3/20/2016Assoc. Prof. Stoyan Bonev1 COS220 Concepts of PLs AUBG, COS dept Lecture 23 OOP Polymorphism…

05/14/23 Assoc. Prof. Stoyan Bonev 1

COS220 Concepts of PLs AUBG, COS dept

Lecture 23

OOPPolymorphism

Reference: R.Sebesta, Chapter 12Lafore, Chapter 11


Lecture Contents:

• Early binding and Late binding;– Regular member functions;–virtual member functions;– Methods accessed with pointers.


The 3 main OOP characteristics• Data Encapsulation and Data Hiding• Inheritance• Polymorphism

– Generally, the ability to appear in many forms– More specifically, in OOP it is the ability to

redefine methods for derived classes– ability to process objects differently depending

on their data type or class– Giving different meanings to the same thing

http://www.webopedia.com/TERM/P/data_type.html

http://www.webopedia.com/TERM/P/class.html


What is polymorphism?

• The concept of polymorphism (giving different meanings to the same thing).

• Early binding and Late binding. • Regular /normal/ and virtual methods

accessed via pointers. Examples: Base

Derived1, Derived2classes

Person Professor, Student

classes


Member Functions accessed with pointers

Introduction to virtual functions: Virtual means existing in appearance (effect) but not in reality.

Early binding operates on regular /normal/ member functions (methods) accessed with pointers.

Late binding operates on virtual member functions (methods) accessed with pointers.


Early binding / Late binding

virtual vs. non virtual methodsFirst example on Polymorphism: method Show() class Base { . . . }; class Derived1 : public Base { . . . }; class Derived2 : public Base { . . . };

Derived1 drv1; Derived2 drv2; Base *ptr; ptr = &drv1; ptr->Show();

ptr = &drv2; (*ptr).Show();


Early Binding (at compile time)

Normal, regular, non virtual methods class Base {public: void Show(){ cout << "\n Base:" ; }}; class Derived1 : public Base {public: void Show(){

cout << "\n Derived1:" ; } }; class Derived2 : public Base {public: void Show(){

cout << "\n Derived2:" ; } };Major factor/condition: the type of the ptr pointer – object of Base class Derived1 drv1; Derived2 drv2; Base *ptr; ptr = &drv1; ptr->Show();


OOP3aEarlyBinding.cpp



The compiler ignores the contents of the pointer ptr and selects the member function that matches the type of the pointer.

See figure on next slide.




Late Binding (at run time)

Virtual methods class Base {public: virtual void Show(){ cout << "\n Base:" ; } }; class Derived1 : public Base {public: void Show(){

cout << "\n Derived1:" ; } }; class Derived2 : public Base {public: void Show(){

cout << "\n Derived2:" ; } };Major factor/condition: contents of the ptr pointer – obj of derived class Derived1 drv1; Derived2 drv2; Base *ptr; ptr = &drv1; ptr->Show();


OOP3aLateBinding.cpp



The compiler selects the function based on the contents of the pointer ptr, not on the type of the pointer.

See figure on next slide.





Pure virtual methodsA virtual function with no body that is never executed. class Base { public: virtual void Show() = 0 ;};

class Derived1 : public Base {public: void Show(){ cout << "\n Derived1:" ;} };

class Derived2 : public Base {public: void Show(){ cout << "\n Derived2:" ;} };


Early binding / Late binding

virtual vs. non virtual methods Second example on Polymorphism:

method isOutstanding()

class Person { . . . }; class Professor: public Person { . . . }; class Student: public Person { . . . };

OOP3bIsOutStanding.cpp


Virtual method IsOutstanding()class Person { protected: char name[20]; public:

void GetName() { cout << "\nEnter name:"; cin >> name; } void ShowName() { cout << "\n Name is:" << name << " "; } bool virtual isOutStanding() = 0;

};


Virtual method IsOutstanding()class Student : public Person{ private: float score; public:

void GetScore() { cout << "\n Enter student's score:"; cin >> score; } bool isOutStanding() {return (score > 98.0) ? true: false; }

};


Virtual method IsOutstanding()class Professor : public Person{ private: int NumPubs; public:

void GetNumPubs() { cout << "\n Enter number of professor's publications:"; cin >> NumPubs; } bool isOutStanding() { return (NumPubs > 100) ? true: false; }

};


Virtual method IsOutstanding()void main (){ Person *PersPtr[100]; Student *StuPtr; Professor *ProPtr; int n=0; char choice; do {

cout << "\n Enter student or professor (s/p)?:"; cin >> choice; if (choice == 's') { StuPtr = new Student; StuPtr->GetName(); StuPtr->GetScore(); PersPtr[n++] = StuPtr; } else { ProPtr = new Professor; ProPtr->GetName(); ProPtr->GetNumPubs(); PersPtr[n++] = ProPtr; } cout << "\n\n Enter another (y/n)?:"; cin >> choice;} while (choice == 'y'); // end of do

for (int j=0; j<n; j++) {PersPtr[j]->ShowName();if (PersPtr[j]->isOutStanding() == true) cout << " --outstanding person";

} // end of for }// end of main()

Polymorphism in C++

Conclusion


Polymorphism in C++ classified

• Dynamic polymorphism – present lecture– Based on late binding and virtual methods

• Static or ad-hoc polymorphism – lecture on the subroutine concept– Based on overloaded functions concept

• Parametric (generic) polymorphism – lecture on generic programming– Based oh template reserved word

21

More on OOP polymorphism •Polymorphism in VBasic

•Polymorphism in C#

•Polymorphism in Java

22

Java // file OOPPolymorphism.javaclass Base { public void Show() { System.out.println( "\n Base:");}};

class Derived1 extends Base { public void Show() { System.out.println( "\n Derived1:");}};

class Derived2 extends Base { public void Show() { System.out.println( "\n Derived2:");}};public class OOPPolymorhismJava { public static void main(String[] args) { Base ptr = new Base(); ptr.Show(); ptr = new Derived1(); ptr.Show(); ptr = new Derived2(); ptr.Show(); }}

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3/20/2016Assoc. Prof. Stoyan Bonev1 COS220 Concepts of PLs AUBG, COS dept Lecture 23 OOP Polymorphism…