7.1.1. Designing the Sales_data Class

Ultimately, we want Sales_data to support the same set of operations as the Sales_item class. The Sales_item class had one member function (§ 1.5.2, p. 23), named isbn, and supported the +, =, +=, <<, and >> operators.

We’ll learn how to define our own operators in Chapter 14. For now, we’ll define ordinary (named) functions for these operations. For reasons that we will explain in § 14.1 (p. 555), the functions that do addition and IO will not be members of Sales_data. Instead, we’ll define those functions as ordinary functions. The function that handles compound assignment will be a member, and for reasons we’ll explain in § 7.1.5 (p. 267), our class doesn’t need to define assignment.

Thus, the interface to Sales_data consists of the following operations:

• An isbn member function to return the object’s ISBN

• A combine member function to add one Sales_data object into another

• A function named add to add two Sales_data objects

• A read function to read data from an istream into a Sales_data object

• A print function to print the value of a Sales_data object on an ostream

Using the Revised Sales_data Class

Before we think about how to implement our class, let’s look at how we can use our interface functions. As one example, we can use these functions to write a version of the bookstore program from § 1.6 (p. 24) that works with Sales_data objects rather than Sales_items:

Sales_data total;         // variable to hold the running sum
if (read(cin, total))  {  // read the first transaction
    Sales_data trans;     // variable to hold data for the next transaction
    while(read(cin, trans)) {      //  read the remaining transactions
        if (total.isbn() == trans.isbn())   // check the isbns
            total.combine(trans);  // update the running total
        else {
            print(cout, total) << endl;  // print the results
            total = trans;               // process the next book
        }
    }
    print(cout, total) << endl;          // print the last transaction
} else {                                 // there was no input
    cerr << "No data?!" << endl;         // notify the user
}

We start by defining a Sales_data object to hold the running total. Inside the if condition, we call read to read the first transaction into total. This condition works like other loops we’ve written that used the >> operator. Like the >> operator, our read function will return its stream parameter, which the condition checks (§ 4.11.2, p. 162). If the read fails, we fall through to the else to print an error message.

If there are data to read, we define trans, which we’ll use to hold each transaction. The condition in the while also checks the stream returned by read. So long as the input operations in read succeed, the condition succeeds and we have another transaction to process.

Inside the while, we call the isbn members of total and trans to fetch their respective ISBNs. If total and trans refer to the same book, we call combine to add the components of trans into the running total in total. If trans represents a new book, we call print to print the total for the previous book. Because print returns a reference to its stream parameter, we can use the result of print as the left-hand operand of the <<. We do so to print a newline following the output generated by print. We next assign trans to total, thus setting up to process the records for the next book in the file.

After we have exhausted the input, we have to remember to print the data for the last transaction, which we do in the call to print following the while loop.