A constructor turns raw bits into a living object. Until the ctor has finished, you don't have a complete 'object'. In particular, while the base class' ctor is working, the object isn't yet a Derived class object, so the call of the base class' virtual fn defn is correct.
Similarly dtors turn a living object into raw bits (they 'blow it to bits'), so the object is no longer a Derived during Base's dtor. Therefore the same thing happens: when Base::~Base() calls 'virt()', Base::virt() gets control, not the Derived::virt() override. (Think of what would happen if the Derived fn touched a subobject from the Derived class, and you'll quickly see the wisdom of the approach).
