C基础第25课--# 和 ## 操作符使用分析

学习自狄泰软件学院唐佐林老师C语言课程，文章中图片取自老师的PPT，仅用于个人笔记。

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实验1 ：#运算符分使用

25-1.c

#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;

}

gcc -E 25-1.c -o 25-1.i

单步编译，看预处理结果，此处提前删除 包含的库头文件

# 1 "25-1.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "25-1.c"




int main()
{

    printf("%s\n", "Hello world!");
    printf("%s\n", "100");
    printf("%s\n", "while");
    printf("%s\n", "return");

    return 0;
}

. 编译运行

mhr@ubuntu:~/work/C$ gcc 25-1.c
mhr@ubuntu:~/work/C$ ./a.out 
Hello world!
100
while
return
mhr@ubuntu:~/work/C$ 

实验2：#运算符分使用2

#include <stdio.h>

// 使用 逗号表达式 以及 #操作符 组合
#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;
}

gcc -E 25-2.c -o 25-2.i

单步编译，看预处理结果，此处提前删除 包含的库头文件

# 1 "25-2.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "25-2.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;
}

编译运行

mhr@ubuntu:~/work/C$ gcc 25-2.c
mhr@ubuntu:~/work/C$ 
mhr@ubuntu:~/work/C$ 
mhr@ubuntu:~/work/C$ ./a.out 
Call function square
result = 16
Call function func
result = 10
mhr@ubuntu:~/work/C$ 
mhr@ubuntu:~/work/C$ 

实验3： ## 操作符使用学习

#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;
}

gcc -E 25-3.c -o 25-3.i

单步编译，看预处理结果，此处提前删除 包含的库头文件

# 1 "25-3.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "25-3.c"




int main()
{

    int name1;
    int name2;

    name1 = 1;
    name2 = 2;

    printf("%d\n", name1);
    printf("%d\n", name2);

    return 0;
}

编译运行

mhr@ubuntu:~/work/C$ gcc 25-3.c
mhr@ubuntu:~/work/C$ ./a.out 
1
2
mhr@ubuntu:~/work/C$ 

实验4：##操作符实际使用
在工程中，有成千上万个结构体需要定义，但是每一次定义结构体变量的时候都要写 struct xxx xxx 很麻烦，可以用 typedef + ##操作符 组合使用 简化工程中 结构体的定义

#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("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;
}

gcc -E 25-4.c -o 25-4.i

单步编译，看预处理结果，此处提前删除 包含的库头文件

# 1 "25-4.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "25-4.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;
}

编译运行

mhr@ubuntu:~/work/C$ 
mhr@ubuntu:~/work/C$ gcc 25-4.c
mhr@ubuntu:~/work/C$ 
mhr@ubuntu:~/work/C$ ./a.out 
s1.name = s1
s1.id = 0
s2.name = s2
s2.id = 1
mhr@ubuntu:~/work/C$