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

Donald E. Knuth在1992年CSLI出版的《识字编程》一书中回答了这个问题。在第17页有一篇文章“使用goto语句的结构化编程”(PDF)。我认为这篇文章也可能发表在其他书中。

本文描述了Dijkstra的建议,并描述了这种建议有效的情况。但他也给出了一些反例(问题和算法),这些反例仅用结构化循环是无法轻易重现的。

这篇文章包含了对问题、历史、例子和反例的完整描述。

有时候,在一个函数中使用GOTO作为异常处理的替代是有效的:

    if (f() == false) goto err_cleanup;
    if (g() == false) goto err_cleanup;
    if (h() == false) goto err_cleanup;
    
    return;
    
    err_cleanup:
    ...

COM代码似乎经常陷入这种模式。

我避免使用它,因为同事/经理无疑会在代码评审或偶然发现它时质疑它的使用。虽然我认为它有一些用途(例如错误处理案例),但你会遇到一些其他开发人员,他们会遇到一些类型的问题。

这不值得。

goto有一些问题。一是很难看到代码是如何流动的。因为花括号,所以更容易看到if-block,但是goto隐藏了它。此外,while和if本质上也是goto,但它们有助于解释为什么要在代码中来回跳转。有了一个固定的目标,你必须自己拼凑起来。

作为练习,试着编写一些计算斐波那契数列的代码,看看当你完成后,它有多难阅读。

如果你要处理这些代码,那么我建议你写一些单元测试,然后重写它。否则,就顺其自然吧。

尽管如此,有时出于性能原因,使用goto“可能”是合适的。

如果你用C写一个VM,使用(gcc的)计算gotos是这样的:

char run(char *pc) {
    void *opcodes[3] = {&&op_inc, &&op_lda_direct, &&op_hlt};
    #define NEXT_INSTR(stride) goto *(opcodes[*(pc += stride)])
    NEXT_INSTR(0);
    op_inc:
    ++acc;
    NEXT_INSTR(1);
    op_lda_direct:
    acc = ram[++pc];
    NEXT_INSTR(1);
    op_hlt:
    return acc;
}

工作速度比循环内的传统开关快得多。