Note
C++ programmers tend to speak of users interchangeably as users of the application or users of a class.
C++程序员们倾向于将用户（可交换地interchangeably）称为应用程序的用户或者类的用户的一个。
In simple applications, the user of a class and the designer of the class might be one and the same person. Even in such cases, it is useful to keep the roles distinct. When we design the interface of a class, we should think about how easy it will be to use the class. When we use the class, we shouldn't think about how the class works.
在一些简单的应用程序中，类的用户和类的设计者常常是同一个人。即使在这种情况下，把角色区分开来是有好处的。当我们设计一个类的接口时，我们应当考虑怎样才能使这个类易于使用。当我们使用这个类时，我们不应该考虑这个类是如何工作的（我们不应该顾及这个类的实现机理）。
Authors of successful applications do a good job of understanding and implementing the needs of the application's users. Similarly, good class designers pay close attention to the needs of the programmers who will use the class. A well-designed class has an interface that is intuitive and easy to use and has an implementation that is efficient enough for its intended use.
要想开发一款成功的应用程序，其作者必须出色地（充分地）理解并实现用户的需求。同理，优秀的类设计者也会密切关注那些可能使用此类的程序员的需求。一个设计良好的类，既要有直观且易于使用的接口，也必须具备高效的实现过程。