如何在C预处理器中生成唯一值?

问题描述

我正在写一堆相关的预处理器宏，其中一个会生成标签，另一个会跳转到该标签.我以这种方式使用它们:

I'm writing a bunch of related preprocessor macros, one of which generates labels which the other one jumps to. I use them in this fashion:

MAKE_FUNNY_JUMPING_LOOP(
  MAKE_LABEL();
  MAKE_LABEL();
)

我需要某种方式来生成唯一标签，使用预处理器为每个内部MAKE_LABEL调用一个标签.我尝试使用__LINE__，但是由于我在另一个宏中调用了MAKE_LABEL，因此它们都具有相同的行，并且标签会发生冲突.

I need some way to generate unique labels, one for each inner MAKE_LABEL call, with the preprocessor. I've tried using __LINE__, but since I call MAKE_LABEL inside another macro, they all have the same line and the labels collide.

我希望将其扩展为以下内容:

What I'd like this to expand to is something like:

MAKE_FUNNY_JUMPING_LOOP(
  my_cool_label_1:  // from first inner macro
  ...
  my_cool_label_2:  // from second inner macro
  ...
)

是否可以使用预处理器生成哈希或自动递增整数?

推荐答案

如其他人所述，__COUNTER__是执行此操作的简单但非标准的方法.

As others noted, __COUNTER__ is the easy but nonstandard way of doing this.

如果您需要额外的可移植性，或者需要其他出色的预处理器技巧，请 Boost预处理程序库(适用于C和C ++)将起作用.例如，以下头文件将在包含头文件的任何地方输出一个唯一的标签.

If you need extra portability, or for other cool preprocessor tricks, the Boost Preprocessor library (which works for C as well as C++) will work. For example, the following header file will output a unique label wherever it's included.

#include <boost/preprocessor/arithmetic/inc.hpp>
#include <boost/preprocessor/slot/slot.hpp>

#if !defined(UNIQUE_LABEL)
#define UNIQUE_LABEL
#define BOOST_PP_VALUE 1
#include BOOST_PP_ASSIGN_SLOT(1)
#undef BOOST_PP_VALUE
#else
#define BOOST_PP_VALUE BOOST_PP_INC(BOOST_PP_SLOT(1))
#include BOOST_PP_ASSIGN_SLOT(1)
#undef BOOST_PP_VALUE
#endif


BOOST_PP_CAT(my_cool_label_, BOOST_PP_SLOT(1)):

示例:

int main(int argc, char *argv[]) {
    #include "unique_label.h"
    printf("%x\n", 1234);
    #include "unique_label.h"
    printf("%x\n", 1234);
    #include "unique_label.h"
    return 0;
}

预处理为

int main(int argc, char *argv[]) {
    my_cool_label_1:
    printf("%x\n", 1234);
    my_cool_label_2:
    printf("%x\n", 1234);
    my_cool_label_3:
    return 0;
}

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