CMPT 212 Object-Oriented Applications Design in C++ Spring 1998

Modelling Object Composition

• Recall that object composition is where an object is made up of parts, and these parts are other objects.
  • This is the is-a-part-of relationship.

  • Example: A car is made up of an engine, some doors, some tires, etc. An engine is made up of some pistons, some valves, etc.

• We model composition by defining the data members of a class to be instances of another class.
  • Example:

       class door {
          ...
          };
    
       class tire {
          ...
          };
    
       class piston {
          ...
          };
    
       class valve {
          ...
          };
    
       class engine {
          piston p[6];
          valve  v[24];
          ...
          };
    
       class car {
          engine e;
          door   d[2];
          tire   t[4];
          ...
          };
    
                   

Modelling Object Membership

• Recall that object membership is where a class is a special case of another class.
  • This is the is-a relationship.

  • Example: An elephant is an animal. An animal is a living thing.

• Problem: How do we model this relationship? What exactly is this relationsihp? What does it mean for a class to be a "special case" of another class?

• Question: What makes an elephant a "special case" of an animal?
  • It walks.
  • It lives.
  • It is grey.
  • It has a trunk.
  In general, it is everything a animal is, and it has some other features that are unique to it.

• General principle: If a class A is a special case of a class B, then an instance of A has all of the properties of class B, plus some additional ones.
  • This is the principle of inheritance. An instance of A inherits properties from B, and it has some additional properties because it is also an A.

  • Class B is called the base class and class A is called the derived class.

  • Example: "elephants" is a class derived from the base class "animals", and "animals" is a class derived from the base class "living things".

Inheritance in C++

• We can create a derived C++ class from a base C++ class.

• We specify only the differences between the derived class and the base class.
  • There are three kinds of differences:
    1. The derived class may have additional properties (data members).
    2. The derived class may have a different implementation of a member function.
    3. The derived class may have addition member functions.

  • Note that we are not allowed to change the type of data members.

• Besides public and private, data members and member functions can also be protected. protected members are like private members except that they can also be accessed by member functions and friend functions of any class derived from the class.
  • Example:

       class myClass {
          public:
    	 int a;
          protected:
    	 int b;
          private:
    	 inc c;
          };
    	       

    a can be accessed publicly.
    c can only be accessed by member functions and friend functions of myClass.
    b can only be accessed by member functions and friend functions of myClass and by member functions and friend functions of any class derived from myClass.

• Example: Suppose we have this hierarchy of objects: point, circle, and disk.
  • A point has properties x_coord, y_coord, and colour, and functions move, setColour, and display.
    We can use this diagram to show a point object:

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