CMPT 212 Object-Oriented Applications Design in C++ Fall 1997

Read chapter 12 from C++ Primer Plus (2nd ed.), by Stephen Prata.

Static Members

• If a class data member or member function is declared with the static keyword, then that member is a static member.

• If a class A has a static member M, then all instances of A share the same M. There is not one M for each instance, like there would be for nonstatic members.
  • Static members belong to the class, not to the individual instances.

• A static function can be called even if no instances exist.
  • To specify the name of the static member function, we must qualify it using the name resolution operator and the name of the class.

• Example: Suppose we have a person class which represents a person. It stores a person's name and weight (in kilograms). We want to be able to output the person's weight in either kilograms or pounds, so we will use a boolean value which will be shared by all instances of person.

     class person {
        public:
           int getWeight() const;
           static void setUnits(boolean kilograms);
           ...
        private:
           static bool kgs;
           ...
        };
  
     ...
  
     person::kgs = true;
  
     ...
     
     int main() {
        ...
        person::setUnits(true);
        ...
        person p;
        cout << p.getWeight();
        ...
        p.setUnits(false);    // changes "kgs" for all persons, not just p.
        ...
     }
           

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".

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