Question

Parent class has a Virtual function. Is it necessary to have a virtual destructor in the parent class ?

So the questions are
1. Now considering rule of three , should we declare the other two?

Answer 1

A virtual destructor is required any time delete is called on a pointer to that class where the object being deleted is actually of a more derived type. If your base class may be used in this sort of situation then you must provide a virtual destructor.

It is usually advisable to add a virtual destructor to any class that has virtual functions as this gives safety and flexibility to future users of the class and the additional cost of adding a virtual destructor to an already polymorphic class is typically low.

This last rule is just a rule of thumb, though. It may be necessary to have a virtual destructor in a class even if it doesn’t have any other virtual functions and, conversely, a virtual destructor may not be needed for a polymorphic class if it is never used in a context that requires this (typically the destructor would be made protected or even private to enforce this).

You only need to provide a user-defined copy constructor and a copy assignment operator if the compiler provided default implementations won’t do the correct thing. If you’ve added a destructor with an empty implementation purely to make it virtual, this is unlikely to have any bearing on the need for you to provide a copy constructor and a copy assignment operator.