Constructors can't be virtual in C++.It is an error.The
destructors on the other hand can be virtual.

there is no need to use vitual constructors in C++.
because the construction of the class tree will be happened
from the base class to child class.

C++ doesn’t give any direct support to declare a virtual 
constructor as such. But we can have a virtual constructor 
by indirectly providing the mechanism.

As like ordinary virtual functions, a virtual constructor 
can exhibit different behaviors depending upon the request. 
That is, a virtual constructor can construct different 
types of objects on different requests. This implies the 
class hierarchy should be polymorphical.

This technique resembles two stage construction technique. 
To avoid breaking at the constructor of an object, we keep 
the heavy initialization code in a separate initializer 
function so that we get the minimum guarantee that the 
constructor won’t fail.
In this case, we first invoke the constructor and then 
invoke the initializer function. The advantage of this 
method is that we will have a better control of the 
construction.

For example,

CBrush blueBrush;  // Just an object. Must be initialized!
blueBrush.CreateSolidBrush(RGB(0,0,255));  // Blue brush.

Since I am running short of time, I hang on with this for 
the time being. I will add more details about the virtual 
constructor theory soon.

Virtual constructors are not used in any case in c++, bcz
the construction order in c++ should always remains from
base to derived , no one can changed their construction
order, so no need to declare constructor as virtual. But if
anyone wants to make it virtual then it  is possible to make
a constructor virtual but it is against the OOPs.