Introduction

• Have we established copying code is bad?
• Copying classes that do the same thing is bad too!
• As we have seen, we can even create a new class from one where we don't have source code.
• This also leads to polymorphism
• So we can
  • Start with a base class that contains the common members.
    • Also called a parent class.
    • Or a superclass.
    • We can even add methods that we need all derived classes to implement but we don't have the data to implement at this time.
  • Create one or more derived classes also called child classes.
• class identifier : access_specifier baseclass {};
  • There are other values that can go at the end
  • We will discuss these as we encounter them.
• You can do this as many times as you wish.
  • You can build a tree, where classes are derived from derived classes.
    • a spring peeper is a frog is an amphibian is an animal
    • In this case they just keep inheriting different methods and data.
  • Or you can build "siblings"
    • A spring peeper is a frog.
    • A bullfrog is a frog
    • IN this case they share a common ancestor, but the new traits are not shared.
  • you can even derive one class from two.
    • A bullfrog is a jumping animal
    • A bullfrog is a swimming animal
    • This is called Multiple Inheritance, and is problematic.
    • But we will approach that later as well.
• A cool thing here, is that client code does not know who added the methods.
  • From the prospective of a client, an object belongs to it's defined class, as well as any base classes.
  • Pointers to an object become more interesting.
  • As we saw in the example, the pointer to a base class can point to an instance of a derived class.
  • But slicing removes the new members.

    • 
      Container * tmp = new EvenContainer;
      
      cout << "The base pointer can get to the size " << tmp->Size() <<< endl;
      tmp->InsertNextEven(100); 

• He states "From the perspective of a derived class, all public and protected members from the base class are available for use."
  • And being a derived class really is nothing special.
  • Well for a few exceptions we will discuss.
• One last point

  • class identifier final {
    ...
    }
    
    class identifier final : ParentClass {
    ...
    }; 

  • Marks a class so nothing can be derived from it.