cpp中指针的类型转换

Question

Can anybody tell me what's going on with the following code:

int compareX(const void* a, const void* b)  
 {  
    Point *p1 = (Point *)a, *p2 = (Point *)b;  
    return (p1->x - p2->x);  
 }

I have taken this code from geeks for geeks, the closest pair of points . can anyone explain to me the similar code in a much simpler and easy way with line by line explanation

also, I am unable to understand this piece of code as well:

float bruteForce(Point P[], int n)  
{  
    float min = FLT_MAX;
    for (int i = 0; i < n; ++i)  
        for (int j = i+1; j < n; ++j)  
            if (dist(P[i], P[j]) < min)  
                min = dist(P[i], P[j]);  
    return min;  
}  

What's the use of FLT_MAX here?

Answer 1

Point *p1 = (Point *)a, *p2 = (Point *)b;

This line is initialising two variables in the same line. Like

int a = 2, b = 3;

For pointers, the * is kept near the variable. The following will give you an int * and an int :

 int *a = nullptr, b = 2;

https://cdecl.org

---

what's the use of FLT_MAX here?

Everything is smaller than FLT_MAX . So this is safe to check against in the first comparison for the minimum number.

Answer 2

FLT_MAX是这里定义的常量，如果用于查找数组中所有点对之间距离最小的点的逻辑，使用 FLT_MAX 设置变量 min 的想法是定义可以超过的阈值限制值通过第一次 2 点的第一次比较...

Answer 3

The compareX function in your first code snippet has the signature required if it is to be passed as the comparator in a call to the standard qsort() function (or other, similar sorting routines provided by the Standard Library). The two pointer arguments must be given as const void* types.

However, when that function is used, the code will actually be passing pointers to Point objects, so the function must first explicitly cast them as such, so that they can be dereferenced in order to do the comparison of their respective x members.

Note: Although using "C-Style" casts will work here, many would frown on such usage. It is, perhaps, more in keeping with "Modern C++" to use static_cast operations:

int compareX(const void* a, const void* b)
{
    const Point *p1 = static_cast<const Point*>(a), *p2 = static_cast<const Point*>(b);
    return (p1->x - p2->x);
}