Implementation inheritance vs. Subtyping

Subtyping
	IS-A relationship

	superclass defines method interface

	subclasses override superclass methods

	subclass objects can be substituted for superclass objects

	you want subclass methods to be called when invoked through superclass pointer

	static binding vs. dynamic binding

	subtyping + dynamic binding = polymorphism

Polymorphism in C++
	Chess example
		call Piece Factory
		ask Piece for legal moves
		delete Piece

	By default, C++ uses static binding

	Virtual methods use dynamic binding

	How about destructors?  same as other methods
		virtual destructors

Pure virtual methods
	Often times a virtual method can only be defined in the subclass,
	and it is undesirable to provide a default implementation in the superclass

	You can make a method "pure virtual" in the superclass

Abstract and Interface classes
	A superclass defines the method interface for all subclasses.

	For each method, the superclass can choose whether or not to provide
		a default implementation

	If a superclass has at least one pure virtual method, it is called
		an "abstract class" because it cannot be instantiated
		
		Examples: Window superclass, Piece superclass

	If a superclass only has pure virtual methods, it is called an
		"interface class" (relate to Java interfaces)

		Example: InputStream in CS240 Utilities

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