Why does an abstract class have a vtable?

Question

Regarding this post:

For implementations that use vtable, the answer is: Yes, usually. You might think that vtable isn't required for abstract classes because the derived class will have its own vtable, but it is needed during construction: While the base class is being constructed, it sets the vtable pointer to its own vtable. Later when the derived class constructor is entered, it will use its own vtable instead.

I'm assuming the answer is correct, but I don't quite get it. Why is the vtable needed exactly for construction?

Answer 1

Because the standard says so.

[class.cdtor]/4

When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class's non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor's or destructor's class and not one overriding it in a more-derived class.

The rationale is that first the base class is constructed, then the derived one. If a virtual function is called inside the base class' constructor, it would be bad to call the derived class, since the derived class isn't initialized yet.

Remember that an abstract class may have non-pure virtual functions. Also, for debugging purposes, it is good to point pure virtual functions to a debugging trap (e.g. MSVC calls _purecall()).

If all virtual functions are pure, in MSVC you can omit the vtable with __declspec(novtable). If you use a lot of interface classes, this can lead to significant savings because you omit vfptr initialization. But if you accidentally call a pure virtual function, you'll get a hard to debug access violation.