在C宏的范围是什么？ [英] Scope of macros in C?

问题描述

如何将这些宏进行评估？

How these macros are evaluated?

# define i 20
void fun();

int main(){
  printf("%d",i);
  fun();
  printf("%d",i);
  return 0;
}

void fun(){
  #undef i
  #define i 30
}

给输出2020年，但而，

gives output as 2020 but whereas,

# define i 20
void fun(){
  #undef i
  #define i 30
}

int main(){
  printf("%d",i);
  fun();
  printf("%d",i);
  return 0;
}

给出输出3030。
请解释。谢谢你。

gives output as 3030. Please Explain. Thanks.

推荐答案

çpreprocessor作品从上到下不论函数调用。它是从点（线）中的任何定义宏的文件有效，直到相应民主基金或到翻译单元结束。

C Preprocessor works top to bottom irrespective of function calls. It is effective from that point (line) in whatever the file that macro is defined, until corresponding undef or till end of the translation unit.

所以，你的code将成为

So, your code would become,

# define i 20
               // from now on, all token i should become 20
void fun();
int main()
{
  printf("%d",i);   // printf("%d",20);
  fun();
  printf("%d",i);   // printf("%d",20);
  return 0;
}
void fun()
{
#undef i
              // from now on, forget token i
#define i 30
              // from now on, all token i should become 30
}

你的第二个code将成为

Your second code would become,

# define i 20
               // from now on, all token i should become 20
void fun()
{
#undef i
               // from now on, forget i
#define i 30
               // from now on, all token i should become 30
}
int main()
{
  printf("%d",i);    //  printf("%d",30);
  fun();
  printf("%d",i);    // printf("%d",30);
  return 0;
}

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