C语言进阶:25、#和##操作符使用分析

#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;

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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


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转载自blog.csdn.net/qq_28388835/article/details/80388690