Question

So suppose I have a class like this one:

class Point
{
   private:
      int x, y;
   public:
      void setX(int arg_x) { x = arg_x; }
      void sety(int arg_y) { y = arg_y; }
      int getX() const { return x; }
      int gety() const { return y; }
};

Now I want to have a map like this one:

map<Point, Point> m;

But I need a third parameter. I read in cplusplus that this third parameter is to compare something, but I didn’t understand what that something was. Can anyone explain that for me?

Answer 1

You can extend your class with such a method if you don’t need a separate compare function

class Point
{
   private:
      int x, y;
   public:

      bool operator<( const Point& other) const
      {
          if ( x == other.x )
          {
              return y < other.y;
          }

          return x < other.x;
      }
};

By default the stl map orders all elements in it by some notion of ordering. In this case this operator is used. Sometimes you dont have control over the Point class or you might want to use it in two different maps each defines its own ordering. For example one map might sort points by x first and other one might sort by y first. So it might be helpful if the comparison operator is independent of the class Point. You can do something like this.

class Point
{
   public:
      int x, y;
};


struct PointComparer
{
    bool operator()( const Point& first , const Point& second) const
    {
        if ( first.x == second.x )
        {
            return first.y < second.y;
        }

        return first.x < second.x;
    }
};

map<Point, Point , PointComparer> m;