Help Understanding Classes

I am reading "Tech Yourself C++ in 21 Days." In the book is the following code (it takes the form of two listings, one is a header file and the other actual code including the header file)

Rect.hpp

Code:

1:   // Begin Rect.hpp
2:   #include <iostream.h>
3:   class Point     // holds x,y coordinates
4:   {
5:      // no constructor, use default
6:      public:
7:         void SetX(int x) { itsX = x; }
8:         void SetY(int y) { itsY = y; }
9:         int GetX()const { return itsX;}
10:        int GetY()const { return itsY;}
11:     private:
12:        int itsX;
13:        int itsY;
14:  };    // end of Point class declaration
15:
16:
17:  class  Rectangle
18:  {
19:     public:
20:        Rectangle (int top, int left, int bottom, int right);
21:        ~Rectangle () {}
22:
23:        int GetTop() const { return itsTop; }
24:        int GetLeft() const { return itsLeft; }
25:        int GetBottom() const { return itsBottom; }
26:        int GetRight() const { return itsRight; }
27:
28:        Point  GetUpperLeft() const { return itsUpperLeft; }
29:        Point  GetLowerLeft() const { return itsLowerLeft; }
30:        Point  GetUpperRight() const { return itsUpperRight; }
31:        Point  GetLowerRight() const { return itsLowerRight; }
32:
33:        void SetUpperLeft(Point Location)  {itsUpperLeft = Location;}
34:        void SetLowerLeft(Point Location)  {itsLowerLeft = Location;}
35:        void SetUpperRight(Point Location)  {itsUpperRight = Location;}
36:        void SetLowerRight(Point Location)  {itsLowerRight = Location;}
37:
38:        void SetTop(int top) { itsTop = top; }
39:        void SetLeft (int left) { itsLeft = left; }
40:        void SetBottom (int bottom) { itsBottom = bottom; }
41:        void SetRight (int right) { itsRight = right; }
42:
43:        int GetArea() const;
44:
45:     private:
46:        Point  itsUpperLeft;
47:        Point  itsUpperRight;
48:        Point  itsLowerLeft;
49:        Point  itsLowerRight;
50:        int    itsTop;
51:        int    itsLeft;
52:        int    itsBottom;
53:        int    itsRight;
54:  };
55: // end Rect.hpp

Rect.cpp

Code:

1:   // Begin rect.cpp
2:   #include "rect.hpp"
3:   Rectangle::Rectangle(int top, int left, int bottom, int right)
4:   {
5:         itsTop = top;
6:         itsLeft = left;
7:         itsBottom = bottom;
8:         itsRight = right;
9:
10:        itsUpperLeft.SetX(left);
11:        itsUpperLeft.SetY(top);
12:
13:        itsUpperRight.SetX(right);
14:        itsUpperRight.SetY(top);
15:
16:        itsLowerLeft.SetX(left);
17:        itsLowerLeft.SetY(bottom);
18:
19:        itsLowerRight.SetX(right);
20:        itsLowerRight.SetY(bottom);
21:  }
22:
23:
24:  // compute area of the rectangle by finding corners,
25:  // establish width and height and then multiply
26:  int Rectangle::GetArea() const
27:  {
28:        int Width = itsRight-itsLeft;
29:        int Height = itsTop - itsBottom;
30:        return (Width * Height);
31:  }
32:
33:  int main()
34:  {
35:        //initialize a local Rectangle variable
36:        Rectangle MyRectangle (100, 20, 50, 80 );
37:
38:        int Area = MyRectangle.GetArea();
39:
40:        cout << "Area: " << Area << "\n";
41:        cout << "Upper Left X Coordinate: ";
42:        cout << MyRectangle.GetUpperLeft().GetX();
43:      return 0;
44: }

That is the exact code from the book. Anyway, I am pretty confused by how it is using the Point class...

Thank you.

Note that the Point class has x and y member variables.
A Point object must be seen as a co-ordinate.
A rectangle has four corners, so the Rectangle class must have has four Points/Co-ordinates, top-left, top-right, bottom-left, bottom-right.

I hope this helps.

What about on line 28-31 on the header file - why is it prefixed by 'Point' ? Is it using the point class for something I cannot see?

Oh, because all of the function from line 28-31 return private member variables that are Point - why are they using Point?

No.

Point GetUpperLeft() const { return itsUpperLeft; }
Point GetLowerLeft() const { return itsLowerLeft; }
Point GetUpperRight() const { return itsUpperRight; }
Point GetLowerRight() const { return itsLowerRight; }

These functions return a Point object, so that you'd have access to the appropriate x and y co-ordinate.

Eg.

Code:

Rectangle one;
cout << one.GetUpperLeft().GetX();

That would print the x-coordinate of the upper-left coner of the rectangle.

This code is just for illustration. Don't do stuff like that because the variables have not been initialized.

Ahh! I finaly get it now!!

I feel a great overwhelming power of joy!



Thanks The Dog!

Anytime!

btw you might wanna get yourself another book.

I've noticed that the teach yourself in 21 day series is a bit crappy for beginners.

Just a suggestion.

Suggestion noted

Any suggestion on waht book - because I actually am thinking of getting one.

Here's one:

C++ From the Beginning by Jan Skansholm

Do a google search, you're bound to find something.