GOTO仍然被认为有害?

每个人都知道Dijkstra的《致编辑的信》:goto语句被认为是有害的(这里。html transcript和这里。pdf)，从那时起，就有一种强大的推动力，尽可能避免使用goto语句。虽然可以使用goto来生成不可维护的、庞大的代码，但它仍然存在于现代编程语言中。即使Scheme中先进的连续控制结构也可以被描述为复杂的后向。

在什么情况下需要使用goto?什么时候最好避免?

作为一个后续问题:C提供了一对函数setjmp()和longjmp()，它们不仅提供了在当前堆栈帧内进行跳转的能力，还提供了在任何调用帧内进行跳转的能力。这些应该被认为和goto一样危险吗?更危险?

Dijkstra自己也对这个头衔感到后悔，因为这不是他的责任。在EWD1308的结尾(也在这里。pdf)，他写道:

Finally a short story for the record. In 1968, the Communications of the ACM published a text of mine under the title "The goto statement considered harmful", which in later years would be most frequently referenced, regrettably, however, often by authors who had seen no more of it than its title, which became a cornerstone of my fame by becoming a template: we would see all sorts of articles under the title "X considered harmful" for almost any X, including one titled "Dijkstra considered harmful". But what had happened? I had submitted a paper under the title "A case against the goto statement", which, in order to speed up its publication, the editor had changed into a "letter to the Editor", and in the process he had given it a new title of his own invention! The editor was Niklaus Wirth.

Donald E. Knuth写了一篇关于这个主题的经典论文，它与Dijkstra的论文相匹配，那就是结构化编程与语句。阅读既有助于重建上下文，也有助于对主题的非教条性理解。在本文中，Dijkstra对这个案例的观点被报道，并且更加强烈:

Donald E. Knuth:我相信通过呈现这样一个事实上，我并不是不同意 Dijkstra的观点，因为他最近写道: “请不要落入。相信我是可怕的教条的关于[去]的教条的声明)。我觉得很不舒服感觉别人在做宗教出来了，好像编程的概念问题只用一个小技巧就能解决，用什么一种简单的编码纪律!”

当前回答

您可以使用它来打破深度嵌套循环，但大多数情况下，无需深度嵌套循环，代码可以重构得更干净。

2008-09-05 19:08:31

其他回答

是的，GOTO仍然被认为是有害的。当您发现自己处于使用GOTO可能有效的罕见情况时，您应该对自己的编程技能有足够的信心，而不需要其他人的验证。任何类似GOTO的函数允许你在GOTO允许的范围内跳得更远，都应该被认为比GOTO更危险。

2009-04-23 03:37:27

实际上，我发现自己不得不使用goto，因为我真的想不出更好(更快)的方法来编写这段代码:

我有一个复杂的对象，我需要对它做一些操作。如果对象处于一种状态，那么我就可以进行快速操作，否则我就必须进行慢速操作。问题是，在某些情况下，在缓慢的手术过程中，可能会意识到这可以用快速手术来完成。

SomeObject someObject;    

if (someObject.IsComplex())    // this test is trivial
{
    // begin slow calculations here
    if (result of calculations)
    {
        // just discovered that I could use the fast calculation !
        goto Fast_Calculations;
    }
    // do the rest of the slow calculations here
    return;
}

if (someObject.IsmediumComplex())    // this test is slightly less trivial
{
    Fast_Calculations:
    // Do fast calculations
    return;
}

// object is simple, no calculations needed.

这是一个实时UI代码的速度关键部分，所以我真的认为GOTO在这里是合理的。

Hugo

2009-01-17 18:55:14

Since I began doing a few things in the linux kernel, gotos don't bother me so much as they once did. At first I was sort of horrified to see they (kernel guys) added gotos into my code. I've since become accustomed to the use of gotos, in some limited contexts, and will now occasionally use them myself. Typically, it's a goto that jumps to the end of a function to do some kind of cleanup and bail out, rather than duplicating that same cleanup and bailout in several places in the function. And typically, it's not something large enough to hand off to another function -- e.g. freeing some locally (k)malloc'ed variables is a typical case.

I've written code that used setjmp/longjmp only once. It was in a MIDI drum sequencer program. Playback happened in a separate process from all user interaction, and the playback process used shared memory with the UI process to get the limited info it needed to do the playback. When the user wanted to stop playback, the playback process just did a longjmp "back to the beginning" to start over, rather than some complicated unwinding of wherever it happened to be executing when the user wanted it to stop. It worked great, was simple, and I never had any problems or bugs related to it in that instance.

Setjmp /longjmp有它们自己的位置——但那个位置是你不太可能只在很长一段时间内访问一次的地方。

编辑:我只是看了一下代码。实际上，我使用的是siglongjmp()，而不是longjmp(不是说这是一个大问题，但我已经忘记了siglongjmp的存在。)

2009-04-23 03:53:02

几乎所有可以使用goto的情况，都可以使用其他结构实现相同的效果。无论如何，编译器都会使用Goto。

我个人从来没有明确地使用它，也不需要。

2010-06-17 07:41:44

我唯一同意使用Goto的地方是当你需要处理错误时，每一个特定的错误点都需要特殊的处理。

例如，如果您正在获取资源并使用信号量或互斥，您必须按顺序获取它们，并且应该始终以相反的方式释放它们。

一些代码需要一种非常奇怪的模式来获取这些资源，并且您不能仅仅编写一个易于维护和理解的控制结构来正确处理这些资源的获取和释放以避免死锁。

在没有goto的情况下总是可以做得很好，但在这种情况下和其他一些情况下，goto实际上是更好的解决方案，主要是为了可读性和可维护性。

亚当

2008-09-05 19:45:19