Virtual Functions

• Typically, pointer-class determines functions
• virtual functions; object (not pointer) determines function called
• Why useful?
  • Suppose Circle, Triangle, Rectangle derived from Shape; each has own draw function
  • To draw any shape
    • Have base class Shape pointer, call draw
    • Program determines proper draw function at run time (dynamically)
    • Treat all shapes generically
• Declare draw as virtual in base class
  • Override draw in each derived class; like redefining, but new function must have same signature
  • If function declared virtual, can only be overridden
    • virtual void draw() const;
    • Once declared virtual, virtual in all derived classes; good practice to explicitly declare virtual
• Dynamic binding
  • Choose proper function to call at run time
  • Only occurs off pointer handles; if function called from object, uses that object"s definition
• Example
  • Redo Point, Circle example with virtual functions
  • Base-class pointer to derived-class object; will call derived-class function

  Figure 10: Point class header file declares print function as virtual (upper) and Circle class header file declares print function as virtual.

  

  

  Figure 11: Demonstrating polymorphism by invoking a derived-class virtual function via a base-class pointer to a derived-class object. (part 1 of 2)

  

  

  Figure 12: Demonstrating polymorphism by invoking a derived-class virtual function via a base-class pointer to a derived-class object. (part 2 of 2)

  

  
• Polymorphism
  • Same message, "print", given to many objects; all through a base pointer
  • Message takes on "many forms"
• Summary
  • Base-pointer to base-object, derived-pointer to derived; straightforward
  • Base-pointer to derived object; can only call base-class functions
  • Derived-pointer to base-object
    • Compiler error
    • Allowed if explicit cast made (more in section 10.9)