C中的#和##宏 [英] # and ## macros in C

问题描述

程序1:

#include <stdio.h>
#define foo(x, y) #x #y

int main()
{
  printf("%s\n", foo(k, l)); //prints kl
  return 0;
}

计划2:

#include <stdio.h>
#define foo(m, n) m ## n

int main()
{
  printf("%s\n", foo(k, l)); //compiler error
}

为什么两个程序的输出都有这种变化? 这两个程序之间确切的区别是什么?

Why such variation in the output of both programs? What is the exact difference between these two programs?

推荐答案

是 字符串化" 操作符;事实证明它的参数成一个字符串.

# is the "stringizing" operator; it turns its argument into a string literal.

##是令牌粘贴"运算符；它加入它的两个参数为单个令牌，不一定字符串文字.

## is the "token-pasting" operator; it joins its two arguments into a single token, not necessarily a string literal.

一个例子:

#include <stdio.h>

#define foo(m, n) m ## n

int main(void) {
    char *kl = "token pasting";
    printf("%s\n", foo(k, l));
}

其中打印:

token pasting

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