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

跳跃的例子在Java字符串类源代码:

int firstUpper;

/* Now check if there are any characters that need to be changed. */
scan: {
    for (firstUpper = 0 ; firstUpper < count; ) {
         char c = value[offset+firstUpper];
         if ((c >= Character.MIN_HIGH_SURROGATE) &&
                 (c <= Character.MAX_HIGH_SURROGATE)) {
             int supplChar = codePointAt(firstUpper);
             if (supplChar != Character.toLowerCase(supplChar)) {
                  break scan;
             }
             firstUpper += Character.charCount(supplChar);
         } else {
             if (c != Character.toLowerCase(c)) {
                  break scan;
             }
             firstUpper++;
         }
     }
     return this;
}
[... subsequent use of firstUpper ...]

这可以用很少的开销重写,例如:

 int firstUpper = indexOfFirstUpper();
 if (firstUpper < 0) return this; 

即使在现代语言中,即使我实际上不喜欢使用gotos,但我认为它们在许多情况下是可以接受的,在像这样的低级情况下,我看起来更好(它不仅仅是退出循环)。

没有激起宗教战争的意图。

其他回答

Using a goto makes it far too easy to write "spaghetti code" which is not particularly maintainable. The most important rule to follow is to write readable code, but of course it depends on what the goals of the project are. As a "best practice" avoiding a goto is a good idea. It's something extreme programming types would refer to as "code smell" because it indicates that you may be doing something wrong. Using a break while looping is remarkably similar to a goto, except it isn't a goto, but again is an indication that the code may not be optimal. This is why, I believe, it is also important to not find more modern programming loopholes which are essentially a goto by a different name.

我们已经讨论过这个问题了,我坚持我的观点。

此外,我受够了人们将高级语言结构描述为“伪装的goto”,因为他们显然根本没有抓住重点。例如:

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。

那完全是胡说八道。每个控制结构都可以在goto方面实现,但这个观察完全是微不足道和无用的。Goto被认为是有害的,因为它的积极影响,而是因为它的消极后果,这些已经被结构化编程消除了。

Similarly, saying “GOTO is a tool, and as all tools, it can be used and abused” is completely off the mark. No modern construction worker would use a rock and claim it “is a tool.” Rocks have been replaced by hammers. goto has been replaced by control structures. If the construction worker were stranded in the wild without a hammer, of course he would use a rock instead. If a programmer has to use an inferior programming language that doesn't have feature X, well, of course she may have to use goto instead. But if she uses it anywhere else instead of the appropriate language feature she clearly hasn't understood the language properly and uses it wrongly. It's really as simple as that.

今天,很难看出GOTO语句有什么大不了的,因为“结构化编程”的人赢得了这场辩论,今天的语言有足够的控制流结构来避免GOTO。

计算现代C程序中goto的数量。现在添加break、continue和return语句的数量。此外,加上你使用if、else、while、switch或case的次数。这是1968年Dijkstra写这封信时,如果你用FORTRAN或BASIC语言编写程序,你的程序会有多少个goto。

当时的编程语言缺乏控制流程。例如,在最初的达特茅斯BASIC中:

IF statements had no ELSE. If you wanted one, you had to write: 100 IF NOT condition THEN GOTO 200 ...stuff to do if condition is true... 190 GOTO 300 200 REM else ...stuff to do if condition is false... 300 REM end if Even if your IF statement didn't need an ELSE, it was still limited to a single line, which usually consisted of a GOTO. There was no DO...LOOP statement. For non-FOR loops, you had to end the loop with an explicit GOTO or IF...GOTO back to the beginning. There was no SELECT CASE. You had to use ON...GOTO.

因此,您的程序中最终出现了许多goto。并且您不能依赖于goto限制在单个子例程中(因为GOSUB…RETURN是一个非常弱的子例程概念),所以这些goto可以去任何地方。显然,这使得控制流难以遵循。

这就是反goto运动的由来。

用于调度的计算gotos通常比非常大的switch语句更容易理解。

对于错误和联合线程,我认为setcontext或setjmp(如果可用)是“更好的”。

一个现代的GOTO用法是由c#编译器为yield return定义的枚举器创建状态机。

GOTO应该由编译器而不是程序员使用。