Comp 2673, Spring 2002

April 19, lecture notes



Reading:
     Begin reading D&D, chapter 9

Now we begin learning new features of the C++ language
    - Inheritance and polymorphism are two important capabilities of
      object-oriented programming.
    - The idea of inheritance is to create new classes out of existing classes.
    - If you want to use an existing class, but it's missing some attributes
      or behaviors that you need, you can create a new class by starting with
      the original one, and embellishing it with new attributes and behaviors
      or overriding existing behaviors.
    - The original class (that you build on top of) is called the base class,
      and the new one is called the derived class.
    - A derive class is a class in its own right, so you can derive yet a new
      class from it.  This gives a hierarchy of classes.
    - An example that you've already seen is ostream and ofstream.  ofstream
      is derived from the base class ostream.  This means that an ofstream
      object is a special case of an ostream object.  (In fact, there is
      a hierarchy of classes that ostream and ofstream are part of - see
      page 761 of D&D for more details on this.)
    - You can derive a class from multiple classes at once.  This is called
      multiple inheritance and we will not study it.

Example and syntax:

// base class
Class Shape {
public:
    Shape(float x=0, float y=0);
    Shape(const Shape &);
    void display() const;
    float get_xpos() const {return ypos;}
    float get_ypos() const {return xpos;}
    void move(float x_trans, float y_trans);  // translate by given distance
    void resize(float scale);  // changes size by factor scale
protected:
    float x_pos;  // x coordinate of the center of the shape
    float y_pos;  // y coordinate of the center of the shape
};

Class Square : public Shape {
public:
   Square(float s=0, float x=0, float y=0);
   Square(const Square &);
   void set_side(float s) {if (s >= 0) side = s; else side=0;}
   float get_side() const {return side;}
   float area() const {return side*side;}
   float perimeter() const {return 4*side};
private:
   float side;   // length of the side of the square
};