可能重复： C预处理程序：在#define中使用#if吗？
在define的rhs中包含预处理器指令是否有任何技巧？问题是，预处理器将所有rhs折叠成一条长行。但是也许有一个窍门？
Is there any trick to have preprocessor directives inside rhs of define ? The problem is, preprocessor folds all rhs into one long line. But maybe there is a trick ?
在rhs中我想要的例子是
Example of what I'd want in the rhs is
#define MY_CHECK \ #ifndef MY_DEF \ # error MY_DEF not defined \ #endif？ 目的是一个简短之处：使用1行快捷方式而不是多行检查顺序。
? The purpose is a shortness: to have 1-line shortcut instead of multiline sequence of checks.
推荐答案正如其他人指出的那样，预处理器宏不能扩展为任何其他预处理器指令。如果这样，您通常会在输入中误输入＃字符。但是，有时您可以做一些事情来获得想要的东西。如果您想将宏扩展为以下内容：
As others have noted, preprocessor macros cannot expand into any other preprocessor directives; if they do you'll generally get odd errors about stray '#' characters in the input. However, sometimes there are things you can do to get what you want. If you want a macro that expands to something like:
#ifdef SOMETHING ...some code... #endif其中某些代码不包含任何预处理程序指令，可以定义一个IFDEF宏：
where some code doesn't include any preprocessor directives, you can define an IFDEF macro:
#ifdef SOMETHING #define IFDEF_SOMETHING(X) X #else #define IFDEF_SOMETHING(X) #endif然后在另一个宏中使用 IFDEF_SOMETHING（...一些代码...）。
and then use IFDEF_SOMETHING(...some code...) in your other macro.
如果您有一个一堆要重复多次的预处理器碎片，您可以将其粘贴到自己的文件中，然后在需要的每个位置使用 #include stuff 。
If you have a bunch of preprocessor cruft that you want to repeat multiple times, you can stick it in its own file and then use #include "stuff" in each spot you need it.
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预处理指令内部定义？
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