本文学习自狄泰软件学院 唐佐林老师的 C语言进阶课程，图片全部来源于课程PPT，仅用于个人学习记录

宏定义的本质

实验-字面量比较

#define ERROR -1
#define PATH1 "D:\test\test.c"
#define PATH2 D:\test\test.c
#define PATH3 D:\test\
test.c

int main()
{
    int err = ERROR;
    char* p1 = PATH1;
    //char* p2 = PATH2;//error: 'D' undeclared (first use in this function)|
    //char* p3 = PATH3;//error: 'D' undeclared (first use in this function)|
}

宏定义表达式

实验-表达式有效性

#define _SUM_(a, b) (a) + (b)
#define _MIN_(a, b) ((a) < (b) ? (a) : (b))
#define _DIM_(a) sizeof(a)/sizeof(*a)


int main()
{
    int a = 1;
    int b = 2;
    int c[4] = {0};

    int s1 = _SUM_(a, b);(a)+(b)
    int s2 = _SUM_(a, b) * _SUM_(a, b);//为什么不是9??  (a)+(b)*(a)+(b)=1+2+2
    int m = _MIN_(a++, b);// 为啥不是1??  ((a++) < (b) ? (a++) : (b))
    int d = _DIM_(c);//求数组大小

     printf("s1 = %d\n", s1);
     printf("s2 = %d\n", s2);
     printf("m = %d\n", m);
     printf("d = %d\n", d);

    return 0;
}

/*output:
s1 = 3
s2 = 5
m = 2
d = 4

*/

宏的作用域

实验-作用域分析

#include <stdio.h>

void def()
{
    #define PI 3.1415926
    #define AREA(r) r * r * PI
}

double area(double r)
{
    return AREA(r);
}

int main()
{
    double r = area(5);

    printf("PI = %f\n", PI);
    printf("d = 5; a = %f\n", r);

    return 0;
}

/*output:
PI = 3.141593
d = 5; a = 78.539815

*/

内置宏

内置宏演示

#include <stdio.h>
#include <malloc.h>

#define MALLOC(type, x) (type*)malloc(sizeof(type)*x)

#define FREE(p) (free(p), p=NULL)

#define LOG(s) printf("[%s] {%s:%d} %s \n", __DATE__, __FILE__, __LINE__, s)

#define FOREACH(i, m) for(i=0; i<m; i++)
#define BEGIN {
#define END   }

int main()
{
    int x = 0;
    int* p = MALLOC(int, 5);

    LOG("Begin to run main code...");

    FOREACH(x, 5)
    BEGIN
        p[x] = x;
    END

    FOREACH(x, 5)
    BEGIN
        printf("%d\n", p[x]);
    END

    FREE(p);

    LOG("End");

    return 0;
}

/*output:
[Apr  4 2024] {D:\Users\cy\Test\test1.c:139} Begin to run main code...
0
1
2
3
4
[Apr  4 2024] {D:\Users\cy\Test\test1.c:153} End

*/

小结

预处理器直接对宏进行文本替换
宏使用时的参数不会进行求值和运算
预处理器不会对宏定义进行语法检查
宏定义时出现的语法错误只能被编译器检测
宏定义的效率高于函数调用
宏的使用会带来一定的副作用