Virtual functions allow dynamic binding in C++ by allowing a function to be overridden in a derived class. A function is declared virtual by using the virtual keyword. At runtime, the compiler will call the version of a virtual function that is defined in the object's own class, rather than being bound at compile time like regular functions. This allows a base class pointer or reference to call a derived class implementation of a virtual function. Classes that contain virtual functions are called polymorphic classes.

1. Virtual Functions
09/04/131 VIT - SCSE
By
G.SasiKumar., M.E., (Ph.D).,
Assistant Professor
School of Computing Science and Engineering
VIT University

2. 09/04/132 VIT - SCSE
Virtual Functions

C++ matches a function call with the correct function definition
at compile time
− known as static binding

the compiler can match a function call with the correct function
definition at run time
− known as dynamic binding.
− declare a function with the keyword virtual if you want the
compiler to use dynamic binding for that specific function.

3. 09/04/133 VIT - SCSE
Virtual Functions

The virtual keyword indicates to the compiler that
− it should choose the appropriate definition of a function not by the
type of reference, but by the type of object that the reference refers
to.

4. 09/04/134 VIT - SCSE
Virtual Methods

Therefore,
− a virtual function is a member function you may redefine for
other derived classes,
− can ensure that the compiler will call the redefined virtual
function for an object of the corresponding derived class,
− even if you call that function with a pointer or reference to a base
class of the object.

A class that declares or inherits a virtual function is called a
polymorphic class.

5. Declaring virtual

prefix declaration with the virtual keyword

redefine a virtual member function in any derived class

this is called overriding
− understand the contrast with overloading
For example, the general syntax to declare a Virtual Function uses:
class classname
//This denotes the base class of C++ virtual function
{
public:
virtual void memberfunctionname() //This denotes the C++ virtual function
{
.............
............
}
};

6. More on definition

overridden function must have same name and same parameter
list
− no need to use the virtual keyword again
− return type can be different

if the parameter lists are different, they are considered different
− in this case, it is not overridden, but hidden
− hidden methods cannot be called

7. Example
class Vehicle //This denotes the base class of
C++ virtual function
{
public:
virtual void Make() //This denotes the C++
virtual function
{
cout <<"Member function of Base Class Vehicle
Accessed"<<endl;
}
};
class FourWheeler : public Vehicle
{
public:
void Make()
{
cout<<"Virtual Member function of Derived class
FourWheeler Accessed"<<endl;
}
};
void main()
{
Vehicle *a, *b;
a = new Vehicle();
a->Make();
b = new FourWheeler();
b->Make();
}