Re: abstract sub programs overriding

[Hyman Rosen <hyrosen@mail.com>]

Robert I. Eachus wrote:
> No, Ada semantics is that each object of a tagged type contains a tag.

Yeah, I actually know that. It's the same in C++. The standard doesn't
require any particular implementation. I think you could even do DK's
fat pointers if you wanted. But it's so conventional to have a vtable
pointer in the object that I just used that as shorthand.

> To answer your specific question

I don't think you're answering to the issue at hand. The first question
is, if you have a class hierarchy where each class has an Initialize
defined for itself, and you create an object of the most derived class,
does Ada arrange for all of the Initialize procedures to be called, or
does each Initialize need to explicitly invoke its parent? The second
question, or observation, is that however it's called, a parent Initialize
can view-convert its object to classwide type and call a dispatching
procedure on it. This means that a derived class procedure might be
called from a parent Initialize before its own Initialize has prepared
the class. I'm not saying this is common, but DK was saying that this
couldn't happen in Ada, and I'm saying that it can. It can't happen in
C++, but in excahnge, C++ can dispatch to an abstract procedure, which
is formally undefined behavior, but which implementations generally turn
into an immediate abort, so that the problematic call can be found.