测试是否在宏内部定义了预处理器符号 [英] Test if preprocessor symbol is defined inside macro

问题描述

测试预处理器符号是否定义的常用方法是使用#ifdef.但是，#ifdef 不能在宏中使用.如果该宏的参数是已定义的预处理器符号，我需要一种方法来检查该宏.

The usual way to test whether a preprocessor symbol is defined is using #ifdef. However, #ifdef cannot be used in a macro. What I need is a way to check in a macro if an argument of that macro is a defined preprocessor symbol.

例如:

#define TRACE(x,y) if(IS_DEFINED(x)){ std::cout << y; }

这里，TRACE 有两个参数，第一个x 应该是预处理器符号的名称.如果定义了这样的符号，则应打印第二个参数.我正在寻找不存在的 IS_DEFINED 函数/宏.

Here, TRACE takes two arguments, the first x should be the name of a preprocessor symbol. If such symbol is defined, the second argument should be printed. The non-existing IS_DEFINED function/macro is what I am looking for.

用法如下:

#undef BLA
TRACE(BLA,"abc") // "abc" won't be printed, as BLA is not defined
#define BLA 1
TRACE(BLA,"xyz") // "xyz" will be printed, as BLA is a defined symbol

有没有办法做到这一点?也许一些宏观魔术?当然，这些解决方案应该适用于任何符号，而不仅仅是 BLA 或一组硬编码的符号.如果事先知道要检查的符号集，显然很容易.

Is there a way to achieve this? Maybe some macro magic? Of course, thes solution should work for any symbol, not only BLA or a hardcoded set of symbols. It is obviously quite easy if the set of symbols to be checked is known in advance.

推荐答案

Linux' kgconfig.h 为此用例定义了一个 __is_defined 宏:

 #define __ARG_PLACEHOLDER_1 0,
 #define __take_second_arg(__ignored, val, ...) val

/*
 * Helper macros to use CONFIG_ options in C/CPP expressions. Note that
 * these only work with boolean and tristate options.
 */

/*
 * Getting something that works in C and CPP for an arg that may or may
 * not be defined is tricky.  Here, if we have "#define CONFIG_BOOGER 1"
 * we match on the placeholder define, insert the "0," for arg1 and generate
 * the triplet (0, 1, 0).  Then the last step cherry picks the 2nd arg (a one).
 * When CONFIG_BOOGER is not defined, we generate a (... 1, 0) pair, and when
 * the last step cherry picks the 2nd arg, we get a zero.
 */
#define __is_defined(x)         ___is_defined(x)
#define ___is_defined(val)      ____is_defined(__ARG_PLACEHOLDER_##val)
#define ____is_defined(arg1_or_junk)    __take_second_arg(arg1_or_junk 1, 0)

它是 C99，适用于三态选项(未定义、定义为 0、定义为 1).

It's C99 and works for tristate options (undefined, defined to 0, defined to 1).

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