Abstract Classes

Abstract Classes

• Used to generalize the characteristics of a hierarchy
  • Defines the basic set of member data and methods(messages)
  • Declares virtual functions that each derived class will support
• Are not intended to have objects instantiated
  • Define partial or incomplete “objects”
  • Data or methods may not be sufficient for use in a real object
• Provide the essential features for real objects of a hierarchy
  • Identifies minimum data and methods that the class family will contain
  • Useful as program design tool

Pure Virtual Methods

• Special virtual methods
• Forces a class to be abstract class (partial class)
  • Compiler won’t let objects of the class be instantiated
• Syntax - declare a virtual method with an assignment value
  • class figure { … virtual int draw ( ) = 0; // assign 0 to function name ( no body) };
• Can have a definition that provides a default function body for derived class objects