Question

今天,我尝试使用窒息性,但我发现,最复杂的变量仍然可以修改,使我感到困惑。

下面是该守则,相比之下,(最无效的*a,*b)功能,我试图修改a指的是:

#include <stdio.h>
#include <stdlib.h>

int values[] = {40, 10, 100, 90, 20, 25};

int compare (const void *a, const void*b)
{
    *(int*)a=2;
/* Then the value that a points to will be changed! */
    return ( *(int*)a - *(int*)b);
}

int main ()
{
    int n;
    qsort(values, 6, sizeof(int), compare);
    for (n = 0; n < 6; n++)
        printf("%d ", values[n]);
    return 0;
}

然后,我也试图改变a的价值。本身:

#include <stdio.h>
#include <stdlib.h>

int values[] = {40, 10, 100, 90, 20, 25};

int compare (const void *a, const void*b)
{
    a=b;
    return ( *(int*)a - *(int*)b);
}

int main ()
{
    int n;
    qsort(values, 6, sizeof(int), compare);
    for (n = 0; n < 6; n++)
        printf("%d ", values[n]);
    return 0;
}

However, I found both of them works.. Can anyone explain to me why I need to use const in the parameter list of compare if they can still be changed?

Answer 1

案例1:你正在使用静态的 cast子来消除 con。您违反了该方法界定的合同。

案例2:你没有改变(主轴)的内容,而是分配包含一个最空白点的变数。

对实际影响: 如果有1起案件的话,你就可以在脚步中击.,如果真的看不到变。

建议: 只有当你知道你正在做些什么时,才摆脱混乱。

Answer 2

It only works in this case because the pointer you re working against wasn t originally constant. Casting away constness and then modifying a value is undefined behaviour. UB means that the app can do anything from succeed to crash to make purple dragons fly out of your nostrils.

Answer 3

Its protecting you from silly mistakes, not when you try hard to make a mistake.
(int *)a when a is const something * is a bad practice, use (const int *)a instead.
a = b when a is const void * is ok because only the value pointed to is const. if you want both *a = x and a = x disallowed, declare a as const void * const.

Answer 4

实际上。

int compare (const void *a, const void*b);

这里需要考虑两点,点人和点人所指的记忆地点。点人不是固定的,而是记忆地点。

如果您将签名改为:

int compare (const void *const a, const void *const void);

那么,所有东西都会合在一起。在你看来,你可以改变点,而不是价值。因此,贵点人可以指出不同的记忆点。

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