Thread: How does virtual function works

How does virtual function works in C++?

Whenever a program has a virtual function declared, a v - table is constructed for the class. The v-table consists of addresses to the virtual functions for classes that contain one or more virtual functions. The object of the class containing the virtual function contains a virtual pointer that points to the base address of the virtual table in memory. Whenever there is a virtual function call, the v-table is used to resolve to the function address. An object of the class that contains one or more virtual functions contains a virtual pointer called the vptr at the very beginning of the object in the memory. Hence the size of the object in this case increases by the size of the pointer. This vptr contains the base address of the virtual table in memory. Note that virtual tables are class specific, i.e., there is only one virtual table for a class irrespective of the number of virtual functions it contains. This virtual table in turn contains the base addresses of one or more virtual functions of the class. At the time when a virtual function is called on an object, the vptr of that object provides the base address of the virtual table for that class in memory. This table is used to resolve the function call as it contains the addresses of all the virtual functions of that class. This is how dynamic binding is resolved during a virtual function call.

The following code shows how we can write a virtual function in C++ and then use the same to achieve dynamic or runtime polymorphism.

Code:

#include <iostream.h>
class base
{
public:
virtual void display()
{
  cout<<”\nBase”;
}
};
class derived : public base
{
  public:
  void display()
  {
     cout<<”\nDerived”;
  }
};

void main()
{

  base *ptr = new derived();
  ptr->display();
}

In the above example, the pointer is of type base but it points to the derived class object. The method display() is virtual in nature. Hence in order to resolve the virtual method call, the context of the pointer is considered, i.e., the display method of the derived class is called and not that of the base. If the method was non virtual in nature, the display() method of the base class would have been called.