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Lesson 2 Designing Reusable Polymorphism
Objective Define polymorphism and explain why it is central to OOP.

Polymorphism and Object Oriented Programming

Polymorphism is a way of giving different meanings to the same message. The meanings are dependent on the type of data being processed. Object orientation takes advantage of polymorphism by linking behavior to the object's type. In addition, polymorphism localizes responsibility for behavior. The client code frequently requires no revision when additional functionality is added to the system through implementor-provided code additions.
In effect, polymorphism is the genie in OOP, taking instruction from a client and properly interpreting its wishes.

Polymorphism in the context of object-oriented programming is the ability of one type alpha to appear as and be used like another type beta. The purpose of polymorphism is to implement a style of programming called message-passing in which objects of various types define a common interface of operations for users. In strongly typed languages, polymorphism usually means that type alpha somehow derives from type beta, or type gamma implements an interface that represents type beta. In weakly typed languages types are implicitly polymorphic.

Why is polymorphism central to Object Oriented Programming?

Polymorphism is considered central to object-oriented programming (OOP) because it allows for code reuse, flexibility, and abstraction. By allowing different objects to respond differently to the same message, polymorphism enables programmers to write code that can work with a variety of different objects without needing to know their specific types. This makes code more flexible and easier to maintain, as it can be easily extended to work with new types of objects. Polymorphism also promotes code reuse by allowing the same code to be used with different objects, rather than needing to write separate code for each object type. This can significantly reduce code duplication and development time. Additionally, polymorphism enables abstraction, which is a key principle of OOP. Abstraction allows programmers to focus on the essential characteristics and behaviors of an object, rather than its implementation details. Polymorphism supports abstraction by allowing objects to be treated as instances of a more general class, rather than their specific implementation class. Overall, polymorphism is essential to OOP because it promotes code reuse, flexibility, and abstraction, making it easier to write, maintain, and extend complex programs.

To begin, examine this short example:
#include <iostream>
using namespace std;
class base {
 virtual void vfunc() {
  cout << "This is base's vfunc().\n";
class derived1 : public base {
  void vfunc() {
  cout << "This is derived1's vfunc().\n";
class derived2 : public base {
 void vfunc() {
 cout < <"This is derived2's vfunc().\n";

int main(){
 base *p, b;
 derived1 d1;
 derived2 d2;
 // point to base
 p = &b;
 p->vfunc(); // access base's vfunc()
 // point to derived1
 p = &d1;
 p->vfunc(); // access derived1's vfunc()
 // point to derived2
 p = &d2;
 p->vfunc(); // access derived2's vfunc()
 return 0;

This program displays the following:
This is base's vfunc().
This is derived1's vfunc().
This is derived2's vfunc().

As the program illustrates, inside base, the virtual function vfunc() is declared. Notice that the keyword virtual precedes the rest of the function declaration. When vfunc() is redefined by derived1 and derived2, the keyword virtual is not needed. (However, it is not an error to include it when redefining a virtual function inside a derived class; it's just not needed.)