Can you have virtual constructor in cpp?

A virtual function is a method defined and implemented in a base class that we are expected to override in our derived classes.

When we derive a class from a base class, we automatically inherit all the public and protected members of the base class, but we are free to override any and all the methods, including private methods (we can even change the access type if we wish).

However, when we override a method in the base class, we can only call that override when we actually have a reference or pointer to the derived class itself. If we cast the reference or pointer to the base class we will end up calling the base class method. This is expected behaviour and is normally fine, but what happens if we have a pointer to a base class but we actually want to call the derived class method? This is particularly important when that call comes from the base class itself. How will it determine its actual type and make the correct call?

We could use runtime information within the base class to determine the actual type, but this would break a fundamental rule of encapsulation: a base class should NEVER be concerned about the inner workings of any of its derived classes. If we allow this, we'd be forced to update the base class every time we derived a new class from it. Apart from the performance penalty incurred with accessing runtime information, maintaining the base class code will quickly become unmanageable.

That is where virtual functions come in. Even though we are pointing at a base class, when we call a virtual function we actually call the derived class method instead. This is extremely powerful: we are no longer concerned with the actual type of object any more. We can treat the base class as if it were a generic data type, and the compiler will know exactly which version of a function to call regardless of whether the call was made from the base class or not.

But what if we want to call the base class method? Simple: make an explicit call to it. We don't need runtime information for this since every derived class is also a base class as well. The only exception is when the virtual function is declared private in the base class. In this case, we are not expected to call the base class method at all (only the base class and friends of the base class can make an explicit call to its private methods).