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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Problem in "X (1).Re := X (1).Re + 1"
Date: Tue, 8 May 2012 17:29:14 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2012-05-08T17:29:14-05:00
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"Adam Beneschan" <adam@irvine.com> wrote in message 
news:5209872.2691.1336497260879.JavaMail.geo-discussion-forums@ynlq12...
...
>In fact, looking into this further, I think there's a problem with the AARM 
>in this regard.
>A.18.2(254.a), which was from Ada 2005, says "an implementation that 
>avoided copying
>until one of the containers is modified would be allowed".  It seems to me 
>that this is no longer
>true.  I'm going to send something to Ada-Comment about this.

I think it is true, it's just that creating a reference (or for that matter, 
calling Query_Element) has to be dealt with specially; they're both somewhat 
like modifying an element. Query_Element is a bit less of a problem as most 
ways (but not all) to modify the container while the call is active are 
invalid. But it seems that the same problems can come up for it.

(Note that a copy-on-write implementation is not a good idea for the 
standard containers for other reasons as well, the main one being that the 
usual uses don't create many copies, so having overhead for that can't be 
justified. But it should be possible to create a container with that 
semantics.)

                      Randy.