4.3 Abstractions 抽象

The term abstraction is closely related to the idea of modular design. An abstraction is a simplified view of an entity, which omits unimportant details. Abstractions are useful because they make it easier for us to think about and manipulate complex things.

术语抽象与模块化设计的思想紧密相关。抽象是实体的简化视图，其中省略了不重要的细节。抽象是有用的，因为它们使我们更容易思考和操纵复杂的事物。

In modular programming, each module provides an abstraction in form of its interface. The interface presents a simplified view of the module’s functionality; the details of the implementation are unimportant from the standpoint of the module’s abstraction, so they are omitted from the interface.

在模块化编程中，每个模块以其接口的形式提供抽象。该接口提供了模块功能的简化视图；从模块抽象的角度来看，实现的细节并不重要，因此在接口中将其省略。

In the definition of abstraction, the word “unimportant” is crucial. The more unimportant details that are omitted from an abstraction, the better. However, a detail can only be omitted from an abstraction if it is unimportant. An abstraction can go wrong in two ways. First, it can include details that are not really important; when this happens, it makes the abstraction more complicated than necessary, which increases the cognitive load on developers using the abstraction. The second error is when an abstraction omits details that really are important. This results in obscurity: developers looking only at the abstraction will not have all the information they need to use the abstraction correctly. An abstraction that omits important details is a false abstraction: it might appear simple, but in reality it isn’t. The key to designing abstractions is to understand what is important, and to look for designs that minimize the amount of information that is important.

在抽象的定义中，“无关紧要”一词至关重要。从抽象中忽略的不重要的细节越多越好。但是，如果细节不重要，则只能将其从抽象中省略。抽象可以通过两种方式出错。首先，它可以包含并非真正重要的细节。当这种情况发生时，它会使抽象变得不必要的复杂，从而增加了使用抽象的开发人员的认知负担。第二个错误是抽象忽略了真正重要的细节。这导致模糊不清：仅查看抽象的开发人员将不会获得正确使用抽象所需的全部信息。忽略重要细节的抽象是错误的抽象：它可能看起来很简单，但实际上并非如此。

As an example, consider a file system. The abstraction provided by a file system omits many details, such as the mechanism for choosing which blocks on a storage device to use for the data in a given file. These details are unimportant to users of the file system (as long as the system provides adequate performance). However, some of the details of a file system’s implementation are important to users. Most file systems cache data in main memory, and they may delay writing new data to the storage device in order to improve performance. Some applications, such as databases, need to know exactly when data is written through to storage, so they can ensure that data will be preserved after system crashes. Thus, the rules for flushing data to secondary storage must be visible in the file system’s interface.

例如，考虑一个文件系统。文件系统提供的抽象省略了许多细节，例如用于选择存储设备上的哪些块用于给定文件中的数据的机制。这些详细信息对于文件系统的用户而言并不重要（只要系统提供足够的性能即可）。但是，文件系统实现的一些细节对用户很重要。大多数文件系统将数据缓存在主内存中，并且它们可能会延迟将新数据写入存储设备以提高性能。一些应用程序（例如数据库）需要确切地知道何时将数据写入存储设备，因此它们可以确保在系统崩溃后将保留数据。因此，将数据刷新到辅助存储的规则必须在文件系统的接口中可见。

We depend on abstractions to manage complexity not just in programming, but pervasively in our everyday lives. A microwave oven contains complex electronics to convert alternating current into microwave radiation and distribute that radiation throughout the cooking cavity. Fortunately, users see a much simpler abstraction, consisting of a few buttons to control the timing and intensity of the microwaves. Cars provide a simple abstraction that allows us to drive them without understanding the mechanisms for electrical motors, battery power management, anti-lock brakes, cruise control, and so on.

我们不仅依靠抽象来管理复杂性，而且不仅在编程中，而且在日常生活中无处不在。微波炉包含复杂的电子设备，可将交流电转换为微波辐射并将该辐射分布到整个烹饪腔中。幸运的是，用户看到了一个简单得多的抽象，它由几个按钮控制微波的定时和强度。汽车提供了一种简单的抽象概念，使我们可以在不了解电动机，电池电源管理，防抱死制动，巡航控制等机制的情况下驾驶它们。