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

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。)

其他回答

在我看来,“goto有害”更多的是关于状态的封装和一致性。

许多代码,甚至是'oo'代码,都有像意大利面条代码一样糟糕的混乱状态封装。

“goto有害”的问题是,它让程序员只看机制规则而不理解这样的印象,即唯一可用的流控制应该是返回方法,这很容易导致通过引用传递许多状态——这又导致缺乏状态封装,而这正是“goto有害”试图摆脱的东西。

遵循典型的“OO”代码库中的控制流,并告诉我我们仍然没有意大利面条代码....(顺便说一下,我并不是指那些经常让人讨厌的“馄饨”代码——馄饨代码的执行路径通常是非常简单的,即使对象关系不是立即明显的)。

或者,换一种说法,避免gotos而将所有东西都作为子例程,只有在每个子例程只修改局部状态时才有用,只有通过该子例程(或至少该对象)才能修改局部状态。

我只记得用过一次goto。我有一系列五个嵌套计数循环,我需要能够根据某些条件从内部打破整个结构:

    for{
      for{
        for{
          for{
            for{
              if(stuff){
                GOTO ENDOFLOOPS;
              }
            }
          }
        }
      }
    }
    
    ENDOFLOOPS:

我可以很容易地声明一个布尔中断变量,并将其用作每个循环的条件的一部分,但在这种情况下,我认为GOTO是一样实用和一样可读的。

没有迅猛龙攻击我。

c++包含构造函数和析构函数。这允许一种称为RAII(资源分配是初始化)的模式。基本上,您创建一个本地堆栈变量,创建堆栈变量的行为打开一个文件,分配内存,锁定一个互斥锁,或以其他方式获取一个稍后必须释放的资源。

当变量超出作用域时,析构函数将运行并释放资源。

C语言没有这个特性。但您仍然经常需要在函数开始时获取资源,并在结束时释放它们。

你的函数可能有一个或多个错误条件导致它提前返回。您不希望重复资源释放代码。解决方案是使用goto。

例子:

int
foo(const char *arg)
{
    char *argcopy = strdup(arg);

    if (!isvalid(argcopy))
        goto out1;

    FILE *myfile = fopen(argcopy, "r");
    if (myfile == NULL)
      goto out1;

    char bytes[10];
    if (fread(bytes, sizeof(bytes), 1, myfile) != sizeof(mybytes))
        goto out2;

    /* do some actual work */
    /* .... */
    /* end of actual work */

    out2:
    fclose(myfile);

    out1:
    free(argcopy);

    return 0;
 }

以下陈述是概括;尽管抗辩例外总是可能的,但通常(以我的经验和拙见)不值得冒险。

Unconstrained use of memory addresses (either GOTO or raw pointers) provides too many opportunities to make easily avoidable mistakes. The more ways there are to arrive at a particular "location" in the code, the less confident one can be about what the state of the system is at that point. (See below.) Structured programming IMHO is less about "avoiding GOTOs" and more about making the structure of the code match the structure of the data. For example, a repeating data structure (e.g. array, sequential file, etc.) is naturally processed by a repeated unit of code. Having built-in structures (e.g. while, for, until, for-each, etc.) allows the programmer to avoid the tedium of repeating the same cliched code patterns. Even if GOTO is low-level implementation detail (not always the case!) it's below the level that the programmer should be thinking. How many programmers balance their personal checkbooks in raw binary? How many programmers worry about which sector on the disk contains a particular record, instead of just providing a key to a database engine (and how many ways could things go wrong if we really wrote all of our programs in terms of physical disk sectors)?

以上附注:

关于第2点,考虑以下代码:

    a = b + 1
    /* do something with a */

在代码中的“do something”点,我们可以高度自信地声明a大于b。(是的,我忽略了未捕获整数溢出的可能性。我们不要拘泥于一个简单的例子。)

另一方面,如果代码是这样读的:

    ...
    goto 10
    ...
    a = b + 1
    10: /* do something with a */
    ...
    goto 10
    ...

标记10的方法的多样性意味着我们必须更加努力才能确信a和b在这一点上的关系。(事实上,在一般情况下,这是不可判断的!)

关于第4点,代码中“去某个地方”的整个概念只是一个比喻。除了电子和光子(用于余热),CPU内部没有任何东西真正“去”到任何地方。有时候,我们会放弃一个比喻,转而使用另一个更有用的比喻。我记得(几十年前!)遇到过一种语言

    if (some condition) {
      action-1
    } else {
      action-2
    }

通过将action-1和action-2编译为行外无参数例程,然后使用单个双参数VM操作码(使用条件的布尔值来调用其中一个)在虚拟机上实现。这个概念只是“选择现在调用什么”,而不是“去这里或去那里”。再一次,换一个比喻。

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。