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

Posted 2023-02-18

【中文标题】qsort 给出 [错误]：从 `int (*)(cricketer*, cricketer*)\' 到 `int (*)(const void*, const void*)\' 的无效转换

【英文标题】：qsort gives [Error] : invalid conversion from `int (*)(cricketer*, cricketer*)' to `int (*)(const void*, const void*)'qsort 给出 [错误]：从 `int (*)(cricketer*, cricketer*)' 到 `int (*)(const void*, const void*)' 的无效转换

【发布时间】：2016-02-14 19:53:39

【问题描述】：

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：

[错误] C:\Users\Encoder\Documents\C-Free\Temp\Untitled3.cpp:29: 错误：从int (*)(cricketer*, cricketer*) 到的无效转换 int (*)(const void*, const void*)

[错误] C:\Users\Encoder\Documents\C-Free\Temp\Untitled3.cpp:29: 错误：初始化 `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;

【参考方案1】：

你传递给qsort的函数必须是

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

因为这是qsort 所期望的。然后在该功能中，您必须在开始时执行

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-> 访问

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

在reference查看示例

关于错误消息，这个 int (*)(cricketer*, cricketer*) 是一个指向函数的指针，该函数将两个指向 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*)，它告诉您它不能将前者转换为后者。另请注意，您如何需要指向 const 数据的指针，因为 sort 不应该修改数据。

【讨论】：

理想情况下，您希望在这些指针上保留 const 限定符。