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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: What is the best method for transmitting objects/tagged records?
Date: Mon, 6 Jun 2011 22:25:15 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2011-06-06T22:25:15-05:00
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"Shark8" <onewingedshark@gmail.com> wrote in message 
news:5e5bc0bb-8b39-4655-8c9d-f93ec24b3529@p21g2000yqh.googlegroups.com...
>...
>"(75)
>    S'External_Tag
>
>        S'External_Tag denotes an external string representation for
>S'Tag; it is of the predefined type String. External_Tag may be
>specified for a specific tagged type via an
>attribute_definition_clause; the expression of such a clause shall be
>static. The default external tag representation is implementation
>defined. See 3.9.2 and 13.13.2."
>Per 
>http://www.univ-orleans.fr/mapmo/systeme/Recup/systeme/logiciels/GRASP_Documentation/ada95lrm/rm9x-13-03.html
>
>* I had an application that did some working on external tags to check/
>change the structure a bit; then one day I updated GNAT and, lo and
>behold, everything there quit working because GNAT's external-tag
>formatting had been changed.

If you are depending on the format of the external tags, you surely should 
not be depending on the default values for external tag for those particular 
tags. But you can specify these youself, and if you do that compiler won't 
change them. Note that the text you quote says that 'External_Tag can be 
specified.

This is exactly how you stream tagged values -- there too it is a bad idea 
to depend on the default external tags unless everything will always use the 
same compiler version everywhere. Specifying the tags is better.

                                          Randy.