Why a virtual call to a pure virtual function from a constructor is UB and a call to a non-pure virtual function is allowed by the Standard?

Question

From 10.4 Abstract Classes parag. 6 in the Standard :

\"Member functions can be called from a constructor (or destructor) of an abstract class; the effe

Answer 1

Because a virtual call can NEVER call a pure virtual function -- the only way to call a pure virtual function is with an explicit (qualified) call.

Now outside of constructors or destructors, this is enforced by the fact that you can never actually have objects of an abstract class. You must instead have an object of some non-abstract derived class which overrides the pure virtual function (if it didn't override it, the class would be abstract). While a constructor or destructor is running, however, you might have an object of an intermediate state. But since the standard says that trying to call a pure virtual function virtually in this state results in undefined behavior, the compiler is free to not have to special case things to get it right, giving much more flexibility for implementing pure virtual functions. In particular, the compiler is free to implement pure virtuals the same way it implements non-pure virtuals (no special case needed), and crash or otherwise fail if you call the pure virtual from a ctor/dtor.