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From: Tucker Taft <stt@averstar.com>
Subject: Re: meaning of "current instance"
Date: 1999/11/11
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Organization: AverStar (formerly Intermetrics)  Burlington, MA  USA
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Date: 1999-11-11T00:00:00+00:00
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Matthew Heaney wrote:
> ...
> > But I don't see what the type of the expression T'Access has to do with
> > it, so perhaps I'm misunderstanding your question.
> 
> Perhaps I stated my question incorrectly.
> 
> I do not care about the type of the expression T'Access.
> 
> I do care about the type of the object designated by the access object
> returned by T'Access.
> 
> I care because I want to call an operation that dispatches on the tag of
> the designated object:
> 
>   function Init (O : access T'Class) return Integer is
>   begin
>     return Do_Init (O);
>   end;
> 
> where Do_Init is a (private) primitive operation:
> 
>   function Do_Init (O : access T) return Integer;
> 
> For object O1 (of type T), I want T's Do_Init to be called.
> 
> For object O2 (of type NT), I want NT's Do_Init to be called.

You should probably be saying "with tag NT" rather than "of type NT."
For dispatching, the "type" (which is basically a compile-time concept)
is not particularly important, so long as it is class-wide.
What matters is the "tag" (which is fundamentally a run-time concept).

At least when talking to Bob and I ;-), it would behoove you
to reserve the term "type" for the compile-time view of the object,
and the term "tag" for the relevant run-time characteristics.

Another way to be clear is to say the "run-time type" or even
the "underlying run-time type" but that is pretty much what
"tag" means, so why not use that nice 3-letter word?

> 
> My compiler calls T's Do_Init for both O1 and O2.

Sounds like a bug, though I haven't personally analyzed the
situation in depth (it seems like Bob did...).

-Tuck
-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA