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From: Martin Krischik <martin@krischik.com>
Newsgroups: comp.lang.ada
Subject: Re: OO problem: Performing actions on messages (very long, sorry)
Date: Wed, 22 Dec 2004 19:16:30 +0100
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Date: 2004-12-22T19:16:30+01:00
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per wrote:

> Hi Dimitri, thanx for your answers.
> 
> Sorry for the yucky code/text. All indentation and line breaks was
> screwed up:( I'm using google groups (for the first time). Not very
> user friendly. Had to compose this answer in a separate file and
> manually paste in the citations so this answer is also a bit yucky...
> 
>>> Each action child shall be able to "do something" with a message in
> the
>>> virtual (is that the Ada term?) procedure Execute.
>>
>>No. It is called "primitive operation".
> 
> OK. But is there no distinction between operations that may dispatch
> and those those who cannot? Execute, for example, may be a primitive
> operation of each child regardless of if the parent has declared it or
> not. So "primitive operation" doesn't tell that dispatching will be
> possible or not, right?

All primitve operations dispach.

>>> The action Action.Override serves as an example in this post. This
>>> action shall override a field in a message.
>>
>>One cannot override a field. Presently Ada's inheritance model is a
> type
>>extension one. I.e. you can add a field, but not replace it with
> another.
> 
> No! I was unclear. I'm not trying to override anything in that sense.
> 
> The messages are basically records filled with values. It's the
> _values_ in the existing message structures the Action.Override is
> supposed to override:
 
> type Instance is new Message.Instance with
> record
> A : Float;
> B : Integer;
> ...
> end record;

Maybe you want to read about a "variant record":

http://en.wikibooks.org/wiki/Programming:Ada:Types:record#Variant_Record

It just might get you a new idea. 

> Somewhere some mechanism assigns the initial values:
> M1 : Message.M1.Instance;
> Put_A(M1, 1.0);
> Put_B(M1, 1);

You could try a qualified expression:

http://en.wikibooks.org/wiki/Programming:Ada:Subtypes#Evaluate_As
 
>>procedure Put (M : Instance; A : ArgumentType) is
>>begin
>>  M.Extension := A;
>>end Put;
> 
> You're saying I should replace every single field in every message
> record with an extension as above?

Well, a variant record initialized by a qualified expression might be the
better option.
 
With Regards

Martin
-- 
mailto://krischik@users.sourceforge.net
http://www.ada.krischik.com