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From: ytomino <aghia05@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Problem in "X (1).Re := X (1).Re + 1"
Date: Wed, 9 May 2012 04:03:22 -0700 (PDT)
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Date: 2012-05-09T04:03:22-07:00
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On Wednesday, May 9, 2012 5:05:50 PM UTC+9, ytomino wrote:
> 
> I still don't think it's impossible.
> How about inserting the copy-on-write code into Constant_Reference?

I reconsidered.

X : Vector;
-- insert items into X
Ref1 := V.Constant_Reference (X, 1); -- reference counter = 1
Z := X; -- reference counter = 2
Ref2 := V.Constant_Reference (X, 2); -- (E) what should it do? 

If simple copy-on-write is executed at E, Ref1.Element will be an old value.
Probably, Reference/Constant_Reference should disable the container from increasing the reference counter.

X : Vector;
-- insert items into X
Ref1 := V.Constant_Reference (X, 1); -- reference counter = 1, and locking it
Z := X; -- deep copy, keeping reference counter = 1
Ref2 := V.Constant_Reference (X, 2); -- OK

Otherwise, more complicated mechanism like ownership control is required.