`用C;对`（{}）的更多信息？ [英] More information on `({});` in C?

问题描述

我注意到，有时，C宏被写成这样的：

I've noticed that sometimes, C macros are written as something like this:

#define foo(bar) ({ ++bar; })

一些实验后，我发现：

After some experimentation, I've found that:

1. （{}）; 将编译，但也无可奈何。 （正如预期的那样。）


2. 离开; 关闭将导致语法错误。这样做的副作用是确保富（）看起来像在code函数。 （虽然，如果你离开分号了，错误不是诊断问题非常有用！）


3. 收益率（{}）; 抱怨没有被忽视，一个空值就像如果我曾试图用一个void函数

1. ({}); will compile, but do nothing. (As expected.)
2. Leaving the ; off will cause a syntax error. A side effect of this is ensuring that foo() looks like a function in your code. (Although, if you leave the semicolon off, the error isn't very useful for diagnosing the problem!)
3. return ({}); complains about a void value not being ignored, just like if I had tried to use a void function.

这是只是为了开发一个分号添加到自己的宏，还是有其他目的？我尝试过谷歌，但它与标点符号悲惨的失败了。是否有此名字？

Is this just to make developers add a semicolon to their macros, or does it have another purpose? I've tried Google, but it fails miserably with punctuation. Is there a name for this?

推荐答案

这是一个GNU扩展名为的语句前pressions 的。

This is a GNU extension called statement expressions.

在声明宏在标准C，你经常会看到做...而（0）用于类似用途循环（即创建一个块范围内）。声明前pression优于循环黑客，因为它可以返回一个值。如果你想要做标准-C类似的东西，你必须定义一个附加功能，失去词法范围的便利性。

When declaring macros in standard-C, you often see do...while(0) loops used for similar purposes (ie creating a block scope). A statement expression is superior to the loop hack because it can return a value. If you want to do something similar in standard-C, you'd have to define an additional function and lose the convenience of lexical scoping.

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