1.5.1. The Sales_item Class

The purpose of the Sales_item class is to represent the total revenue, number of copies sold, and average sales price for a book. How these data are stored or computed is not our concern. To use a class, we need not care about how it is implemented. Instead, what we need to know is what operations objects of that type can perform.

Every class defines a type. The type name is the same as the name of the class. Hence, our Sales_item class defines a type named Sales_item. As with the built-in types, we can define a variable of a class type. When we write

Sales_item item;

we are saying that item is an object of type Sales_item. We often contract the phrase “an object of type Sales_item” to “a Sales_item object” or even more simply to “a Sales_item.”

In addition to being able to define variables of type Sales_item, we can:

• Call a function named isbn to fetch the ISBN from a Sales_item object.

• Use the input (>>) and output (<<) operators to read and write objects of type Sales_item.

• Use the assignment operator (=) to assign one Sales_item object to another.

• Use the addition operator (+) to add two Sales_item objects. The two objects must refer to the same ISBN. The result is a new Sales_item object whose ISBN is that of its operands and whose number sold and revenue are the sum of the corresponding values in its operands.

• Use the compound assignment operator (+=) to add one Sales_item object into another.

Now that we know what operations we can use with Sales_item objects, we can write programs that use the class. For example, the following program reads data from the standard input into a Sales_item object and writes that Sales_item back onto the standard output:

#include <iostream>
#include "Sales_item.h"
int main()
{
    Sales_item book;
    // read ISBN, number of copies sold, and sales price
    std::cin >> book;
    // write ISBN, number of copies sold, total revenue, and average price
    std::cout << book << std::endl;
    return 0;
}

If the input to this program is

0-201-70353-X 4 24.99

then the output will be

0-201-70353-X 4 99.96 24.99

Our input says that we sold four copies of the book at $24.99 each, and the output indicates that the total sold was four, the total revenue was $99.96, and the average price per book was $24.99.

This program starts with two #include directives, one of which uses a new form. Headers from the standard library are enclosed in angle brackets (< >). Those that are not part of the library are enclosed in double quotes (" ").

Inside main we define an object, named book, that we’ll use to hold the data that we read from the standard input. The next statement reads into that object, and the third statement prints it to the standard output followed by printing endl.

Adding Sales_items

A more interesting example adds two Sales_item objects:

#include <iostream>
#include "Sales_item.h"
int main()
{
    Sales_item item1, item2;
    std::cin >> item1 >> item2;   // read a pair of transactions
    std::cout << item1 + item2 << std::endl; // print their sum
    return 0;
}

0-201-78345-X 3 20.00
0-201-78345-X 2 25.00

our output is

0-201-78345-X 5 110 22

This program starts by including the Sales_item and iostream headers. Next we define two Sales_item objects to hold the transactions. We read data into these objects from the standard input. The output expression does the addition and prints the result.

Our program that adds two Sales_items should check whether the objects have the same ISBN. We’ll do so as follows:

#include <iostream>
#include "Sales_item.h"
int main()
{
    Sales_item item1, item2;
    std::cin >> item1 >> item2;
    // first check that item1 and item2 represent the same book
    if (item1.isbn() == item2.isbn()) {
        std::cout << item1 + item2 << std::endl;
        return 0;   // indicate success
    } else {
        std::cerr << "Data must refer to same ISBN"
                  << std::endl;
        return -1;  // indicate failure
    }
}

The difference between this program and the previous version is the if and its associated else branch. Even without understanding the if condition, we know what this program does. If the condition succeeds, then we write the same output as before and return 0, indicating success. If the condition fails, we execute the block following the else, which prints a message and returns an error indicator.

What Is a Member Function?

The if condition

item1.isbn() == item2.isbn()

The right-hand operand of the equality operator executes in the same way—it returns the ISBN stored in item2. If the ISBNs are the same, the condition is true; otherwise it is false.