Question

I have a vector filled with some vertex object instances and need to sort it, according to its ‘x’ and after it its ‘y’ coordinate.

vertex.h

#ifndef VERTEX_H
#define VERTEX_H 1

class Vertex
{
private:
  double __x;
  double __y;
public:
  Vertex(const double x, const double y);
  bool operator<(const Vertex &b) const;
  double x(void);
  double y(void);
};

#endif // VERTEX_H

vertex.cpp

#include "vertex.h"

Vertex::Vertex(const double x, const double y) : __x(x), __y(y)
{
}

bool Vertex::operator<(const Vertex &b) const
{
  return __x < b.x() || (__x == b.x() && __y < b.y());
}

double Vertex::x(void)
{
  return __x;
}

double Vertex::y(void)
{
  return __y;
}

run.cpp

#include <algorithm>
#include <stdio.h>
#include <vector>

#include "vertex.h"

void prnt(std::vector<Vertex *> list)
{
  for(size_t i = 0; i < list.size(); i++)
    printf("Vertex (x: %.2lf y: %.2lf)\n", list[i]->x(), list[i]->y());
}

int main(int argc, char **argv)
{
  std::vector<Vertex *> list;
  list.push_back(new Vertex(0, 0));
  list.push_back(new Vertex(-3, 0.3));
  list.push_back(new Vertex(-3, -0.1));
  list.push_back(new Vertex(3.3, 0));

  printf("Original:\n");
  prnt(list);

  printf("Sorted:\n");
  std::sort(list.begin(), list.end());

  prnt(list);

  return 0;
}

What I expect as output is:

Original:
Vertex (x: 0.00 y: 0.00)
Vertex (x: -3.00 y: 0.30)
Vertex (x: -3.00 y: -0.10)
Vertex (x: 3.30 y: 0.00)
Sorted:
Vertex (x: -3.00 y: -0.10)
Vertex (x: -3.00 y: 0.30)
Vertex (x: 0.00 y: 0.00)
Vertex (x: 3.30 y: 0.00)

But what I actually get is:

Original:
Vertex (x: 0.00 y: 0.00)
Vertex (x: -3.00 y: 0.30)
Vertex (x: -3.00 y: -0.10)
Vertex (x: 3.30 y: 0.00)
Sorted:
Vertex (x: 0.00 y: 0.00)
Vertex (x: -3.00 y: -0.10)
Vertex (x: -3.00 y: 0.30)
Vertex (x: 3.30 y: 0.00)

I don’t know what exactly is going wrong, any idea?

Answer 1

You are storing Vertex * in the container not Vertex. When you call std::sort, you’re actually sorting the value of the pointers, not the items themselves.

If you really need to be storing pointers (which I doubt), you can use a workaround like this (untested):

struct less_than_key {
    inline bool operator() (const Vertex*& v1, const Vertex*& v2) {
        return ((*v1) < (*v2));
    }
};
std::sort(list.begin(), list.end(), less_than_key());