No Virtual constructors but virtual destructor

Question

If we dont have virtual constructors then why we have virtual destructors? Can constructors also be virtual?

Answer 1

Virtual destructors are needed because at destruction time, you don't always know what type you're dealing with:

Base *make_me_an_object()
{
    if (the_moon_is_full())
        return new Derived();
    else
        return new Base();
}

int main()
{
    Base *p = make_me_an_object();
    delete p;
}

The delete in the above program's main doesn't know whether its p points to a Base or a Derived object, but if the Base destructor is virtual (as it should be), then delete can use *p's vtable to find the right destructor.

By contrast, at construction time, you always know what kind of object you're creating. (And in case you don't, then you can create a factory or "virtual constructor" that does know.)