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

问题描述

测试是否定义了预处理器符号的常用方法是使用#ifdef.但是，不能在宏中使用#ifdef.我需要的是一种方法来检查宏，如果该宏的参数是已定义的预处理器符号.

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; }

在这里，TRACE带有两个参数，第一个x应该是预处理器符号的名称.如果定义了此类符号，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

有没有办法实现这一目标?也许一些宏观魔术?当然，该解决方案应适用于任何符号，不仅适用于BLA或一组硬编码符号.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'

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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