L20#和##使用

1. #

    #include <stdio.h>    #define CALL(f, p) (printf("Call function %s\n", #f), f(p))    int square(int n)    {        return n * n;    }    int f(int x)    {        return x;    }    int main()    {        printf("1. %d\n", CALL(square, 4));        printf("2. %d\n", CALL(f, 10));        return 0;    }

作用： #在宏定义中将后面的任何元素转换为一个字符串

2. ##

作用： ##用于在预编译期粘连两个符号
范例：结构体定义

    #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("%s\n", s1.name);        printf("%d\n", s1.id);        printf("%s\n", s2.name);        printf("%d\n", s2.id);        return 0;    }