chapter10

Introduc tion • virtual functions and polymorphism -Design and implement systems that are more easily extensible -Programs written to generically process objects of all existing classes in a hierarchy 3 © 2000 Prentice Hall, Inc. All rights reserved. Type Fie lds a nd switch Sta te me nts • switch statement -Take an action on a object based on its type -A switch structure could determine which print function to call based on which type in a hierarchy of shapes • Problems with switch -Programmer may forget to test all possible cases in a switch • Tracking this down can be time consuming and prone to error -virtual functions and polymorphic programming can eliminate the need for switch logic 4 © 2000 Prentice Hall, Inc. All rights reserved. virtual Func tions • virtual functions -Suppose a set of shape classes such as Circle, Triangle, etc. -Every shape has own unique draw function but possible to call them by calling the draw function of base class Shape • Compiler determines dynamically (i.e., at run time) which to call -In base-class declare draw to be virtual -Override draw in each of the derived classes -virtual declaration: • Keyword virtual before function prototype in base-class virtual void draw() const; -A base-class pointer to a derived class object will call the correct draw function Shape->draw(); -If a derived class does not define a virtual function, the function is inherited from the base class 5 © 2000 Prentice Hall, Inc. All rights reserved. Virtua l Func tions ShapePtr->Draw(); - Abstra ct a nd Concrete Cla sses • Abstract classes -Sole purpose is to provide a base class for other classes -No objects of an abstract base class can be instantiated • Too generic to define real objects (i.e., TwoDimensionalShape) • Can have pointers and references • Concrete classes -Classes that can instantiate objects -Provide specifics to make real objects (i.e., Square, Circle) • Making abstract classes -Declare one or more virtual functions as "pure" by initializing the function to zero -Example of a pure virtual function: virtual double earnings() const = 0; } Notice the overriden earnings and print functions. They were declared virtual in the base class. © 2000 Prentice Hall, Inc. All rights reserved. Notice the overriden earnings and print functions. They were declared virtual in the base class. © 2000 Prentice Hall, Inc. All rights reserved. Call function print using the object itself. Call function print, using a baseclass pointer. This uses virtual functions and dynamic binding. Call function print, using a baseclass reference. This uses virtual functions and dynamic binding. Ne w Cla sse s a nd Dyna mic Binding Virtua l De struc tors • Problem: -If a base-class pointer to a derived object is deleted, the base-class destructor will act on the object • Solution: -declare a virtual base-class destructor to ensure that the appropriate destructor will be called Notice the virtual functions which will be overridden by each class. © 2000 Prentice Hall, Inc. All rights reserved. Create an array of base class pointers. Assign these to the objects, then call the print functions again, using the base class pointers. The appropriate virtual functions will be called