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1. 回调函数是什么？

回调函数就是⼀个通过函数指针调⽤的函数。
如果你把函数的指针（地址）作为参数传递给另⼀个函数，当这个指针被⽤来调⽤其所指向的函数时，被调⽤的函数就是回调函数。回调函数不是由该函数的实现方直接调用，⽽是在特定的事件或条件发生时由另外的⼀⽅调用的，用于对该事件或条件进行响应。
深入理解指针（4）中我们写的计算器的⼀般实现代码中是重复出现的，其中虽然执⾏计算的逻辑是区别的，但是输⼊输出操作是冗余的，有没有办法，简化⼀些呢？
只有调⽤函数的逻辑是有差异的，我们可以把调⽤的函数的地址以参数的形式传递过去，使⽤函数指针接收，函数指针指向什么函数就调⽤什么函数，这⾥其实使⽤的就是回调函数的功能。

#include <stdio.h>//使用回调函数改造前
int Add(int x, int y)
{return x + y;
}int Sub(int x, int y)
{return x - y;
}int Mul(int x, int y)
{return x * y;
}int Div(int x, int y)
{return x / y;
}void eum()
{printf("*******************************\n");printf("***1. add   2. sub   **********\n");printf("***3. mul   4. div   **********\n");printf("***     0. exit     **********\n");printf("*******************************\n");
}//简易的计算器实现
int main()
{int input = 0;int x = 0;int y = 0;int ret = 0;do{eum();printf("请选择:>");scanf("%d", &input);switch (input){case 1:printf("请输入两个操作数:>");scanf("%d %d", &x, &y);ret = Add(x, y);printf("%d\n", ret);break;case 2:printf("请输入两个操作数:>\n");scanf("%d %d", &x, &y);ret = Sub(x, y);printf("%d\n", ret);break;case 3:printf("请输入两个操作数:>\n");scanf("%d %d", &x, &y);ret = Mul(x, y);printf("%d\n", ret);break;case 4:printf("请输入两个操作数:>\n");scanf("%d %d", &x, &y);ret = Div(x, y);printf("%d\n", ret);break;case 0:printf("退出程序\n");break;default:printf("输入错误请重新输入\n");break;}} while (input);return 0;
}

1.1 使用回调函数修改

//使用回调函数改造后
int Add(int x, int y)
{return x + y;
}int Sub(int x, int y)
{return x - y;
}int Mul(int x, int y)
{return x * y;
}int Div(int x, int y)
{return x / y;
}void eum()
{printf("*******************************\n");printf("***1. add   2. sub   **********\n");printf("***3. mul   4. div   **********\n");printf("***     0. exit     **********\n");printf("*******************************\n");
}void Clcs(int (*pf)(int, int))
{int x = 0;int y = 0;int ret = 0;printf("输入两个操作数:>");scanf("%d %d", &x, &y);ret = (*pf)(x, y);printf("%d\n", ret);
}int main()
{int input = 0;int x = 0;int y = 0;int ret = 0;do{eum();printf("请选择:>");scanf("%d", &input);switch (input){case 1:Clcs(Add);break;case 2:Clcs(Sub);break;case 3:Clcs(Mul);break;case 4:Clcs(Div);break;case 0:printf("退出程序\n");break;default:printf("输入错误请重新输入\n");break;}} while (input);return 0;
}

2. qsort使⽤举例

C/C++函数介绍: https://legacy.cplusplus.com/
使用说明

这里的头文件是#include <stdlib.h>

2.1 使⽤qsort函数排序整型数

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int cmp_int(const void* p1, const void* p2)
{return (*(int*)p1) - (*(int*)p2);
}void print1(int* p, int sz)
{for (int i = 0; i < sz; i++){printf("%d ", *(p + i));}
}//测试qsort排序整型结构体
void test1()
{int arr[10] = { 9,8,7,6,5,4,3,2,1,0 };int sz = sizeof(arr) / sizeof(arr[0]);//默认是升序的qsort(arr, sz, sizeof(arr[0]), cmp_int);print1(arr, sz);
}int main()
{test1();//打印整型数组return 0;
}

运行结果：

2.2 使⽤qsort排序结构数据按年龄排序

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Stu
{char name[20];int age;
};int cmp_int(const void* p1, const void* p2)
{return (*(int*)p1) - (*(int*)p2);
}cmp_stu_by_age(const void* p1, const void* p2)
{return ((struct Stu*)p1)->age - ((struct Stu*)p2)->age;
}test2()
{struct Stu arr[] = { {"zhangsan",20},{"lisi",50},{"wangwu",18} };int sz = sizeof(arr) / sizeof(arr[0]);qsort(arr, sz, sizeof(arr[0]), cmp_stu_by_age);print2(arr, sz);
}//打印结构体age
print2(struct Stu* p, int sz)
{for (int i = 0; i < sz; i++){printf("%d ", (p + i)->age);}
}int main()
{test2();//打印结构体agereturn 0;
}

运行结果：

2.3 使⽤qsort排序结构数据按名字排序

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Stu
{char name[20];int age;
};int cmp_stu_by_name(const void* p1, const void* p2)
{return ((struct Stu*)p1)->name - ((struct Stu*)p2)->name;
}void print3(int* p, int sz)
{for (int i = 0; i < sz; i++){printf("%s ", ((struct Stu*)p + i)->name);}
}test3()
{struct Stu arr[3] = { {"zhangsan",18},{"wangwu",30},{"lisi",20} };int sz = sizeof(arr) / sizeof(arr[0]);qsort(arr, 3, sizeof(arr[0]), cmp_stu_by_name);print3(arr, sz);
}//qsort使用
int main()
{	test3();//打印结构体namereturn 0;
}

运行结果：

2.4整体代码

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int cmp_int(const void* a, const void* b)
{return *((int*)a) - *((int*)b);
}void print1(int arr[], int sz)
{for (int i = 0; i < sz; i++){printf("%d ", arr[i]);}
}test1()
{int arr[] = { 9,8,7,6,5,4,3,2,1,0 };int sz = sizeof(arr) / sizeof(arr[0]);//默认是升序的qsort(arr, sz, sizeof(arr[0]), cmp_int);print1(arr, sz);
}struct Stu
{char name[20];int age;
};int cmp_stu_by_age(const void* p1, const void* p2)
{return ((struct Stu*)p1)->age - ((struct Stu*)p2)->age;
}void print2(int* p, int sz)
{for (int i = 0; i < sz; i++){printf("%d ", ((struct Stu*)p + i)->age);}
}test2()
{struct Stu arr[3] = {{"zhangsan",18},{"wangwu",30},{"lisi",20}};int sz = sizeof(arr) / sizeof(arr[0]);qsort(arr, 3, sizeof(arr[0]), cmp_stu_by_age);print2(arr, sz);
}int cmp_stu_by_name(const void* p1, const void* p2)
{return ((struct Stu*)p1)->name - ((struct Stu*)p2)->name;
}void print3(int* p, int sz)
{for (int i = 0; i < sz; i++){printf("%s ", ((struct Stu*)p + i)->name);}
}test3()
{struct Stu arr[3] = { {"zhangsan",18},{"wangwu",30},{"lisi",20} };int sz = sizeof(arr) / sizeof(arr[0]);qsort(arr, 3, sizeof(arr[0]), cmp_stu_by_name);print3(arr, sz);
}//qsort使用
int main()
{	test1();//打印整型数组//test2();//打印结构体age//test3();//打印结构体namereturn 0;
}

3. qsort函数的模拟实现

使⽤回调函数，模拟实现qsort（采⽤冒泡的⽅式）。
注意：这⾥使⽤的是 void$\ast$ 的指针

3.1 整型数组的实现

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
int int_cmp(const void* p1, const void* p2)
{return (*(int*)p1 - *(int*)p2);
}void Swap(char* buf1, char* buf2, int size)
{for (int i = 0; i < size; i++){char tmp = *buf1;*buf1 = *buf2;*buf2 = tmp;buf1++;buf2++;}
}
void bubble_sort(void* base, int num, int size, int(*cmp)(const void*, const void*))
{//趟数for (int i = 0; i < num - 1; i++){//一趟内部比较的对数for (int j = 0; j < num - 1 - i; j++){//假设需要升序cmp返回>0，交换if (cmp((char*)base + j * size, (char*)base + (j + 1) * size) > 0)//两个元素比较{//交换Swap((char*)base + j * size, (char*)base + (j + 1) * size, size);}}}
}void print1(int* arr, int sz)
{for (int i = 0; i < sz; i++){printf("%d ", *(arr + i));}
}void test1()
{int arr[] = { 1, 3, 5, 7, 9, 2, 4, 6, 8, 0 };int sz = sizeof(arr) / sizeof(arr[0]);bubble_sort(arr, sz, sizeof(int), int_cmp);print1(arr, sz);
}int main()
{test1();return 0;
}

运行结果：

3.2 结构体按名字排序实现

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
void Swap(char* buf1, char* buf2, int size)
{for (int i = 0; i < size; i++){char tmp = *buf1;*buf1 = *buf2;*buf2 = tmp;buf1++;buf2++;}
}void bubble_sort(void* base, int num, int size, int(*cmp)(const void*, const void*))
{//趟数for (int i = 0; i < num - 1; i++){//一趟内部比较的对数for (int j = 0; j < num - 1 - i; j++){//假设需要升序cmp返回>0，交换if (cmp((char*)base + j * size, (char*)base + (j + 1) * size) > 0)//两个元素比较{//交换Swap((char*)base + j * size, (char*)base + (j + 1) * size, size);}}}
}struct Stu
{int age;char name[10];
};void com_name(const void* p1, const void* p2)
{return ((struct Stu*)p1)->name - ((struct Stu*)p2)->name;
}void print2(struct Stu* p1, int sz)
{for (int i = 0; i < sz; i++){printf("%s ", (p1 + i)->name);}
}void test2()
{struct Stu arr[] = { {18,"zhangsan"},{30,"lisi"},{20,"wangwu"}};int sz = sizeof(arr) / sizeof(arr[0]);bubble_sort(arr, sz, sizeof(arr[0]), com_name);print2(arr, sz);
}//qsort实现
int main()
{test2();//结构体打印return 0;
}

运行结果：

3.3 结构体按年龄排序实现

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
void Swap(char* buf1, char* buf2, int size)
{for (int i = 0; i < size; i++){char tmp = *buf1;*buf1 = *buf2;*buf2 = tmp;buf1++;buf2++;}
}void bubble_sort(void* base, int num, int size, int(*cmp)(const void*, const void*))
{//趟数for (int i = 0; i < num - 1; i++){//一趟内部比较的对数for (int j = 0; j < num - 1 - i; j++){//假设需要升序cmp返回>0，交换if (cmp((char*)base + j * size, (char*)base + (j + 1) * size) > 0)//两个元素比较{//交换Swap((char*)base + j * size, (char*)base + (j + 1) * size, size);}}}
}struct Stu
{int age;char name[10];
};
//按年龄排序
void com_age(const void* p1, const void* p2)
{return ((struct Stu*)p1)->age - ((struct Stu*)p2)->age;
}
//按年龄打印
void print3(struct Stu* p1, int sz)
{for (int i = 0; i < sz; i++){printf("%d ", (p1 + i)->age);}
}void test3()
{struct Stu arr[] = { {18,"zhangsan"},{30,"lisi"},{20,"wangwu"} };int sz = sizeof(arr) / sizeof(arr[0]);bubble_sort(arr, sz, sizeof(arr[0]), com_age);print3(arr, sz);
}//qsort实现
int main()
{test3();//结构体打印agereturn 0;
}

运行结果：

3.4 整体代码

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
int int_cmp(const void* p1, const void* p2)
{return *((int*)p1) - *((int*)p2);
}void Swap(char* buf1, char* buf2, int size)
{for (int i = 0; i < size; i++){char tmp = *buf1;*buf1 = *buf2;*buf2 = tmp;buf1++;buf2++;}
}void bubble_sort(void* base, int num, int size, int(*cmp)(const void*, const void*))
{//趟数for (int i = 0; i < num - 1; i++){//一趟内部比较的对数for (int j = 0; j < num - 1 - i; j++){//假设需要升序cmp返回>0，交换if (cmp((char*)base + j * size, (char*)base + (j + 1) * size) > 0)//两个元素比较{//交换Swap((char*)base + j * size, (char*)base + (j + 1) * size, size);}}}
}void print1(int* arr, int sz)
{for (int i = 0; i < sz; i++){printf("%d ", *(arr + i));}
}void test1()
{int arr[] = { 7,9,3,5,4,6,2,1,0,8 };int sz = sizeof(arr) / sizeof(arr[0]);bubble_sort(arr, sz, sizeof(arr[0]), int_cmp);print1(arr, sz);
}struct Stu  //学生
{int age;//年龄char name[10];//名字
};
//假设按照名字来⽐较
void com_name(const void* p1, const void* p2)
{return ((struct Stu*)p1)->name - ((struct Stu*)p2)->name;
}void print2(struct Stu* p1, int sz)
{for (int i = 0; i < sz; i++){printf("%s ", (p1 + i)->name);}
}
//按照名字来排序
void test2()
{struct Stu arr[] = { {18,"zhangsan"},{30,"lisi"},{20,"wangwu"}};int sz = sizeof(arr) / sizeof(arr[0]);bubble_sort(arr, sz, sizeof(arr[0]), com_name);print2(arr, sz);
}void com_age(const void* p1, const void* p2)
{return ((struct Stu*)p1)->age - ((struct Stu*)p2)->age;
}void print3(struct Stu* p1, int sz)
{for (int i = 0; i < sz; i++){printf("%d ", (p1 + i)->age);}
}void test3()
{struct Stu arr[] = { {18,"zhangsan"},{30,"lisi"},{20,"wangwu"} };int sz = sizeof(arr) / sizeof(arr[0]);bubble_sort(arr, sz, sizeof(arr[0]), com_age);print3(arr, sz);
}//qsort实现
int main()
{//test1();//整型打印//test2();//结构体打印test3();return 0;
}