Question

If i call an inherited method on a derived class instance does the code require the use of a vtable? Or can the method calls be ‘static’ (Not sure if that is the correct usage of the word)

For example:

Derived derived_instance;
derived_instance.virtual_method_from_base_class();

I am using msvc, but i guess that most major compilers implement this roughly the same way.

I am (now) aware that the behavior is implementation-specific, i’m curious about the implementation.

EDIT:

I should probaby add that the reason that we are interested is that the function is called a lot of times, and it is very simple, and i am not allowed to edit the function itself in any way, i was just wondering if would be possible, and if there would be any benifit to eliminating the dynamic-dispach anyway.

I have profiled and counted functions etc etc before you all get on my backs about optomization.

Answer 1

Both of your examples will require that Derived has a constructor accepting a Base and create a new instance of Derived. Assuming that you have such a constructor and that this is what you want, then the compiler would “probably” be able to determine the dynamic object type statically and avoid the virtual call (if it decides to make such optimizations).

Note that the behavior is not undefined, it’s just implementation-specific. There’s a huge difference between the two.

If you want to avoid creating a new instance (or, more likely, that’s not what you want) then you could use a reference cast static_cast<Derived&>(base_instance).virtual_method_from_base_class(); but while that avoids creating a new object it won’t allow you to avoid the virtual call.

If you really want to cast at compile time what you’re looking for is most likely the CRTP http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern which allows you to type everything at compile time, avoiding virtual calls.

EDIT for updated question: In the case you’ve shown now, I would suspect many compilers capable of statically determining the dynamic type and avoiding the virtual call.