Use class in other class definition

i would like to use a class Point in an other class Rect.

class Point
{
    int x, y;
    public:
    Point (int px, int py){
        x = px;
        y = py;
    }
};

class Rect
{
    Point top_left; 
    Point bottom_right;
    public:
    Rect (Point p1, Point p2){
        top_left = p1;
        bottom_right = p2;
    }
};

error message is: "main.cpp:31:30: error: no matching function for call to ‘Rect::Point::Point()’". In my understanding the constructor method of the Rect class uses two parameters of type Point to instantiate a Rect object. I guess that i cannot use "Point" as type since it sounds to me as the compiler wants to call a function. The error message doesn’t help me so i hope you will. Thanks for that in advance.

>Solution :

Members are initialized before the body of the constructor runs. When you write:

Rect (Point p1, Point p2){
    top_left = p1;
    bottom_right = p2;
}

Then before the constructor is executed the members top_left and bottom_right are initialized. Because Point has no default constructor the members cannot be initialized.

To initialize members with a constructor you should use the member initializer list:

Rect (Point p1, Point p2) : top_left(p1), bottom_right(p2) { }

The error can be prevented also by providing default initializers for the members:

class Rect
{
    Point top_left{0,0}; 
    Point bottom_right{0,0};
    public:
    Rect (Point p1, Point p2){
        top_left = p1;
        bottom_right = p2;
    }
};

Or by providing a default constructor for Point. A default constructor is one that can be called without parameters. However, in any case you should prefer the member initializer list over assignment in the constructor body, because initialization + assignment is more expensive than initialization only.