什么是'＃'正是用C呢？ [英] What does '#' exactly do in C?

问题描述

我有一个计划。

#include <stdio.h>

#define f(a,b) a##b
#define g(a)   #a
#define h(a) g(a)

int main()
{
      printf("%s\n",h(f(1,2)));
      printf("%s\n",g(f(1,2)));
      return 0;
}

这个程序正常工作，并给予作为输出：

This program working properly and giving output as:

12
f(1, 2)

我不明白编译器如何给这个输出。

I don't understand how compiler giving this output.

什么是＃中的函数 A ## b 和＃A ？

推荐答案

让我分解为您：

#define f(a,b) a##b //2 this macro is evaluated first with a = 1 and b = 2 it concatenates them and returns 12
#define g(a)   #a //4 g turns 12 into "12" (string)
#define h(a) g(a) //3 back to h which now has a = 12 and call g()

int main()
{
      printf("%s\n",h(f(1,2)));//1 printf calls the macro h() and gives it the macro f() as an argument 
      printf("%s\n",g(f(1,2)));// g here turns f(1,2) into "f(1,2)" (string)
      return 0;
}

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