Polymorphism - C++

POLYMORPHISM AND DATA ABSTRACTION

POLYMORPHISM

^§    many forms

^§    Greek

o  "poly" – many

o   "morph" form

^§    the same method can be called on different objects

^§    they may respond to it in different ways

^§    all Vehicles have a move method

^§    Cars and Truck drive

^§    Airplanes fly

#include "Vehicle.h"

int main(){

Vehicle v ("Transporter 54");

Airplane a("Tornado 2431", 14);

LandVehicle lv("My wheels");

Car c("Ford Anglia 22");

Truck t("Red pickup");

v.move();

a.move();

lv.move();

c.move();

t.move();

}

OUTPUT

Vehicle constructor

Vehicle constructor

Airplane constructor

Vehicle constructor

Land vehicle constructor

Vechicle constructor

Land vehicle constructor

Car constructor

Vechicle constructor

Land vehicle constructor

Truck constructor

Vehicle Transporter 54 moving

Airplane Tornado 2431 flying

Land Vehicle My wheels driving

Land Vehicle Ford Anglia 22 driving

Land Vehicle Red pickup driving

Polymorphic behaviour

^§    to get polymorphic behaviour, we would like the version of move() to be determined at run-time

^§    if moveVehicle is sent an Airplane object, it should get it to fly

^§    do this by using the virtual keyword in the first (base class) declaration of the polymorphic method

class Vehicle { protected:

string name;

public:

// other members

virtual void move() { cout << "Vehicle " << name << " moving" << endl; } };

•         now it works

•         Vehicle Transporter 54 moving

•         Airplane Tornado 2431 flying

•         Land Vehicle My wheels driving

•         Land Vehicle Ford Anglia 22 driving

•         Land Vehicle Red pickup driving

•         polymorphism allows us to use a pointer to a derived type object wherever a pointer to base type is expected

Car c("Ford Anglia 22"); Vehicle * v2 = &c;

v2->move(); Vehicle & v3 = c; v3.move();

•         only works for pointer and reference types

•         they store an address – same size for all objects Airplane a("Tornado 2431", 14);

Vehicle v2 = a; v2.move();

•         trying to fit an airplane into a space meant for any vehicle

•         can call the move() method, but we've lost the wingspan member variable

•         Polymorphism:

•         Ability for objects of different classes to respond differently to the same function call

•         Base-class pointer (or reference) calls a virtual function

•           C++ chooses the correct overridden function in object

•         Suppose print not a virtual function

Employee e, *ePtr = &e;                            

HourlyWorker h, *hPtr = &h;                   

ePtr->print();        //call  base-class   print  function

hPtr->print();       //call  derived-class        print  function

ePtr=&h;              //allowable  implicit       conversion

ePtr->print();        // still calls base-class print