constructors are invoked at the time of object creation.
The v-table for virtual function mechanism is not 
intialised properly during the constructor invocation.
So constructors cannot be virtual

But,destructors can be virtual.Because the V-table is 
properly initialised at the time of object creation.
So the v-table is available for the destructors.
Destructors can be made pure virtual also.But unlike other 
member functions,it must be redfined outside the class.

Since OOPS basics says always base class object to be 
created and than its derivatives. So just consider the case 
where we try to create object of derived class with virtual 
contructor since pointer points to derived type object it 
fetches derived constructor directly, without its base 
class object constructed( instantiated )...
So Constructor cannot be made virtual bt when comes to case 
of Destructor object destuction process must be reverse...
So we must make destructor as virtual.
This applies only when we going for Dynamic Memory 
Allocation...

If, on the other hand, you were to have a pointer to object 
A and assign it a dynamic object B, when you destroy it, 
only object A's destructor is called and object B part of 
the object is never destroyed. Virtual destructors are used 
to solve this problem and destroy the objects in the right 
order.