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From: "Randy Brukardt" <randy@rrsoftware.com>
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Subject: Re: Controlled types and exception safety
Date: Tue, 6 Dec 2005 13:34:29 -0600
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Date: 2005-12-06T13:34:29-06:00
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"Jeffrey R. Carter" <spam@spam.com> wrote in message
news:2Rklf.171$n1.114@newsread2.news.pas.earthlink.net...
> Personally, I would have preferred
>
> procedure R'Assign (To : in out R; From : in R);
>
> for any record type R. This can be redefined by the user:
>
> for R'Assign use My_Assignment_Procedure;
>
> I have seen objections to this approach, but none that aren't handled by
one of
> the following rules:

This was the original idea for Ada 95, but it doesn't work. That's because
the object on the left-hand side may come into existence because of the
outer assignment, or disappear because of the assignment. The beauty (and
curse) of Adjust is that it can be called by itself when needed, or with an
appropriate Finalize.

You can't, in general, read the object that you're assigning into. That
means that user-defined assignment in Ada can never be as powerful as that
in other languages (unless you somehow prevent the types from being used in
discriminant-dependent components - which would probably be a generic
contract problem).

For an another explanation of this, see ARM 7.6(17.a-17.h).
http://www.adaic.com/standards/95aarm/html/AA-A-5-3.html

The other issues are solveable, but this one is not.

                     Randy Brukardt