Virtual functions allow dynamic binding to occur. This means that the function called is determined at runtime based on the actual object type, rather than static binding which determines the function at compile time. Some key points: 1) A virtual function tells the compiler to create a pointer to the function but not fill it in until called. 2) Classes with virtual functions are called polymorphic classes and allow overriding derived class functions. 3) Virtual functions are useful for polymorphism when subclasses need to provide unique implementations of the same method name.

1. Lecture 21
Virtual Functions

2. Introduction:
Overloading vs. Overriding
• Overloading is a form of polymorphism which means
that a program construct which has a certain
appearance can mean different things (for example, calls
to different functions) depending on the types of the
parameters involved. Example
• Overriding is also a form of polymorphism which means
that a derived class defines a function that has similar
name and no. & types of parameters as its base class
but the implementation of both functions are different.
Example

3. Overloading
Patient:
Data Member:
IdNumber, PatName
Function member:
SetDetails(int , char)
Inpatient:
Data Member:
Wardnumber, Daysinward
Function member:
SetDetails(int, char, int, int)
// overload function member

4. Overriding
Patient:
Data Member:
IdNumber, PatName
Function member:
DisplayDetails() { cout<<IdNumber<<PatNumber; }
Inpatient:
Data Member:
Wardnumber, Daysinward
Function member:
DisplayDetails() {cout<<Wardnumber<<Daysinward;}
// override function member

5. Introduction
• Binding means deciding exactly which form or function
is appropriate.
• Binding occurs during
• during compilation is called static or early binding,
• during program execution is called dynamic or late
binding.
Dynamic Binding
Static Binding In C++, to implement dynamic
Usually is known ad hoc binding, use virtual functions.
polymorphism.
A member function is declared
as virtual by writing the word
Performed at compile-time virtual first in the
when a function is called via declaration.
a specific object or via a
pointer to an object Syntax:
virtual return_type
function_name (arg_list….)

6. Static Binding : Sample Program
#include <iostream.h> void Displaydetails()
{ cout<<endl<<"Inpatient:"<<IdNumber<<
class Patient { Wardnumber<<Daysinward; }
public: };
int IdNumber; char Name;
void Setdetails (int I, char N) void main()
{ IdNumber = I; Name = N; } { Patient p1;
void Displaydetails() p1.Setdetails(111,'a'); // static binding
{ cout<<endl<<"Patient:"<<IdNumber p1.Displaydetails(); // static binding
<<Name; } }; // end class Patient
class InPatient : public Patient { InPatient p2;
private: int Wardnumber; p2.Setdetails(333,'z',12,14); // static binding
int Daysinward; p2.Displaydetails(); // static binding
public: }
void Setdetails (int I, char N, int W, int D)
{ IdNumber = I; Name = N;
Wardnumber = W;
Daysinward = D; }

7. Points on Dynamic Binding
• A virtual function tells the compiler to create a pointer to a function
but not to fill in the value of the pointer until the function is actually
called
• A class which contains at least one virtual function, or which
inherits a virtual function, is called a
polymorphic class.
class Shape class Shape
{ private: …… { protected: ……
public: virtual void display() public: virtual void display()
{ ……..} { ……..}
: :
: :
}; };
class Rectangle: public Shape
{ ………… };

8. Dynamic Binding : Sample Program 1
#include <iostream.h> void Displaydetails()
{ cout<<"Inpatient No: "<<IdNumber<<endl;
class Patient { cout<<“ Name: “<<Name<<endl;
protected: cout<<“Ward No: “<<Wardnumber<<endl;
int IdNumber; char PatName; cout<<“Days: ”<< Daysinward<<endl;};
public: };
void Setdetails (int I, char N)
{……..} void main()
{ Patient *Pat1, Pat2;
virtual void Displaydetails() Pat2.Setdetails(999, ’A');
{ cout<<"Patient No: "<<IdNumber<<endl; Pat1 = &Pat2;
cout<< ”Name: “<<PatName<<endl; } Pat1 -> Displaydetails();
}; //dynamic binding
class InPatient : public Patient { InPatient InPat;
private: InPat.Setdetails(333, ’Z', 12, 14);
int Wardnumber; int Daysinward; Pat1 = &InPat;
public: Pat1 -> Displaydetails();
void Setdetails (int I, char N, int W, int D) //dynamic binding
{ …….. } }

9. Dynamic Binding : Sample Program 2
#include <iostream.h> class Professor : public Person {
#include <string.h> private:
class Person { int publs;
protected: public:
char *name; Professor (char* s, int n) : Person(s), publs(n) {}
public: void print() { cout<<"My name is "<<name
Person(char *s) {………..} << " and I have "<<
virtual void print() { publs<<"publications."; }
cout<<"My name is "<<name<< ".n"; } }; // End class Professor
}; // End class Person void main()
{ Person* P;
class Student : public Person { Person P2("Ali");
private: Student Stud1("Fitri", 3.56);
double gpa; Professor Prof1("Halizah", 5);
public: P = &P2; P->print();
Student (char* s, double g) : Person(s) {….. } //dynamic binding
P = &Stud1; P->print();
void print() { cout<<"My name is "<<name //dynamic binding
<< " and my G.P.A. is "<< gpa<<".n"; } P = &Prof1; P->print();
}; //End class Student //dynamic binding
} // End main block

10. Virtual Functions
• Once a function is declared virtual, it remains virtual all
the way down the inheritance hierarchy when a class
overrides it. All display() are virtual except display()
Person Person
- virtual display() - display()
Lecturer Lecturer
- display() - virtual display()
Full Time Part Time Full Time Part Time
- display() - display() - display() - display()
All display() are virtual

11. Virtual Functions
• When a derived class chooses not to define a virtual
function, the derived class simply inherits its immediate
base class's virtual function definition.
Person Class Lecturer inherits the
- virtual display() virtual function display() from
class Person.
Lecturer
- setvalue()
Full Time Part Time
- getvalue() - getvalue()

12. Virtual Functions: When it is useful?
Shape
virtual void draw();
Circle Triangle Rectangle Square
void draw(); void draw(); void draw(); void draw();
• Each sub-class has its different definition of draw() function.
• Declare the draw() function in class Shape as virtual function.
• Each sub-class overrides the virtual function.