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From: Charles Hixson <charleshixsn@earthlink.net>
Subject: Re: System.Address_to_Access_Conversions
Date: 1998/07/26
Message-ID: <35BB755A.6F8E02F0@earthlink.net>#1/1
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References: <6odddl$k94$1@nnrp1.dejanews.com> <35AB9C59.74E529E0@magic.fr>
 <6ofn8e$5ff$1@nnrp1.dejanews.com> <6ofqvs$alm@hacgate2.hac.com>
 <6ogieq$qlo@drn.newsguy.com> <m3btqd964p.fsf@mheaney.ni.net>
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Date: 1998-07-26T00:00:00+00:00
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Ok.  But they are just saying how THEY did it.  This doesn't imply that
everyone else did the same thing.  Or even that they will continue to do
things that way (although, public documentation is a kind of a
committment).

nababsi@earthlink.net wrote:
> 
> In article <m3btqd964p.fsf@mheaney.ni.net>, Matthew says...
> >
> >nabbasi@earthlink.net writes:
> >
> >> I read somewhere that the tag position within a record is always defined,
> >> it is the first element always, right? not sure what's its size, I assume
> >> it is an address to someother tag related information somewhere else, so its
> >> size is also known, right?
> >
> >This is all wrong.  A compiler writer is free to use any implementation
> >he pleases.
> >
> 
> there is a paper on www.gnat.com called "Ada 9x tagged types and their
> implementation in GNAT".  by Crille COmer and Brett Porter. it says:
> 
> "for many reasons, it turns out to be mandotary that any field in a tagged
> type keep the same location within the record in the any descendant type",
> 
> then later on they say .."this compoenent is inserted at the top of the
> record forcing the additional compoenents at the end"..
> 
> and they show a digram showing the tag field at the top of the record as
> a pointer to dispatch table.
> 
> nasser