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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: asynchronous task communication
Date: Tue, 08 Jan 2013 21:14:31 +0000
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Date: 2013-01-08T21:14:31+00:00
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Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:

> On 01/08/2013 11:18 AM, Simon Wright wrote:
>> Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:
>>
>>> 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.
>>>
>>> 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.
>
> To my mind, that is Cursor behaving similarly. Just as the index 3 is
> no longer the index of the item in question, so the Cursor should not
> be relied on.

In AdaCore's implementation, yes. But another implementation might
validly raise PE. To my mind, that'd be preferable.