Less than operator for a Point class

[daviddoria]

I'm having trouble with this "strict weak ordering". Consider a class Point with members x and y. I tried to make the comparison "If both the x values are less than each other and the y values are less than each other, the Point's are less than each other". However, in the example below I create two points: (10,10) and (10,11) and the second insert fails.

Can someone please explain my blunder?

#include <iostream>
#include <set>

struct Point
{
  float x,y;
};

struct MyComparison
{
  bool operator()(const Point p1, const Point p2) const
  {
    if((p1.x < p2.x) && (p1.y < p2.y))
    {
      return true;
    }
    else
    {
      return false;
    }
  }
};

int main(int argc, char* argv[])
{
  typedef std::set<Point, MyComparison> SetType;
  SetType s;
  std::pair<SetType::iterator, bool> result;
  Point p1;
  p1.x=10;
  p1.y=10;
  result = s.insert(p1);
  std::cout << result.second << std::endl;
  
  Point p2;
  p2.x=10;
  p2.y=11;
  result = s.insert(p2);
  std::cout << result.second << std::endl;


  return 0;
}

Thanks,

David

[mrnutty]

It doesn't work because for an element n to be less than m, both x and y fields of n has to be lower than of m's( by your construct). You should test x first then y. Like so :

if(p1.x < p2.x) return true;
 else return p1.y < p2.y

[VernonDozier]

By your < operator, p1 isn't less than p2 and p2 isn't less than p1, so... they must be equal, so p2 cannot be inserted since it would be a duplicate.

[daviddoria]

Here is what I ended up with (I believe it is the same as what FirstPerson said):

struct MyComparison
{
  bool operator()(const Point p1, const Point p2) const
  {
    if((p1.x < p2.x))
    {
      return true;
    }

    if(p1.x == p2.x)
      {
      if(p1.y < p2.y)
        {
        return true;
        }
      }

    return false;
  }
};