#运算符
1.#运算符用于在预处理期将宏参数转换为字符串。
2.#的作用是在预处理期间完成的,因此只有在宏定义中有效
3.编译器不知道 # 的转换作用
#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;
}
经过预处理器,处理过后的程序如下所示:
//gcc -E main.c -o test.i
# 1 "main.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 31 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 32 "<command-line>" 2
# 1 "main.c"
##include <stdio.h>
int main()
{
printf("%s\n","Hello World!");
printf("%s\n","100");
printf("%s\n","while");
printf("%s\n","return");
return 0;
}
想一个问题,怎么样在程序中,把函数调用的函数名字打印出来?
这里需要要用到宏,而且必须用到 #,否者用函数是实现不了的。
#include <stdio.h>
#define CALL(f,p) (printf("Call function %s\n",#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\n",result);
result = CALL(func,10);
printf("result = %d\n",result);
return 0;
}
程序经过预处理器处理过后,为下面所示:
# 1 "main.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 31 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 32 "<command-line>" 2
# 1 "main.c"
int square(int n)
{
return n*n;
}
int func(int x)
{
return x;
}
int main()
{
int result=0;
result = (printf("Call function %s\n","square"),square(4));
printf("result = %d\n",result);
result = (printf("Call function %s\n","func"),func(10));
printf("result = %d\n",result);
return 0;
}
##运算符
1.##运算符用于在预处理器期间,粘住两个标识符
2.##的连接作用是在预处理器期间完成的,因此只在宏定义中有效
3.编译器不知道##的连接作用
#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;
}
经过预处理器处理之后的代码为:
# 1 "main.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 31 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 32 "<command-line>" 2
# 1 "main.c"
int main()
{
int name1;
int name2;
name1=1;
name2=2;
printf("%d\n",name1);
printf("%d\n",name2);
return 0;
}
再来看一个实例程序:
//#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;
}
同样经过预处理器之后,程序为:
# 1 "main.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 31 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 32 "<command-line>" 2
# 1 "main.c"
typedef struct _tag_Student Student; struct _tag_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;
}
小结:
1.#运算符用于在预处理期间将宏参数转化为字符串。
2.##运算符用于在预处理器期间连接两个标识符
3.编译器不知道#和##运算符的存在
4.#和##运算符只在宏定义中有效。