用C标记粘贴 [英] Token pasting in C

问题描述

阅读 VA_NARG <后/ A>

我想这取决于 C 使用宏数量的参数来实现函数重载。
现在的问题是：

I tried to implement function overloading depending on number of arguments in C using macros. Now the problem is:

void hello1(char *s) { ... }
void hello2(char *s, char *t) { ... }
// PP_NARG(...)           macro returns number of arguments :ref to link above
 // does not work
#define hello(...)         hello ## PP_NARG(__VA_ARGS__)  

int main(void)
{
   hello("hi");   // call hello1("hi");
   hello("foo","bar"); // call hello2("foo","bar");
   return 0;
}

我读过这从C-FAQ。但还是没能得到它的工作...

I've read this from C-faq. But still could not get it to work...

推荐答案

这是因为宏的评价规则。你必须定义一些辅助宏接收数量为令牌：

This is because of the evaluation rules for macros. You would have to define some sort of helper macro that receives the number as a token:

#define HELLO_1(N, ...)         hello ## N
#define HELLO_0(N, ...)         HELLO_1(N, __VARGS__)
#define HELLO(...)         HELLO_0(PP_NARG(__VA_ARGS__), __VARGS__)  

左右。你也可以有一个一目了然进入 P99的文件。这将为您提供更舒适的宏观工具直接做到这一点。

or so. You could also have a glance into the prerelease of the documentation of P99. This will provide you more comfortable macro tools to do that directly.

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