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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: asynchronous task communication
Date: Tue, 08 Jan 2013 18:18:44 +0000
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Date: 2013-01-08T18:18:44+00:00
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Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:

> On 01/08/2013 05:02 AM, Brian Drummond wrote:

>> So, holding onto a cursor would be unsafe (i.e. no longer refer to the
>> original object) after an "insert" operation, possibly earlier in the
>> vector?
>
> If you have an item in your (unbounded) array, indexed by Positive, at
> index 3, it will still be at index 3 if you assign to components at
> other indices, or extend the array by adding new components beyond the
> current end of the array. However, if you insert a new component
> before the existing item at index 3, then all the components after the
> new component will shift up, and the existing item will then be at
> index 4.
>
> This should be pretty obvious if you think about how you would
> implement an unbounded array.
>
> One would think that a Cursor would behave similarly.

Not so! The Cursor becomes ambiguous[1], and using it is a bounded
error. GNAT GPL 2012 implements Cursor using an Index, & behaves the way
you'd expect.

[1] http://www.ada-auth.org/standards/12rm/html/RM-A-18-2.html#p240