做{}而（0）与如（1）{}中的宏 [英] do { } while(0) vs. if (1) { } in macros

问题描述

可能重复：结果
  <一href=\"http://stackoverflow.com/questions/154136/why-are-there-sometimes-meaningless-do-while-and-if-else-statements-in-c-c-mac\">Why在那里，有时毫无意义的do / while和如果C / else语句/ C ++宏？

当一个人需要preprocessor宏内执行多个语句，它通常是这样写

When one needs to execute multiple statements within preprocessor macro, it's usually written like

#define X(a) do { f1(a); f2(a); } while(0)

所以，当这宏内部使用前pressions这样的：

so when this macro is used inside expressions like:

if (...)
    X(a);

它不会被搞砸了。

it would not be messed up.

现在的问题是：无论我见过这样的前pression，它总是做{...}而（0）; 。是否有任何理由preFER这样的概念了（在我看来更清晰的）如果（1）{...} ？还是我错了，我的意见，他们都同样受欢迎？

The question is: wherever I've seen such expression, it's always do { ... } while(0);. Is there any reason to prefer such notion over (in my opinion more clear one) if (1) { ... }? Or am I wrong in my observations and they are equally popular?

推荐答案

不，你没看错。

实际上，有一个很好的理由：

There's actually a nice reason:

#define my_code if (1) { ... }

if (1)
    my_code;

问题是与; ！它不应该在那里...这只是看起来奇怪的，而不是在语言的精神。您可以选择拥有code，它扩展到两个; 成一排，或code，看起来的 UN-C-ISH 的：）

The problem is with the ; ! It shouldn't be there... and that would just look strange and not in the spirit of the language. You can either choose to have a code that expands in to two ; in a row, or a code that looks un-c-ish :)

在另一方面，在做的，而施工不存在这样的问题。

On the other hand, the do-while construction does not have that problem.

此外，如其他人所说，有一个其他问题：

Also, as others mentioned, there's an else problem:

if (1)
    my_code;
else { ... }

忽略; issuse，在其他块现在属于错的，如果

Ignoring the ; issuse, the else block now belongs to the wrong if.

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