Chapter 5

• Back to the book, chapter 5.
• Again, "What are the (real world) objects I am modeling?"
  • I don't believe real world is part of this.
• Classes:
  • Stroustrap thinks class = data type.
  • An instance of a class is a specific item of a class. Frequently referred to as objects.
  • "A class is simply the encapsulation of what defines a classification of objects"
• Classes can be formed from component classes.
  • Composition is another principle of oo design.
  • An airplane class has an engine object, a wing object, ....
  • More on this in a bit.
• A class has properties
  • The values of the member data.
  • What makes this object unique
• Behaviors
  • What does this object do? or What can I do with this object?
• In general, we can build three types of things.
  • If something only has properties, or values it is data.
  • If something only has behavior, it is an algorithm.
  • If something has both, it is a class.
• The trick is deciding what are our objects, our data, our functions.
• Class Relationships or composition
  • There are multiple ways you can combine classes.
  • has-a relationship is composition.
  • is-a relationship is inheritance.
• A frog has-a heart ie the frog is composed of several things, including a heart. (true Composition)
  • In this case, a heart must have a frog, or it can not exist.
• A pond has-a frog, ie a pond can hold a frog, but might have several or none. (Aggregation)
  • In this case a frog can exist without a pond.
• A frog is-a animal, ie the frog has characteristics common to all animals (inheritance)
  • The animal contains the behaviors and properties common to all animals.
  • A frog is less general, it contains behaviors and properties unique to frogs, but inherits the behaviors and properties of an animal.
• In inheritance we can
  • Add or replace properties and functionality of the base class.
  • More on this in chapter 10.