qsort 给出 [错误]：从“int (*)(cricketer*, cricketer*)”到“int (*)(const void*, const void*)”的无效转换

Question

这是代码，它按平均运行次数对板球运动员的数据进行排序。 qsort 函数显示错误：

[Error] C:\Users\Encoder\Documents\C-Free\Temp\Untitled3.cpp:29: error: invalid conversion from int (*)(cricketer*, cricketer*) to int (*)(const void*, const void*)

[Error] C:\Users\Encoder\Documents\C-Free\Temp\Untitled3.cpp:29: error: initializing argument 4 of `void qsort(void*, size_t, size_t, int ()(const void, const void*))'

包括

#include<stdlib.h>
struct cricketer  //structure for details of cricketer
{
    int avg_run;
    char name[20];
    int age;
    int match_no;
} c[4];
int sort(struct cricketer *a, struct cricketer *b);   //pre-defining sort function 
int main()     //main function
{
    int i, s;
    for (i = 0; i < 3; i++)    //enumerating structure records.
    {
        printf("enter the name of cricketer ");
        fflush(stdin);
        gets(c[i].name);
        printf("enter his age,his no of matches and total average runs ");
        scanf("%d%d%d",&c[i].age, &c[i].match_no, &c[i].avg_run);
        }
    printf("records before sorting");  
    for (i = 0; i < 3; i++)
    {
        printf("\n\nname ");
        puts(c[i].name);
        printf("age %d\nno of matches %d\naverage runs %d\n",c[i].age, c[i].match_no, c[i].avg_run);
    }
    qsort(c, 3, sizeof(c[0]), sort); //sorting using qsort
    printf("\nrecords after sorting");
    for (i = 0; i < 3; i++)
    {
        printf("\n\nname ");
        puts(c[i].name);
        printf("age %d\nno of matches %d\naverage runs %d\n",c[i].age, c[i].match_no, c[i].avg_run);
    }
}
int sort(struct cricketer *a, struct cricketer *b)  //sort function definition
{
    if (a->avg_run == b->avg_run)
        return 0;
    else 
        if ( a->avg_run > b->avg_run)
            return 1;
        else 
            return -1;
}

Answer 1

你传递给qsort的函数指针必须是

int sort(const void* va, const void* vb);

因为那是 qsort 所期望的。然后在那个函数中你必须在开始时做

const struct cricketer *a = (struct cricketer*) va;
const struct cricketer *b = (struct cricketer*) vb;

或者如果您更喜欢使用点 . 而不是箭头 ->

const struct cricketer a = *(struct cricketer*) va;
const struct cricketer b = *(struct cricketer*) vb;

在此查看示例 reference

关于错误消息，此 int (*)(cricketer*, cricketer*) 是一个指向函数的指针，该函数获取 2 个指向 cricketer 的指针作为参数。编译器需要这样一个函数指针 int (*)(const void, const void*) 并且它告诉你它不能将前者转换为后者。还要注意您如何需要指向 const 数据的指针，因为排序不应该修改数据。