DiscountSale’s member function bill() implemented ,ifferently than Sale’s Particular to "discounts". Member functions savings and " | Chapter 15 Polymorphism and Virtual Functions Learning Objectives Virtual Function Basics Late binding Implementing virtual functions When to use a virtual function Abstract classes and pure virtual functions Pointers and Virtual Functions Extended type compatibility Downcasting and upcasting C++ "under the hood" with virtual functions Virtual Function Basics Polymorphism Associating many meanings to one function Virtual functions provide this capability Fundamental principle of object-oriented programming! Virtual Existing in "essence" though not in fact Virtual Function Can be "used" before it’s "defined" Figures Example Best explained by example: Classes for several kinds of figures Rectangles, circles, ovals, etc. Each figure an object of different class Rectangle data: height, width, center point Circle data: center point, radius All derive from one parent-class: Figure Require function: draw() Different instructions for each figure Figures Example 2 Each class needs different . | Chapter 15 Polymorphism and Virtual Functions Learning Objectives Virtual Function Basics Late binding Implementing virtual functions When to use a virtual function Abstract classes and pure virtual functions Pointers and Virtual Functions Extended type compatibility Downcasting and upcasting C++ "under the hood" with virtual functions Virtual Function Basics Polymorphism Associating many meanings to one function Virtual functions provide this capability Fundamental principle of object-oriented programming! Virtual Existing in "essence" though not in fact Virtual Function Can be "used" before it’s "defined" Figures Example Best explained by example: Classes for several kinds of figures Rectangles, circles, ovals, etc. Each figure an object of different class Rectangle data: height, width, center point Circle data: center point, radius All derive from one parent-class: Figure Require function: draw() Different instructions for each figure Figures Example 2 Each class needs different draw function Can be called "draw" in each class, so: Rectangle r; Circle c; (); //Calls Rectangle class’s draw (); //Calls Circle class’s draw Nothing new here yet Figures Example: center() Parent class Figure contains functions that apply to "all" figures; consider: center(): moves a figure to center of screen Erases 1st, then re-draws So Figure::center() would use function draw() to re-draw Complications! Which draw() function? From which class? Figures Example: New Figure Consider new kind of figure comes along: Triangle class derived from Figure class Function center() inherited from Figure Will it work for triangles? It uses draw(), which is different for each figure! It will use Figure::draw() won’t work for triangles Want inherited function center() to use function Triangle::draw() NOT function Figure::draw() But class Triangle wasn’t even WRITTEN when Figure::center() was! Doesn’t know "triangles"! Figures Example: Virtual! Virtual functions are the answer Tells
