将 void 指针转换为 long 类型，而不是 long 指针？

Question

 int cmpLong(void *a, void *b) {
    long aa = (long)a;
    long bb = (long)b;
    return aa - bb;
}

我不太明白这段代码的目的是比较什么？当一个 void 指针被转换为 long 时，它是一个任意数字还是它仍然具有某种意义？

我重写了这段代码，以便在 void *a 和 void *b 之间进行比较

这是我重写的代码:

int cmpLong(void *a, void *b) {
    // casting void pointers to long pointers
    long *aa = (long*)a;
    long *bb = (long*)b;
    return *aa - *bb;
}

这对我来说更有意义，但我不知道它是否在做与原始代码相同的事情。

Answer 1

这些完全不同。

 int cmpLong(void *a, void *b) {
    long aa = (long)a;
    long bb = (long)b;
    return aa - bb;
}

这是比较两个地址。它将返回两个地址值之间的字节数差异。为了理解这一点，让我们给他们一些数字并玩电脑。

a = 0x2000;
b = 0x1000;
long aa = (long)a; // So aa = 0x1000.  Casting only changes the type, not the value.
long bb = (long)b; // bb = 0x2000.
return aa - bb;    // 0x2000 - 0x1000 => returns 0x1000

现在让我们看看您的示例，它做了完全不同的事情。

int cmpLong(void *a, void *b) {
    // casting void pointers to long pointers
    long *aa = (long*)a;
    long *bb = (long*)b;
    return *aa - *bb;
}

玩同一个游戏:

a = 0x2000;
b = 0x1000;
long* aa = (long*)a;  // aa = 0x1000.  Casting still doesn't change the value.
long* bb = (long*)b;  // bb = 0x2000.
return *aa - *bb;     // *aa is the data at 0x1000.  This data is treated as a long value.
                      // *bb is the data at 0x2000, again treated as a long value.
                      // so *aa - *bb is the difference between the values at 0x2000 and 0x1000 when read as longs.