Question

Thanks for a solution in C, now I would like to achieve this in C++ using std::sort and vector:

typedef struct {   double x;   double y;   double alfa; } pkt; 

vector< pkt > wektor; filled up using push_back(); compare function:

int porownaj(const void *p_a, const void *p_b) {   pkt *pkt_a = (pkt *) p_a;   pkt *pkt_b = (pkt *) p_b;    if (pkt_a->alfa > pkt_b->alfa) return 1;   if (pkt_a->alfa < pkt_b->alfa) return -1;    if (pkt_a->x > pkt_b->x) return 1;   if (pkt_a->x < pkt_b->x) return -1;    return 0; }  sort(wektor.begin(), wektor.end(), porownaj); // this makes loads of errors on compile time 

What is to correct? How to use properly std::sort in that case?

Answer 1

std::sort takes a different compare function from that used in qsort. Instead of returning –1, 0 or 1, this function is expected to return a bool value indicating whether the first element is less than the second.

You have two possibilites: implement operator < for your objects; in that case, the default sort invocation without a third argument will work; or you can rewrite your above function to accomplish the same thing.

Notice that you have to use strong typing in the arguments.

Additionally, it’s good not to use a function here at all. Instead, use a function object. These benefit from inlining.

struct pkt_less {     bool operator ()(pkt const& a, pkt const& b) const {         if (a.alfa < b.alfa) return true;         if (a.alfa > b.alfa) return false;          if (a.x < b.x) return true;         if (a.x > b.x) return false;          return false;     } };  // Usage:  sort(wektor.begin(), wektor.end(), pkt_less());