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From: Brian Drummond <brian@shapes.demon.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: asynchronous task communication
Date: Tue, 8 Jan 2013 22:26:26 +0000 (UTC)
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Date: 2013-01-08T22:26:26+00:00
List-Id: <comp.lang.ada>

On Tue, 08 Jan 2013 10:12:30 -0700, Jeffrey Carter wrote:

> On 01/08/2013 05:02 AM, Brian Drummond wrote:
>>
>> And the index is a Cursor type.
> 
> No. The index is a discrete type chosen by the user. Cursor is a private
> type defined by the package. Both serve to represent a "location" in the
> array.

The context was Ada.Containers, where other forms of container use Cursor 
for indexing; I've been using List recently so it seemed odd that Vector 
has both index and cursor - but I stand corrected, they do appear to be 
separate, with overlapping functionality.

>> 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 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.

Question answered; whether it raises PE or follows Index in losing its 
place, Cursor cannot be used as a link to an object in the presence of 
Insert or Remove operations.

There is quite a lot of magic already in Ada.Containers (in the sense of 
making storage management easy and reliable) - that would be a bit too 
much to ask. (Not fundamentally impossible, but unlikely to be efficient)

Underlying point : as Dmitry says, Vector is the wrong abstraction for 
some (most?) graphs.

- Brian