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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: discriminant questions
Date: Mon, 19 Sep 2011 18:35:33 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2011-09-19T18:35:33-05:00
List-Id: <comp.lang.ada>

"ytomino" <aghia05@gmail.com> wrote in message 
news:0d272f62-67d0-4905-972c-8a7e912c5531@en1g2000vbb.googlegroups.com...
...
> Return to the first question. An access discriminant can not have
> default value.
> So I can not make smart pointer like syntax of raw access types after all.
> I want to know the reason.

The short answer is that the model of access discriminants is such that 
objects with them can never allow them to be changed. The reasons for this 
are extremely hard to explain (it has to do with accessibility checking - 
the RM text that Steve Baird has started calling the "heart of darkness" 
;-), and I'm not going to try.

If you want to try to understand it, I suggest you read the AARM notes in 
3.7, particularly those following 3.7(10).

And if you succeed at understanding this, I suggest that you immediately 
join the ARG, because we could use the help. :-)

                                         Randy.