第 12 章为什么写注释？四个理由

Chapter 12 Why Write Comments? The Four Excuses

In-code documentation plays a crucial role in software design. Comments are essential to help developers understand a system and work efficiently, but the role of comments goes beyond this. Documentation also plays an important role in abstraction; without comments, you can’t hide complexity. Finally, the process of writing comments, if done correctly, will actually improve a system’s design. Conversely, a good software design loses much of its value if it is poorly documented.

代码内文档在软件设计中起着至关重要的作用。注释对于帮助开发人员理解系统和有效工作至关重要，但是注释的作用不止于此。文档在抽象中也起着重要作用。没有注释，您就无法隐藏复杂性。最后，编写注释的过程（如果正确完成）实际上会改善系统的设计。相反，如果没有很好的文档记录，那么好的软件设计会失去很多价值。

Unfortunately, this view is not universally shared. A significant fraction of production code contains essentially no comments. Many developers think that comments are a waste of time; others see the value in comments, but somehow never get around to writing them. Fortunately, many development teams recognize the value of documentation, and it feels like the prevalence of these teams is gradually increasing. However, even in teams that encourage documentation, comments are often viewed as drudge work and many developers don’t understand how to write them, so the resulting documentation is often mediocre. Inadequate documentation creates a huge and unnecessary drag on software development.

不幸的是，这种观点并未得到普遍认同。生产代码的很大一部分基本上不包含任何注释。许多开发人员认为注释是浪费时间。其他人则看到了注释中的价值，但不知何故从不动手编写它们。幸运的是，许多开发团队认识到了文档的价值，并且感觉这样的团队越来越普及了。但是，即使在鼓励文档的团队中，注释也经常被视为繁琐的工作，而且许多开发人员也不了解如何编写注释，因此生成的文档通常是平庸的。文档不足会给软件开发带来巨大且不必要的拖累。

In this chapter I will discuss the excuses developers use to avoid writing comments, and the reasons why comments really do matter. Chapter 13 will then describe how to write good comments and the next few chapters after that will discuss related issues such as choosing variable names and how to use documentation to improve a system’s design. I hope these chapters will convince you of three things: good comments can make a big difference in the overall quality of software; it isn’t hard to write good comments; and (this may be hard to believe) writing comments can actually be fun.

在本章中，我将讨论开发人员用来避免写注释的借口，以及注释真正重要的原因。然后，第 13 章将描述如何编写好的注释，其后的几章将讨论相关问题，例如选择变量名以及如何使用文档来改进系统的设计。我希望这些章节能使您相信三件事：好的注释可以对软件的整体质量产生很大的影响；写好注释并不难；并且（可能很难相信）写注释实际上很有趣。

When developers don’t write comments, they usually justify their behavior with one or more of the following excuses:

当开发人员不写注释时，他们通常会以以下一种或多种借口为自己的行为辩护：

“Good code is self-documenting.”
“I don’t have time to write comments.”
“Comments get out of date and become misleading.”
“The comments I have seen are all worthless; why bother?” In the sections below I will address each of these excuses in turn.

“好的代码是自解释的。”
“我没有时间写注释。”
“注释过时，并会产生误导。”
“我所看到的注释都是毫无价值的；何必呢？” 在以下各节中，我将依次讨论这些借口。

12.1 Good code is self-documenting 好的代码是自解释的

Some people believe that if code is written well, it is so obvious that no comments are needed. This is a delicious myth, like a rumor that ice cream is good for your health: we’d really like to believe it! Unfortunately, it’s simply not true. To be sure, there are things you can do when writing code to reduce the need for comments, such as choosing good variable names (see Chapter 14). Nonetheless, there is still a significant amount of design information that can’t be represented in code. For example, only a small part of a class’s interface, such as the signatures of its methods, can be specified formally in the code. The informal aspects of an interface, such as a high-level description of what each method does or the meaning of its result, can only be described in comments. There are many other examples of things that can’t be described in the code, such as the rationale for a particular design decision, or the conditions under which it makes sense to call a particular method.

有人认为，如果代码编写得当，那么显而易见，不需要注释。这是一个美味的神话，就像谣言说冰淇淋对您的健康有益：我们真的很想相信！不幸的是，事实并非如此。可以肯定的是，在编写代码时可以做一些事情来减少对注释的需求，例如选择好的变量名（请参阅第 14 章）。尽管如此，仍有大量设计信息无法用代码表示。例如，只能在代码中正式指定类接口的一小部分，例如其方法的签名。接口的非正式方面，例如对每种方法的作用或其结果含义的高级描述，只能在注释中描述。还有许多代码中无法描述的东西，比如特定设计决策的基本原理，或者调用特定方法的条件。

Some developers argue that if others want to know what a method does, they should just read the code of the method: this will be more accurate than any comment. It’s possible that a reader could deduce the abstract interface of the method by reading its code, but it would be time-consuming and painful. In addition, if you write code with the expectation that users will read method implementations, you will try to make each method as short as possible, so that it’s easy to read. If the method does anything nontrivial, you will break it up into several smaller methods. This will result in a large number of shallow methods. Furthermore, it doesn’t really make the code easier to read: in order to understand the behavior of the top-level method, readers will probably need to understand the behaviors of the nested methods. For large systems it isn’t practical for users to read the code to learn the behavior.

一些开发人员认为，如果其他人想知道某个方法的作用，那么他们应该只阅读该方法的代码：这将比任何注释都更准确。读者可能会通过阅读其代码来推断该方法的抽象接口，但这既费时又痛苦。另外，如果在编写代码时期望用户会阅读方法实现，则将尝试使每个方法尽可能短，以便于阅读。如果该方法执行了一些重要操作，则将其分解为几个较小的方法。这将导致大量浅层方法。此外，它并没有真正使代码更易于阅读：为了理解顶层方法的行为，读者可能需要了解嵌套方法的行为。对于大型系统，用户通过阅读代码来学习行为是不切实际的。

Moreover, comments are fundamental to abstractions. Recall from Chapter 4 that the goal of abstractions is to hide complexity: an abstraction is a simplified view of an entity, which preserves essential information but omits details that can safely be ignored. If users must read the code of a method in order to use it, then there is no abstraction: all of the complexity of the method is exposed. Without comments, the only abstraction of a method is its declaration, which specifies its name and the names and types of its arguments and results. The declaration is missing too much essential information to provide a useful abstraction by itself. For example, a method to extract a substring might have two arguments, start and end, indicating the range of characters to extract. From the declaration alone, it isn’t possible to tell whether the extracted substring will include the character indicated by end, or what happens if start > end. Comments allow us to capture the additional information that callers need, thereby completing the simplified view while hiding implementation details. It’s also important that comments are written in a human language such as English; this makes them less precise than code, but it provides more expressive power, so we can create simple, intuitive descriptions. If you want to use abstractions to hide complexity, comments are essential.

此外，注释是抽象的基础。回顾第四章，抽象的目的是隐藏复杂性：抽象是实体的概览，它保留必要的信息，但忽略了可以放心地忽略的细节。如果用户必须阅读方法的代码才能使用它，则没有任何抽象可言：方法的所有复杂性都将暴露出来。没有注释，方法的唯一抽象就是其声明，该声明指定其名称以及其参数和结果的名称和类型。该声明缺少太多基本信息，无法单独提供有用的抽象。例如，提取子字符串的方法可能有两个参数，开始和结束，表示要提取的字符范围。仅凭声明，无法确定提取的子字符串是否将包含 end 指示的字符，或者如果 start > end 会发生什么。注释使我们能够捕获调用者所需的其他信息，从而在隐藏实现细节的同时完成简化的视图。用人类语言（例如英语）写注释也很重要；这使它们不如代码精确，但提供了更多的表达能力，因此我们可以创建简单直观的描述。如果要使用抽象来隐藏复杂性，则注释必不可少。

12.2 I don’t have time to write comments 我没有时间写注释

It’s tempting to prioritize comments lower than other development tasks. Given a choice between adding a new feature and documenting an existing feature, it seems logical to choose the new feature. However, software projects are almost always under time pressure, and there will always be things that seem higher priority than writing comments. Thus, if you allow documentation to be de-prioritized, you’ll end up with no documentation.

与其他开发任务相比，将注释的优先级降低是很诱人的。如果要在添加新功能和为现有功能写注释之间做出选择的话，选择新功能似乎合乎逻辑。但是，软件项目几乎总是处于时间压力之下，并且总会有比编写注释优先级更高的事情。因此，如果您允许取消文档的优先级，则最终将没有文档。

The counter-argument to this excuse is the investment mindset discussed on page 15. If you want a clean software structure, which will allow you to work efficiently over the long-term, then you must take some extra time up front in order to create that structure. Good comments make a huge difference in the maintainability of software, so the effort spent on them will pay for itself quickly. Furthermore, writing comments needn’t take a lot of time. Ask yourself how much of your development time you spend typing in code (as opposed to designing, compiling, testing, etc.), assuming you don’t include any comments; I doubt that the answer is more than 10%. Now suppose that you spend as much time typing comments as typing code; this should be a safe upper bound. With these assumptions, writing good comments won’t add more than about 10% to your development time. The benefits of having good documentation will quickly offset this cost.

反驳该借口的是第 15 页上讨论的投资心态。如果您想要一个干净的软件结构，可以长期有效地工作，那么您必须花一些额外的时间才能创建该结构。好的注释对软件的可维护性有很大的影响，因此花费在它们上面的精力将很快收回成本。此外，撰写注释不需要花费很多时间。询问自己，假设您不包含任何注释，那么您花费了多少开发时间来写代码（与设计，编译，测试等相比）。我怀疑答案是否超过 10％。现在假设您花在写注释上的时间与写代码所花费的时间一样多。这应该是一个安全的上限。基于这些假设，撰写好的注释不会增加您的开发时间约 10％。拥有良好文档的好处将迅速抵消这一成本。

Furthermore, many of the most important comments are those related to abstractions, such as the top-level documentation for classes and methods. Chapter 15 will argue that these comments should be written as part of the design process, and that the act of writing the documentation serves as an important design tool that improves the overall design. These comments pay for themselves immediately.

此外，许多最重要的注释是与抽象有关的，例如类和方法的顶级文档。第 15 章认为，这些评论应该作为设计过程的一部分来写，并且写文档的行为作为一个重要的设计工具来改进整体设计。这些评论马上就有回报了。

12.3 Comments get out of date and become misleading 注释过时并产生误导

Comments do sometimes get out of date, but this need not be a major problem in practice. Keeping documentation up-to-date does not require an enormous effort. Large changes to the documentation are only required if there have been large changes to the code, and the code changes will take more time than the documentation changes. Chapter 16 discusses how to organize documentation so that it is as easy as possible to keep it updated after code modifications (the key ideas are to avoid duplicated documentation and keep the documentation close to the corresponding code). Code reviews provide a great mechanism for detecting and fixing stale comments.

注释有时确实会过时，但这实践上并不是主要问题。使文档保持最新状态并不需要付出巨大的努力。仅当对代码进行了较大的更改时才需要对文档进行大的更改，并且代码更改将比文档的更改花费更多的时间。第 16 章讨论了如何组织文档，以便在修改代码后尽可能容易地对其进行更新（关键的思想是避免重复的文档，并保持文档接近相应的代码）。代码审查提供了一种检测和修复陈旧注释的强大机制。

12.4 All the comments I have seen are worthless 我所看到的所有注释都是毫无价值的

Of the four excuses, this is probably the one with the most merit. Every software developer has seen comments that provide no useful information, and most existing documentation is so-so at best. Fortunately, this problem is solvable; writing solid documentation is not hard, once you know how. The next chapters will lay out a framework for how to write good documentation and maintain it over time.

在这四个借口中，这可能是最有价值的借口。每个软件开发人员都看到过没有提供有用信息的注释，并且大多数现有文档充其量都是这样。幸运的是，这个问题是可以解决的。一旦你知道怎么做，写可靠的文档并不难。下一章将为如何编写良好的文档并随时间进行维护提供一个框架。

12.5 Benefits of well-written comments

Now that I have discussed (and, hopefully, debunked) the arguments against writing comments, let’s consider the benefits that you will get from good comments. The overall idea behind comments is to capture information that was in the mind of the designer but couldn’t be represented in the code. This information ranges from low-level details, such as a hardware quirk that motivates a particularly tricky piece of code, up to high-level concepts such as the rationale for a class. When other developers come along later to make modifications, the comments will allow them to work more quickly and accurately. Without documentation, future developers will have to rederive or guess at the developer’s original knowledge; this will take additional time, and there is a risk of bugs if the new developer misunderstands the original designer’s intentions. Comments are valuable even when the original designer is the one making the changes: if it has been more than a few weeks since you last worked in a piece of code, you will have forgotten many of the details of the original design.

既然我已经讨论了（并希望揭穿了这些）反对撰写注释的论点，让我们考虑一下从良好注释中将获得的好处。注释背后的总体思想是捕获设计者所想的但不能在代码中表示的信息。这些信息从低级详细信息（例如，激发特殊代码的硬件怪癖）到高级概念（例如，类的基本原理）。当其他开发人员稍后进行修改时，这些注释将使他们能够更快，更准确地工作。没有文档，未来的开发人员将不得不重新编写或猜测开发人员的原始知识。这将花费额外的时间，并且如果新开发者误解了原始设计者的意图，则存在错误的风险。即使是原作者在修改代码时注释也是有价值的：如果距离你最后一次在一段代码中工作已经有几个星期了，你会忘记最初设计的许多细节。

Chapter 2 described three ways in which complexity manifests itself in software systems:

第 2 章介绍了在软件系统中表现出复杂性的三种方式：

Change amplification: a seemingly simple change requires code modifications in many places.
Cognitive load: in order to make a change, the developer must accumulate a large amount of information.
Unknown unknowns: it is unclear what code needs to be modified, or what information must be considered in order to make those modifications.

变更放大：看似简单的变更需要在许多地方进行代码修改。
认知负荷：为了进行更改，开发人员必须积累大量信息。
意料之外的或不可预见的情况：尚不清楚需要修改哪些代码，或必须考虑哪些信息才能进行这些修改。

Good documentation helps with the last two of these issues. Documentation can reduce cognitive load by providing developers with the information they need to make changes and by making it easy for developers to ignore information that is irrelevant. Without adequate documentation, developers may have to read large amounts of code to reconstruct what was in the designer’s mind. Documentation can also reduce the unknown unknowns by clarifying the structure of the system, so that it is clear what information and code is relevant for any given change.

好的文档可以帮助解决最后两个问题。通过为开发人员提供他们进行更改所需的信息，并使开发人员容易忽略不相关的信息，文档可以减轻认知负担。没有足够的文档，开发人员可能必须阅读大量代码才能重构设计人员的想法。文档还可以通过阐明系统的结构来减少意料之外的或不可预见的情况，从而可以清楚地了解与任何给定更改相关的信息和代码。

Chapter 2 pointed out that the primary causes of complexity are dependencies and obscurity. Good documentation can clarify dependencies, and it fills in gaps to eliminate obscurity.

第 2 章指出，导致复杂性的主要原因是依赖性和模糊性。好的文档可以阐明依赖关系，并且可以填补空白以消除模糊性。

The next few chapters will show you how to write good documentation. They will also discuss how to integrate documentation-writing into the design process so that it improves the design of your software.