Abstract Classes

• Abstract classes
  • Sole purpose: to be a base class (called abstract base classes)
  • Incomplete; derived classes fill in ``missing pieces''
  • Cannot make objects from abstract class; however, can have pointers and references
• Concrete classes
  • Can instantiate objects
  • Implement all functions they define
  • Provide specifics
• Abstract classes not required, but helpful
• To make a class abstract
  • Need one or more ``pure'' virtual functions
    • Declare function with initializer of 0
    • virtual void draw() const = 0;
  • Regular virtual functions; have implementations, overriding is optional
  • Pure virtual functions; no implementation, must be overridden
  • Abstract classes can have data and concrete functions; required to have one or more pure virtual functions
• Abstract base class pointers; useful for polymorphism
• Application example
  • Abstract class Shape; defines draw as pure virtual function
  • Circle, Triangle, Rectangle derived from Shape; each must implement draw
  • Screen manager knows that each object can draw itself
• Iterators (more Chapter 21)
  • Walk through elements in vector/array
  • Use base-class pointer to send draw message to each