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From: John Harbaugh <john.s.harbaugh2@boeing.com>
Subject: Re: Variant Record Component
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Date: 2003-02-28T16:15:52+00:00
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Here's the situation:  I'm using a variant record to represent a Link 16
(JTIDS) J7.6 message for memory-mapped I/O.  So a message comes in and,
based on the message header, a particular structure is frozen.  Now I
pass the message object to a decode function that is looking for
particular message fields.  Well, surprise surprise, we occasionally
raise Constraint_Error because the field in question is not present.

Several options come to mind (in order of preferance):
* Refactor the function and pass only the variant part instead of the
entire message

* Pass the discriminant value in to the function and have the function
check it before referencing the variant part

* Add a nested declare block in the function to try the variant part and
handle the exception locally

My question has more to do with understanding what language-level tools
that are available.  Ada is pretty slick at testing membership of a
value within/without a subtype range.  I was wondering if there was
something analogous for variant records.  I'm getting the impression
that the answer is no.  Yes?

Thanks for your replies,

	- John

	

"David C. Hoos" wrote:
> 
> ----- Original Message -----
> From: "John Harbaugh" <john.s.harbaugh2@boeing.com>
> Newsgroups: comp.lang.ada
> To: <comp.lang.ada@ada.eu.org>
> Sent: Wednesday, February 26, 2003 12:00 PM
> Subject: Variant Record Component
> 
> > Hi, all -
> >
> > Is there a way to determine if an object of some variant record type
> > contains a specific variant component, other than a trying it and
> > handling a possible constraint error?  I looked for something like and
> > "in" operator or suitable attribute, but found nothing.
> Well, this sort of thing should not be necessary if the accesses to the
> record components are done distinctly for different values of the
> discriminant as part of a case statement;
> >
> > Thanks in advance,
> >
> > - John Harbaugh
> > _______________________________________________
> > comp.lang.ada mailing list
> > comp.lang.ada@ada.eu.org
> > http://ada.eu.org/mailman/listinfo/comp.lang.ada
> >