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

Read chapter 9 from C++ Primer Plus (2nd ed.), by Stephen Prata.

Objects (continued)

• As humans, we tend to categorize objects in order to bring simplicity and order to the world around us. The two most common ways we categorize objects are
  
  1. membership,
  2. composition.
  • In membership categorization, we group together all objects that have something in common.
    • Examples: all bicycles, all children, all green elephants.

  • In composition categorization, we group together all objects that compose another object.
    • Examples: the parts of my bicycle.

    • Note that aside from being a part of a common object, these objects may have nothing in common.

• How do these categorizations simplify our view of the world?
  • By working at a higher, more abstract, level, we don't need to concern ourselves with details that don't matter.

  • Examples:
    
    • Instead of listing everyone's name, I just say "the students in the class".
    • Instead of listing every part of my bicycle, I just say "my bicycle".

• The categorizations form hierarchies.
  • Examples:
    
    • All tigers are cats, all cats are mammals, all mammals are animals.
    • The parts of an engine form an engine, which is one of the parts which form a car.
    
    

• Do not confuse these two types of hierarchies.
  • One is a is-a hierarchy.

  • One is a is-a-part-of hierarchy.

• Both of these hierarchies are modelled and implemented in object-oriented designs and implementations.

Object Modelling

• A model is a representation of an actual thing.

• The model should represent the "essence" of the actual thing -- those qualities that we consider are necessary for the real thing to have.
  • Example: a model car. What qualities of a real car make it a car? How can a model car represent these qualities?

• In systems analysis, we analyze the problem to be solved, define a solution, then model the solution with a program.
  • Example:
    
    1. Problem: keep track of people's names and ages.
    2. Solution: store information about people in a list.
    3. Program: models "information about people" and a "list".

• In object-oriented programming, we model objects in the real world.
  • Example: In the previous example, objects could be "information about a person" and a "list".

  • Note that objects do not have to be physical objects.

• An object class is a kind of object.
  • Examples: rabbits, mammals, or animals.

• An object instance is a particular object.
  • Example: Bugs Bunny.

• When we define an object type in our program, we are defining a class. When we define a variable or a create a particular object in our program, we are creating an object instance (called instantiation).

C++ Classes

• C++ uses classes to model objects. Note that C++ classes model object classes, just like C++ ints model integers and C++ chars model characters.

• A class is very much like a struct.

• We previously said that objects have attributes and the ability to respond to requests.
  • Data members model the attributes and function members model the reactive abilities.

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