如何与C preprocessor两次连接起来，扩大宏如在QUOT;阿根廷## _ ## MACRO＆QUOT;？

问题描述

我试图写一个程序，其中的某些功能的名称是依赖于一定的宏观变量，像这样的宏值：

 ＃定义变量3
的#define NAME（有趣）有趣的## _ ##变INT NAME（some_function）（int类型的）;
 

不幸的是，宏 NAME（）变成了成

  INT some_function_VARIABLE（int类型的）;
 

而不是

  INT some_function_3（int类型的）;
 

所以这显然是错误的方式去了解它。幸运的是，不同的可能值的变量数量很小，所以我可以简单地做一个＃如果变量==ñ并单独列出所有的情况下，但我不知道是否有是一个聪明的办法做到这一点。

解决方案

  $猫xx.c
＃定义变量3
帕斯特的#define（X，Y）×## _ ##ÿ
＃定义EVALUATOR（X，Y）帕斯特（X，Y）
的#define NAME（FUN）EVALUATOR（好玩，可变）EXTERN无效NAME（矿）（字符* X）;
$ gcc的-E xx.c
＃1xx.c
＃1＆LT;内置＆gt;中
＃1＆LT;命令行＆gt;中
＃1xx.cEXTERN无效mine_3（字符* X）;
$
 

---

<一个href=\"http://stackoverflow.com/questions/1489932/c-$p$pprocessor-and-token-concatenation/1489985#comment1342105_1489971\">asked为什么这需要间接两个层次。该轻率的回答是，因为这是标准如何规定它的工作;你会发现你需要用字符串化运营，相当于招了。

C99的标准包括宏替换，并6.10.3.1盖的说法取代第6.10.3。

In the invocation NAME(mine), the argument is 'mine'; it is fully expanded to 'mine'; it is then substituted into the replacement string:

EVALUATOR(mine, VARIABLE)

Now the macro EVALUATOR is discovered, and the arguments are isolated as 'mine' and 'VARIABLE'; the latter is then fully expanded to '3', and substituted into the replacement string:

PASTER(mine, 3)

The operation of this is covered by other rules (6.10.3.3 'The ## operator'):

So, the replacement list contains x followed by ## and also ## followed by y; so we have:

mine ## _ ## 3

and eliminating the ## tokens and concatenating the tokens on either side combines 'mine' with '_' and '3' to yield:

mine_3

This is the desired result.

---

If we look at the original question, the code was (adapted to use 'mine' instead of 'some_function'):

#define VARIABLE 3
#define NAME(fun) fun ## _ ## VARIABLE

NAME(mine)

The argument to NAME is clearly 'mine' and that is fully expanded.
Following the rules of 6.10.3.3, we find:

mine ## _ ## VARIABLE

which, when the ## operators are eliminated, maps to:

mine_VARIABLE

exactly as reported in the question.

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