Virtual function

Introductio
nVirtual functions belongs to the branch of Runtime
Polymorphism in C++
Polymorphism
Runtime/Late BindingCompile Time/EarlyBinding
Function
Overloading
Operator
Overloading
Virtual Functions /
Function Overriding
2

Introductio
n
3
Polymorphism is classified into 2 branches
Compile Time Polymorphism/Early Binding/Static
Binding
Runtime Polymorphism/Late Binding/Dynamic
Binding

What is
Binding?
4
For every function call; compiler binds or links the call to
one function definition.
This linking can happen at 2 different time
At the time of compiling program, or
At Runtime

Compile Time
Polymorphism
5
Function Overloading is an example of Compile Time
Polymorphism.
This decision of binding among several functions is taken by
considering formal arguments of the function, their data type
and their sequence.

nologies.in
Example of compile time
polymorphis
m
i n t main()
{
MyFunction(10);
MyFunction( ‘ x ’ ) ;
}
an int is passed
a char is passed
void MyFunction(int i )
{
cout << “an i n t i s passed”;
}
void MyFunction(char c)
{
cout << “a char i s passed”;
}
Output

Runtime
Polymorphis
m
7
In late binding; call to a function is resolved at Runtime, the
compiler determines the type of object at execution time and
then binds the function call to a function definition.
or
Runtim
e
Late binding is also called as Dynamic
Binding Binding.
Virtual Functions are example of Late Binding in
C++
Runtime polymorphism is achieved using
pointers.

Pointers behaviour in
Polymorphism
8
A baseclasspointer variable can hold address of
derived class object, but it can access only members of
base class.

nologies.in
Polymorphism
class derived : public base
{
public:
void show()
{
cout << “Show from derived”;
}
} ;
Show from base
Output
class base
{
public:
i n t
{
main()
base * p t r ;
void show()
{
derived ob;
cout << “Show
}
from base”; p t r = &ob;
} ; p t r -> show();
}

1
PolymorphismYou can see that even the pointer holds address of derived
class object; it has called the base version of show()
method.
The problem is even a base class pointer holds address of
derived type of object, it can access only members of base
class, because the base pointer variable doesn’t have any
idea about the structure of derived class.
A base class can’t have any idea about what derived class
has added to it.

nologies.in
Using virtual
Keyword
}
} ;
class derived : public base
{
public:
void show()
{
cout << “Show from derived”;
}
} ;
Show from derived
Output
class base i n t main()
{ {
public: base * p t r ;
v i r t u a l void show() derived ob;
{
cout << “Show from base”; p t r = &ob;
p t r - > show( ) ;
}

Using virtual
Keyword
12
Using virtual keyword with base class version of show
function; late binding takes place and derived version of the
function will be called, because base pointer points an
derived type of object.
We know that in runtime polymorphism the call to a function
is resolved at runtime depending upon the type of object.

How virtual function
works?
13
Base class pointer can point to derived class object.
In this case, using base class pointer if we call some
function which is in both classes, then base class function is
invoked.
But if we want to invoke derived class function using base
class pointer, it can be achieved by defining the function as
virtual in base class, this is how virtual functions support
runtime polymorphism.

Virtual
functions.
14
A virtual function is a member function that is declared as
virtual within a base class and redefined by a derived class.
To create virtual function, precede the base version of
function’s declaration with the keyword virtual.
When a class containing virtual function is inherited, the
derived class can redefine (Override) the virtual function to
suit its own unique needs.
The method name and type signature should be same for
both base and derived version of function.

1
Using virtual
Keyword
class c i r c l e
{
protected:
f l o a t radius;
v i r t u a l f l o a t area()
{
f l o a t a;
a = 3.14 * radius * radius;
return a;
}
} ;
class cylinder : public c i r c l e
{
private:
f l o a t height;
public:
c i r c l e ( f l o a t r )
public:
cylinder(float r , f l o a t h) : c i r c l e ( r )
{ {
radius = r ; height = h;
} }
f l o a t area() / / overriding area method
{
f l o a t a;
a = (2 * 3.14 * radius * radius)
+ (2 * 3.14 * radius * height);
return a;
}

Using virtual
Keyword Area of Circle : 200.96
Area of Cylinder : 602.88
Output
16
i n t main()
{
c i r c l e * p t r ;
ci r c le Ci r c leOb( 8);
cy lin der Cy lOb(8, 4);
p t r = &CircleOb;
cout << endl << "Area of Circle : " << p t r -> area();
p t r = &CylOb;
cout << end l << "Area of Cyl i nder : " << p tr - > area ( );
return 0;
}

Why method
overriding
?
17
Method overriding allows a derived class to provide a
specific implementation of a method that is already provided
by one of its base class.
The implementation in the derived class overrides (replaces)
the implementation in the base class by providing a method
that has same name, same parameters (signature), and
same return type as the method in the parent class has.
Using a base class pointer variable, we can point to object
of any child class, depending upon the type of object at
runtime a particular method will be called if that method is
made virtual in base class & overridden in derived class.

Method Overloading vs.
Overriding?
18
Overloading occurs when two or more methods in one
class have the same method name but different parameters
(signature), Overriding means having two methods with
the same method name and parameters (method
signature), one of the method is in the parent class and the
other is in child class.
Call to an Overloaded method is resolved at compile time,
while call to an Overridden method is resolved at runtime
depending upon the type of object.

Method Overloading vs.
Overriding?
19
Overloading Overriding
Definition Methods having same name but
each must have different number
of parameters or parameters
having different types & order.
Sub class have method with
same name and exactly the
same number and type of
parameters and same return
type as super class method.
Meaning More than one method shares
the same name in the class but
having different signature.
Method of base class is re-
defined in the derived class
having same signature.
Behaviour To Add/Extend more to
method’s behaviour.
To Change existing behaviour of
method.
Polymorphism Compile Time Run Time
Inheritance Not Required Always Required
Method
Signature
Must have different signature Must have same signature.

od Overloading vs. Overriding?
Overloading Overriding
tionship Relationship is between methods of
same class.
Relationship is between methods of
super class and sub class.
s Does not require more than one
class for overloading.
Requiresat least 2 classesfor
overriding.

Virtual function

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