[英]qsort-C standard library-how to send a compare function with non const args?
Im working on a homework project. 我正在做一个家庭作业项目。 I need to implement a generic List accoarding to a header that the staff created.
我需要实现一个符合员工创建的标题的通用列表。 They had defined compare functions in the header like this:
他们在标题中定义了比较函数,如下所示:
typedef int(*CompareListElements)(ListElement, ListElement);
(ListElement=void*) (ListElement =无效*)
I am trying to use qsort from the standard library, but I can't compile the program because the compare function I'm trying to pass to qsort
is getting non const arguments. 我试图使用标准库中的qsort,但我无法编译程序,因为我试图传递给
qsort
的compare函数获得非const参数。 Is there a way to pass it anyway? 有没有办法传递它? Thanks ahead.
谢谢你。
The function im trying to implement: 我试图实现的功能:
ListResult listSort(List list, CompareListElements compareElement) {
CHECK_RETURN(((list == NULL)||(compareElement == NULL)),LIST_NULL_ARGUMENT);
int size=0;
ListElement* elementArray=listToArray(list, &size);
CHECK_RETURN((elementArray == NULL), LIST_OUT_OF_MEMORY);
qsort(elementArray, size, sizeof(*elementArray), compareElement);
returnArrayToList(list,elementArray);
return LIST_SUCCESS;
}
The error: passing argument 4 of 'qsort' from incompatible pointer type 错误:从不兼容的指针类型传递'qsort'的参数4
the compare function I am trying to use qsort from the standard library
比较函数我试图使用标准库中的qsort
Not nice, try to avoid this: 不好,试着避免这个:
Just wrap up the compare function into another function that is accepted by the compiler to be passed to qsort()
. 只需将compare函数包装到编译器接受的另一个函数中,然后传递给
qsort()
。
Inside the wrapper you can then use the Casting-Hammer to fit things ... ;-) 在包装内,你可以使用Casting-Hammer来装配东西...... ;-)
#include <stdlib.h>
typedef void* ListElement;
typedef int(*CompareListElements)(ListElement, ListElement);
CompareListElements c = ... /* Initialise properly here. */
int cmp(const void * pv1, const void * pv2)
{
return c((ListElement)pv1, (ListElement)pv2); /* Hammering here ... */
}
int main(void)
{
int a[4] = {3, 0, 2, 1};
qsort(a, sizeof *a, 4, cmp);
}
Referring your edit providing the implementation of listSort()
, you could still use the workaround I propose above by using the GNU extension function qsort_r()
: 参考你的编辑提供
listSort()
的实现,你仍然可以使用我在上面提出的解决方法,使用GNU扩展函数qsort_r()
:
int cmp_r(const void * pv1, const void * pv2, void * pv)
{
CompareListElements c_r = pv;
return c_r((ListElement)pv1, (ListElement)pv2); /* Still hammering here ... */
}
ListResult listSort(List list, CompareListElements compareElement)
{
...
qsort_r(elementArray, size, sizeof(*elementArray), cmp_r, compareElement);
...
}
This example shows how to use the arguments in compare()
provided for qsort
. 此示例显示如何使用为
qsort
提供的compare()
的参数。 It casts the const void*
pointer to the type you are comparing. 它将
const void*
指针强制转换为您要比较的类型。 Note that I could have returned the simple difference between the two numbers, but in some instances that can cause undefined behaviour when the difference "wraps". 请注意,我本可以返回两个数字之间的简单差异,但在某些情况下,当差异“包装”时,可能会导致未定义的行为。
You said little about your linked list, so I stuck to the question title, about qsort()
. 你对你的链表很少说,所以我坚持问题标题,关于
qsort()
。 The reason it takes const
pointers is to prevent your compare()
function from altering the data directly: it returns a value based on reading the data values. 它需要
const
指针的原因是为了防止你的compare()
函数直接改变数据:它根据读取数据值返回一个值。 Obviously the data itself cannot be const
otherwise qsort()
won't be able to manipulate it. 显然数据本身不能是
const
否则qsort()
将无法操纵它。
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define ARRSIZE 10
struct numarr {
int numb;
struct numarr *next;
};
struct numarr array [ARRSIZE];
int compare( const void *arg1, const void *arg2 )
{
if (((struct numarr*)arg1)->numb > ((struct numarr*)arg2)->numb)
return 1;
if (((struct numarr*)arg1)->numb < ((struct numarr*)arg2)->numb)
return -1;
return 0;
}
int main() {
int i;
srand((unsigned)time(NULL));
// set up a random array
printf ("Unsorted: ");
for(i=0; i<ARRSIZE; i++){
array[i].numb = rand() % ARRSIZE;
printf ("%d ", array[i].numb);
}
printf ("%\n");
// sort and print the array
qsort (array, ARRSIZE, sizeof(struct numarr), compare);
printf ("Sorted: ");
for(i=0; i<ARRSIZE; i++){
printf ("%d ", array[i].numb);
}
printf ("%\n");
return 0;
}
Program output 程序输出
Unsorted: 8 7 3 0 7 5 9 2 0 0
Sorted: 0 0 0 2 3 5 7 7 8 9
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