std::find how does it work? operator==

Question

If I have a class

class Point
{
public:
  Point() {}
  Point(int _col, int _row) : row(_row), col(_col) {}
  int row, col;
};

how can I use std

Answer 1

Unless your types are PODs fundamental types, you will need to provide an equality function, member or not.

There are two fundamental versions of std::find, one that assumes an equality operator and the other uses an equality function you supply.

I recommend that you add operator== and operator< to any class that will be compared for equality or ordered.

Here's an updated version of your class:

class Point
{
  int x;  // These are private by default.
  int y; 
  public:  
    Point(int new_x, int new_y) : x(new_x), y(new_y)
    { ; }
    bool operator==(const Point& p) const
    {
      return (x == p.x) && (y == p.y);
    }
};

The member method operator== allows comparison without exposing the values to friends or the public.

If you want to use a free standing comparison function, you will need to either make the values public or make the function a friend:

class Point
{
  int x;  // These are private by default.
  int y; 
  public:  
    Point(int new_x, int new_y) : x(new_x), y(new_y)
    { ; }
    friend bool operator==(const Point& a, const Point& b);
};

bool operator==(const Point& a, const Point& b)
{
    return (a.x == b.x) && (a.y == b.y);
}

If you want to use the free standing function with std::find, the example would be:

std::vector point_container;
//...
Point p;
std::vector::const_iterator iter;
iter = std::find(point_container.begin(), point_container.end(),
                 p,
                 Equal_Points);

Where Equal_Points is a free standing function that can compare the members of two Points.