading concept to operators, letting you assign multiple meanings to C++ operators. Actually, many C++ (and C) operators already are overloaded. For example, the * operator, when applied to an address, yields the value stored at that address. But applying * to two numbers yields the product of the values. C++ uses the number and type of operands to decide which action to take.

C++ lets you extend operator overloading to user-defined types, permitting you, say, to use the + symbol to add two objects. Again, the compiler uses the number and type of operands to determine which definition of addition to use. Overloaded operators can often make code look more natural. For example, a common computing task is adding two arrays. Usually, this winds up looking like the following for loop:

for (int i = 0; i < 20; i++)

       evening[i] = sam[i] + janet[i];  // add element by element

But in C++, you can define a class that represents arrays and that overloads the + operator so that you can do this:

evening = sam + janet;                 // add two array objects

This simple addition notation conceals the mechanics and emphasizes what is essential, and that is another goal of OOP.

To overload an operator, you use a special function form called an operator function. An operator function has the following form, where op is the symbol for the operator being overloaded:

operatorop(argument-list)

For example, operator+() overloads the + operator and operator*() overloads the * operator. The op has to be a valid C++ operator; you can’t just make up a new symbol. For example, you can’t have an operator@() function because C++ has no @ operator. But the operator[]() function would overload the [] operator because [] is the array-indexing operator. Suppose, for example, that you have a Salesperson class for which you define an operator+() member function to overload the + operator so that it adds sales figures of one salesperson object to another. Then, if district2, sid, and sara are all objects of the Salesperson class, you can write this equation:

district2 = sid + sara;

The compiler, recognizing the operands as belonging to the Salesperson class, replaces the operator with the corresponding operator function:

district2 = sid.operator+(sara);

The function then uses the sid object implicitly (because it invoked the method) and the sara object explicitly (because it’s passed as an argument) to calculate the sum, which it then returns. Of course, the nice part is that you can use the nifty + operator notation instead of the clunky function notation.

C++ imposes some restrictions on operator overloading, but they’re easiest to understand after you’ve seen how overloading works. So let’s develop a few examples to clarify the process and then discuss the limitations.

Time on Our Hands: Developing an Operator Overloading Example

If you worked on the Priggs account for 2 hours 35 minutes in the morning and 2 hours 40 minutes in the afternoon, how long did you work altogether on the account? Here’s an example where the concept of addition makes sense, but the units that you are adding (a mixture of hours and minutes) don’t match a built-in type. Chapter 7, “Functions: C++’s Programming Modules,” handles a similar case by defining a travel_time structure and a sum() function for adding such structures. Now let’s generalize that to a Time class, using a method to handle addition. Let’s begin with an ordinary method, called Sum(), and then see how to convert it to an overloaded operator. Listing 11.1 shows the class declaration.

Listing 11.1. mytime0.h

// mytime0.h -- Time class before operator overloading

#ifndef MYTIME0_H_

#define MYTIME0_H_

class Time

{

private:

    int hours;

    int minutes;

public:

    Time();

    Time(int h, int m = 0);

    void AddMin(int m);

    void AddHr(int h);

    void Reset(int h = 0, int m = 0);

    Time Sum(const Time & t) const;

    void Show() const;

};

#endif