C++ Primer（第5版）- Chapter 7. Classes -003

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.

7.1.1 设计Sales_data类

我们的最终目的是令Sales_data类支持与Sales_item类完全一样的操作集合。Sales_item类有一个名为isbn的成员函数（member function）（参见1.5.2节，第23页），并且支持+,=, +=, <<,>>运算符操作。

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.

我们将在第14章学习如何自定义运算符（operators）。现在，我们先为这些运算定义普通的（命名的named）函数形式。由于在第14.1节（第555页）将要解释的原因，执行加法（do addition）和IO的函数不作为Sales_data的成员。相反地，我们将其定义成普通函数（ordinary functions）。执行复合赋值运算（handles compound assignment）的函数是成员函数。我们的类不需要定义赋值运算，其原因将在7.1.5节（第267页）中介绍。

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

综上所述，Sales_data的接口包含（consists of）以下操作：

• 一个isbn成员函数，用于返回对象的ISBN编号
• 一个combine成员函数，用于将一个Sales_data对象添加到另一个对象上
• 一个名为add的函数，执行两个Sales_data对象的加法
• 一个read函数，将数据从istream读入到Sales_data对象
• 一个print函数，将Sales_data对象的值输出到ostream

Key Concept: Different Kinds of Programming Roles

关键概念：不同的编程角色

Programmers tend to think about the people who will run their applications as users. Similarly a class designer designs and implements a class for users of that class. In this case, the user is a programmer, not the ultimate user of the application.

程序员常把运行其程序的人称作用户（user）。类似地，类的设计者也是为其用户设计并实现一个类的人。在这种情况下，类的用户是程序员，而非应用程序的最终使用者（the ultimate user）。

When we refer to a user, the context makes it clear which kind of user is meant. If we speak of user code or the user of the Sales_data class, we mean a programmer who is using a class. If we speak of the user of the bookstore application, we mean the manager of the store who is running the application.

当我们提及（refer to）用户一词时，语境（context）决定了是指那种类型的用户。如果我们说用户代码（user code）或者Sales_data类的用户，指的是使用此类的程序员。如果我们说书店（bookstore）应用程序的用户，则意指运行该应用程序的书店管理者。