blocks|key|2052230|text|这就是C和C%2B%2B表达NOP的方式。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2052231|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/NOP^0|A|3|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

This is the way C and C++ express <a href="http://en.wikipedia.org/wiki/NOP" rel="noreferrer">NOP</a>.

blocks|key|1838094|text|如果定义了NDEBUG，assert(foo+==+bar);如何才能编译为空呢？|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1838095|entityMap^0|5|6|C|J|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

How else could <code>assert(foo == bar);</code> compile down to nothing when <code>NDEBUG</code> is defined?

blocks|key|1837989|text|我不是语言设计师，但我给出的答案是“有何不可？”从语言设计的角度来看，人们希望规则(即语法)尽可能简单。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1837990|更不用说“空表达式”也有用处了。|1837991|for+(i+=+0；i<+INSANE_NUMBER；i%2B%2B)；|1837992|将死掉-等待(不是一个很好的使用，但仍然是一个使用)。|1837993|编辑:正如在对这个答案的评论中指出的那样，任何称职的编译器可能都不会忙于等待这个循环，并将其优化。然而，如果在for头本身中有更有用的东西(而不是i%2B%2B)，我见过(奇怪的)数据结构遍历，那么我想你仍然可以构造一个包含空体的循环(通过使用/滥用"for“结构)。|1837994|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|N|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|O|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|K|$]]

I'm no language designer, but the answer I'd give is "why not?" From the language design perspective, one wants the rules (i.e. the grammar) to be as simple as possible.

Not to mention that "empty expressions" have uses, i.e.

for (i = 0; i &lt; INSANE_NUMBER; i++);

Will dead-wait (not a good use, but a use nonetheless).

EDIT: As pointed out in a comment to this answer, any compiler worth its salt would probably not busy wait at this loop, and optimize it away. However, if there were something more useful in the for head itself (other than i++), which I've seen done (strangely) with data structure traversal, then I imagine you could still construct a loop with an empty body (by using/abusing the "for" construct).

blocks|key|1838343|text|你希望能够做像这样的事情|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1838344|while+(fnorble(the_smurf)+==+FAILED)
++++;|code-block|syntax|javascript|1838345|而不是|1838346|while+(fnorble(the_smurf)+==+FAILED)
++++do_nothing_just_because_you_have_to_write_something_here();|1838347|但!请不要将空语句写在同一行，如下所示：|1838348|while+(fnorble(the_smurf)+==+FAILED);|1838349|这是一个很好的混淆读者的方法，因为很容易错过分号，因此认为下一行是循环的主体。记住:编程实际上是关于交流的--不是与编译器，而是与其他人，他们将阅读你的代码。(或者是三年后的你自己！)|1838350|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

You want to be able to do things like
<pre><code>while (fnorble(the_smurf) == FAILED)
 ;
</code></pre>
and not
<pre><code>while (fnorble(the_smurf) == FAILED)
 do_nothing_just_because_you_have_to_write_something_here();
</code></pre>
But! Please do not write the empty statement on the same line, like this:
<pre><code>while (fnorble(the_smurf) == FAILED);
</code></pre>
That’s a very good way to confuse the reader, since it is easy to miss the semicolon, and therefore think that the next row is the body of the loop. Remember: Programming is really about communication — not with the compiler, but with other people, who will read your code. (Or with yourself, three years later!)

blocks|key|2052449|text|好的，我将把这个添加到你可能实际使用的最坏的情况中：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2052450|for+(int+yy+=+0;+yy+<+nHeight;+%2B%2Byy)+{
++++for+(int+xx+=+0;+xx+<+nWidth;+%2B%2Bxx)+{
++++++++for+(int+vv+=+yy+-+3;+vv+<=+yy+%2B+3;+%2B%2Bvv)+{
++++++++++++for+(int+uu+=+xx+-+3;+uu+<=+xx+%2B+3;+%2B%2Buu)+{
++++++++++++++++if+(test(uu,+vv))+{
++++++++++++++++++++goto+Next;
++++++++++++++++}
++++++++++++}
++++++++}
++++Next:;
++++}
}+++|code-block|syntax|javascript|2052451|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

OK, I’ll add this to the worst case scenario that you may actually use:
<pre><code>for (int yy = 0; yy &lt; nHeight; ++yy) {
 for (int xx = 0; xx &lt; nWidth; ++xx) {
 for (int vv = yy - 3; vv &lt;= yy + 3; ++vv) {
 for (int uu = xx - 3; uu &lt;= xx + 3; ++uu) {
 if (test(uu, vv)) {
 goto Next;
 }
 }
 }
 Next:;
 }
} 
</code></pre>

blocks|key|1838053|text|显然，这是为了让我们可以像这样说|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1838054|for+(;;)+{
++//+stuff
}|code-block|syntax|javascript|1838055|没有它，谁能活下去呢？|1838056|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Obviously, it is so that we can say things like
<pre><code>for (;;) {
 // stuff
}
</code></pre>
Who could live without that?

blocks|key|1838072|text|老实说，我不知道这是否是真正的原因，但我认为更有意义的是从编译器实现者的角度来考虑它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1838073|很大一部分编译器是由分析特殊语法类的自动化工具构建的。有用的语法允许空语句似乎是很自然的。当这种“错误”没有改变代码的语义时，检测它似乎是不必要的工作。空语句不会做任何事情，因为编译器不会为这些语句生成代码。|1838074|在我看来，这只是“不要修复没有损坏的东西”的结果……|1838075|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

I honestly don't know if this is the real reason, but I think something that makes more sense is to think about it from the standpoint of a compiler implementer.

Large portions of compilers are built by automated tools that analyze special classes of grammars. It seems very natural that useful grammars would allow for empty statements. It seems like unnecessary work to detect such an "error" when it doesn't change the semantics of your code. The empty statement won't do anything, as the compiler won't generate code for those statements.

It seems to me that this is just a result of "Don't fix something that isn't broken"...

blocks|key|1838012|text|最常见的情况可能是|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1838013|int+i+=+0;
for+(/*+empty+*/;+i+!=+10;+%2B%2Bi)+{
++++if+(x[i].bad)+break;
}
if+(i+!=+10)+{
++++/*+panic+*/
}|code-block|syntax|javascript|1838014|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

The most common case is probably
<pre><code>int i = 0;
for (/* empty */; i != 10; ++i) {
 if (x[i].bad) break;
}
if (i != 10) {
 /* panic */
}
</code></pre>

blocks|key|1838362|text|在使用;时，请注意一件事。这是可以的：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1838363|a+?+b()+:+c();|code-block|syntax|javascript|1838364|然而，这不会编译：|1838365|a+?+b()+:+;+;|1838366|entityMap^0|3|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|P|$]]

When using <code>;</code>, please also be aware about one thing. This is ok:
<pre><code>a ? b() : c();
</code></pre>
However this won't compile:
<pre><code>a ? b() : ; ;
</code></pre>

blocks|key|1838372|text|已经有很多很好的答案，但还没有看到生产环境示例。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1838373|Here是FreeBSD对strlen的实现：|offset|length|style|CODE|1838374|size_t
strlen(const+char+*str)
{
++++const+char+*s;

++++for+(s+=+str;+*s;+%2B%2Bs)
++++++++;
++++return+(s+-+str);
}|code-block|syntax|javascript|1838375|entityMap|0|LINK|mutability|MUTABLE|url|http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/lib/libc/string/strlen.c?rev=1.7&content-type=text/x-cvsweb-markup^0|0|D|6|0|4|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]]|9|@$D|Y|E|Z|1|10]]|A|$]]|$1|H|3|I|5|J|7|11|8|@]|9|@]|A|$K|L]]|$1|M|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

There are already many good answers but have not seen the productive-environment sample. 

<a href="http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/lib/libc/string/strlen.c?rev=1.7&amp;content-type=text/x-cvsweb-markup" rel="nofollow noreferrer">Here</a> is FreeBSD's implementation of <code>strlen</code>: 

<pre><code>size_t
strlen(const char *str)
{
 const char *s;

 for (s = str; *s; ++s)
 ;
 return (s - str);
}
</code></pre>

blocks|key|2052147|text|while+(1)+{
++++;++/*+do+nothing+*/
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|2052148|有些时候你想坐着什么也不做。一个事件/中断驱动的嵌入式应用程序，或者当你不想让一个函数退出时，比如设置线程和等待第一次上下文切换时。|unstyled|2052149|示例：http://lxr.linux.no/linux%2Bv2.6.29/arch/m68k/mac/misc.c#L523|offset|length|2052150|entityMap|0|LINK|mutability|MUTABLE|url|http://lxr.linux.no/linux%2Bv2.6.29/arch/m68k/mac/misc.c#L523^0|0|0|3|1N|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|T|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|F|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

<pre><code>while (1) {
 ; /* do nothing */
}
</code></pre>
There are times when you want to sit and do nothing. An event/interrupt driven embedded application or when you don't want a function to exit such as when setting up threads and waiting for the first context switch.
example:
<a href="http://lxr.linux.no/linux+v2.6.29/arch/m68k/mac/misc.c#L523" rel="nofollow noreferrer">http://lxr.linux.no/linux+v2.6.29/arch/m68k/mac/misc.c#L523</a>

<pre><code>int main()
{
 int var = 0;; // Typo which compiles just fine
}
</code></pre>

Why are empty expressions legal in C/C++?

数据结构

编译器

FreeBSD

int main(){  int var = 0;; // Typo which compiles just fine}

问为什么在C/C++中空表达式是合法的？
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回答 11

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