I need to use std::qsort() to sort an arry of nd-Point. But I get an error:

no known conversion from 'function<int (const void *, const void )>' to '__compar_fn_t' (aka 'int ()(const void *, const void *)')`

How to solve it, or sort it by dir in another method?

#include <iostream>
#include <functional>

int d, n;
struct Point {
    int *x;
    Point() : x(new int[d]) {};
};

std::function<int(const void *, const void *)> make_cmp(int dir) {
    return [dir](const void *a, const void *b)->int {
        auto a_ = (Point *)a;
        auto b_ = (Point *)b;
        return a_->x[dir] - b_->x[dir];
    };
}

void sort_points(Point *value, int length, int dir) {
    std::qsort(value, length, sizeof(Point), make_cmp(dir));
}

CodePudding user response：

If you must use std::qsort then you are basically writing in C, not C . This might be one of the few situations where a global variable is the least bad approach.

static thread_local int dir;

int cmp(const void *a, const void *b) {
    auto a_ = (const Point *)a;
    auto b_ = (const Point *)b;
    return a_->x[dir] - b_->x[dir]; // beware of overflow
}

void sort_points(Point *value, int length, int d) {
    dir = d;
    std::qsort(value, length, sizeof(Point), cmp);
}

If this will never be called from more than one thread at a time, then you can drop the thread_local.

(Note that if the values might exceed the range [INT_MIN/2, INT_MAX/2] then you need to replace the subtraction with something that isn't vulnerable to overflow.)