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From: "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>
Subject: Re: System.Address_to_Access_Conversions
Date: 1998/07/15
Message-ID: <6ojc8p$7fu@hacgate2.hac.com>#1/1
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Date: 1998-07-15T00:00:00+00:00
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jsanchor@my-dejanews.com wrote in message
<6oj1qi$n37$1@nnrp1.dejanews.com>...
<snip>
>After reading all the comments from all who have responded I am
>worried because I also thought that the TAG was the first element in the
>record. I have tampered with my program ~100 times and it did stay there.
(I
>am not saying you are wrong just pondering).
>
Well, for a given version of a given compiler, that may well be the case.
With gnat, I found (as I said in an earlier post) that I could control where
that tag _isn't_ by using a representation clause to define where everything
else _is_.

As some have pointed out, there is no requirement that the tag even be
present in the memory layout, so I wouldn't depend on the tag being there,
if it were I.

For portability, I would consider using stream I/O to read and write the
data to NVM.  Then you would have a portable representation -- i.e., the
_external_ tag -- written in memory.  You _could_ use the implementation's
default external tag value for each type, but for portability I would define
the external_tag for each type using an attribute definition clause.

In such a case, the data type which you would be referencing in the
System.Address_to_Access_Conversion instantiation would be an array of
stream_elements.

Just for what it's worth -- I hope a little more than two cents!

David C. Hoos, Sr.