What is virtual function?

When derived class overrides the base class method by redefining the same function, then if client wants to access redefined the method from derived class through a pointer from base class object, then you must define this function in base class as virtual function.
class parent
{
   void Show()
{
cout << "i'm parent" << endl;
}
};

class child: public parent
{
   void Show()
{
cout << "i'm child" << endl;
}
};

parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls parent->show() i

now we goto virtual world...
class parent
{
   virtual void Show()
{
cout << "i'm parent" << endl;
}
};

class child: public parent
{
   void Show()
{
cout << "i'm child" << endl;
}
};

parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls child->show()