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> point_container;
//...
Point p;
std::vector<Point>::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.

Answer 2

The C++ standard (draft N3242) says (in section 25.2.5 [alg.find]) that std::find:

Returns: The first iterator i in the range [first,last) for which the following corresponding conditions hold: *i == value[...]. Returns last if no such iterator is found.

Your question of whether it will search based on the value or the address of the object depends on how operator== is implemented. The simple answer is: std::find will return an iterator to the object for which operator== returned true.

Usually, this will just be a value-based comparison (because operator== is usually implemented to compare the values of two objects), and so you should generally expect std::find to search the range for the value you've provided (not the address of the object you provided).

It's possible for operator== to be implemented such that it compares based on address, like so:

bool operator==(const Point& left, const Point& right) {
    return &left == &right;
}

Using this operator== will compare addresses, and so std::find will search for an object that has the same address as the one you've provided. It's generally a bad idea to implement operator== like this, though. Most people would implement operator== like so:

bool operator==(const Point& left, const Point& right) {
    return left.row == right.row && left.col == right.col;
}

which, when used with std::find, will compare Points based on their values.