是否可以将预处理器有条件地放在C宏中? [英] Is it possible to put a preprocessor conditional inside a C macro?

问题描述

有没有一种方法可以编写一个C预处理器宏，该宏根据接收到的参数而扩展为不同的东西?

Is there a way to write a C preprocessor macro that expands to different things depending on what argument it receives?

#define foo() ???

/* 1 */
foo(name)

/* 2 */
foo(_)

所需结果:

/* 1 */
int name;

/* 2 */
/*ignore*/

是的，我知道宏是邪恶的.我问这个问题主要是出于好奇.

Yes, I know macros are evil. I'm asking this mostly out of curiosity.

推荐答案

也许尝试一些多阶段宏扩展?这是提升预处理器/控件所使用的策略/if库.

Perhaps try some multi-stage macro expansion? This is the strategy used by the Boost preprocessor/control/if library.

#define FOO_NAME 1
#define FOO__ 2

#define CONC(a,b) a##_##b
#define FOO(x) CONC(FOO,x)

我认为没有任何方法可以检查C宏扩展中的条件.

I don't think there is any way to check conditions within a C macro expansion.

我想出的最好的办法是使用#字符串化运算符将宏参数隐式转换为字符串文字，然后使用运行时函数进行检查. (不过，在您要输出变量声明的情况下，这不适用于您的情况.)

The best thing I could come up with is to covert the macro arguments to a string literal using the # stringizing operator, and then checking using run-time functions. (This won't work for your case, though, where you want to output variable declarations.)

例如，以下显示"011":

For example, the following prints "011":

#define FOO(x) (strcmp("NAME", #x) ? 1 : 0)

main()
{
    printf("%d", FOO(NAME));
    printf("%d", FOO(1));
    printf("%d", FOO(2));
}

编译器可能会在编译时优化strcmp比较，因此它不会比使用真正的预处理器有条件的情况低效率.但是，将FOO设置为普通函数会更清晰，也可能同样有效.

The compiler would likely optimize the strcmp comparisons at compile-time so it would be no more inefficient than it would have been had genuine pre-processor conditionals been available. However, making FOO a normal function would be clearer and probably just as efficient.

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