18 # 和 ##

目录

• 1 # 运算符
• 2 ## 运算符

1 # 运算符

• # 运算符用于在预处理期将宏参数转换为字符串

• # 的转换作用是在预处理期完成的，因此只在宏定义中有效

• 编译器不知道# 的转换作用

• 用法

  #include <stdio.h>
  
  #define STRING(x) #x
  
  int main()
  {
      
      printf("%s
  ", STRING(Hello world!));
      printf("%s
  ", STRING(100));
      printf("%s
  ", STRING(while));
      printf("%s
  ", STRING(return));
  
      return 0;
  }
  
  //输出结果
  Hello world!
  100
  while
  return
  

• 示例

  #include <stdio.h>
  
  //动态打印被调用函数的函数名
  #define CALL(f, p) (printf("Call function %s
  ", #f), f(p))
     
  int square(int n)
  {
      return n * n;
  }
  
  int func(int x)
  {
      return x;
  }
  
  int main()
  {
      int result = 0;
      
      result = CALL(square, 4);
      
      printf("result = %d
  ", result);
      
      result = CALL(func, 10);
      
      printf("result = %d
  ", result);
  
      return 0;
  }
  
  //运行结果
  Call function square
  result = 16
  Call function func
  result = 10
  

  • 单步编译：gcc -E test.c -o test.i

    int square(int n)
    {
        return n * n;
    }
    
    int func(int x)
    {
        return x;
    }
    
    int main()
    {
        int result = 0;
        
        result = (printf("Call function %s
    ","square"),square(4));
        
        printf("result = %d
    ", result);
        
        result = (printf("Call function %s
    ","func"),func(10));
        
        printf("result = %d
    ", result);
    
        return 0;
    }
    

2 ## 运算符

• ## 运算符用于在预处理期粘贴两个标识符

• ## 的连接作用是在预处理期完成的，因此只在宏中有效

• 编译器不知道## 的连接作用

• 用法

  #define CONNECT(a,b) a##b
  int CONNECT(a,1);  //int a1;
  a1 = 2;
  

• 示例1

  #include <stdio.h>
  
  #define NAME(n) name##n
  
  int main()
  {
      
      int NAME(1);
      int NAME(2);
      
      NAME(1) = 1;
      NAME(2) = 2;
      
      printf("%d
  ", NAME(1));
      printf("%d
  ", NAME(2));
  
      return 0;
  }
  //输出结果
  NAME1 = 1
  NAME2 = 2
  

• 示例2

  • 解决定义完结构体后，每次生成一个结构体变量都需要使用 struct 关键字的问题

  #include <stdio.h>
  
  #define STRUCT(type) typedef struct _tag_##type type;
                       struct _tag_##type
  
  STRUCT(Student)
  {
      char* name;
      int id;
  };
  
  int main()
  {
      
      Student s1;
      Student s2;
      
      s1.name = "s1";
      s1.id = 0;
      
      s2.name = "s2";
      s2.id = 1;
      
      printf("s1.name = %s
  ", s1.name);
      printf("s1.id = %d
  ", s1.id);
      printf("s2.name = %s
  ", s2.name);
      printf("s2.id = %d
  ", s2.id);
  
      return 0;
  }