#运算符
#运算符用在预处理器期间将宏参数转换为字符串
#的转换作用是在预处理器完成的,因此只在宏定义中有效
编译器不知道#的转换作用用法:
#define STRING(x) #x
printf("%s\n", STRING(Hello World!)); //注意,并没有出现双引号
对于下面代码:
#include <stdio.h>
#define STRING(x) #x
int main()
{
printf("%s\n", STRING(Hello world!));
printf("%s\n", STRING(100));
printf("%s\n", STRING(while));
printf("%s\n", STRING(return));
return 0;
}
单步编译:gcc -E test.c -o test.i
printf("%s\n", "Hello world!");
printf("%s\n", "100");
printf("%s\n", "while");
printf("%s\n", "return");
观察下面代码:
#include <stdio.h>
#define CALL(f, p) (printf("Call function %s\n", #f), f(p)) //打印了调用函数的函数名
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\n", result);
result = CALL(func, 10);
printf("result = %d\n", result);
return 0;
}
Linux下使用GCC编译运行:
~/will$ ./a.out
Call function square
result = 16
Call function func
result = 10
单步编译观察宏被预处理后的样子:
result = (printf("Call function %s\n", "square"), square(4));
result = (printf("Call function %s\n", "func"), func(10));
##运算符
##运算符用于在预处理阶段粘连两个标识符##的连接作用是在预处理器完成的,因此只在宏定义中有效;
用法:
#define CONNECT(a,b) a##b
int CONNECT(a,1); // int a1
a1 = 2;
观察下面代码:
//#include <stdio.h>
#define NAME(n) name##n
int main()
{
int NAME(1);
int NAME(2);
NAME(1) = 1;
NAME(2) = 2;
printf("%d\n", NAME(1));
printf("%d\n", NAME(2));
return 0;
}
单步编译:gcc -E 25-3.c -o 25-3.i
int main()
{
int name1;
int name2;
name1 = 1;
name2 = 2;
printf("%d\n", name1);
printf("%d\n", name2);
return 0;
}
#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\n", s1.name);
printf("s1.id = %d\n", s1.id);
printf("s2.name = %s\n", s2.name);
printf("s2.id = %d\n", s2.id);
return 0;
}
单步编译:
typedef struct _tag_Student Student; struct _tag_Student
{
char* name;
int id;
};
小结:
#运算符用在 预处理器期间将宏参数转换为字符串
##运算符用于在预处理阶段粘连两个标识符
编译器不知道#和##的存在
#和##只在 宏定义中存在