Strange class statement...

While I was looking at an example of class operations I saw something I never seen before. Where the constructor for class Position is declared there is a “:x(xx),y(yy)” statement and I can’t understand what it is doing. I seems like it is using float x and y as functions, and this even before the “{}”. Please explain this statement, what it does, how it does it, what else strange statements like this you can do and if there is any where I can read more about this?

Code:

---

#include<iostream.h>

class Position {
        float x,y;
public:
        Position(float xx, float yy=0):x(xx),y(yy) {}
        Position operator + (const Position& e) const;
        Position operator - (const Position& e) const;
       
        void Display(void)
        { cout << "(" << x << ", " << y << ")" << endl; }
        void Input(void)
        { cout << "enter x och y:"; cin >> y;; }
};

Position Position::operator + (const Position& e) const
{
        Position res = *this;
        res.x += e.x;
        res.y += e.y;
        return (res);
}

Position Position::operator - (const Position& e) const
{
        Position res = *this;
        res.x -= e.x;
        res.y -= e.y;
        return (res);
}

---

Code:

---


class A
{
public:
  A() : x(0),y(0) {}
private:
  int x;
  int y;
};

class B
{
public:
  B() { x = 0; y=0; }
private:
  int x;
  int y;
};

---

are identical in meaning.