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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Examining individual bytes of an integer
Date: Wed, 17 Oct 2018 12:04:21 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:54616
Date: 2018-10-17T12:04:21+02:00
List-Id: <comp.lang.ada>

On 2018-10-17 11:47, briot.emmanuel@gmail.com wrote:
>> Meaning the first address of the object, e.g. of the dope of an
>> indefinite array as it was returned by the allocator:
>>
>>      Allocation_Address := Pointer_To_Object.all'Address - Offset;
>>
>> Presently the offset to the object beginning must be estimated during
>> run-time.
> 
> GNATCOLL has a package GNATCOLL.Storage_Pools.Headers which does some similar
> low-level tricks.
> When you use this storage pool, it will allocate extra bytes (for a header type that you
> provide), before the object'Address.

Right, this is the major use case: to allocate something in front of 
each object.

> GNATCOLL uses it for instance to store reference counting information without impacting
> the declaration of objects, or changing what their 'Address means.

I have an implementation of the linked list with links allocated in pool 
so that the list nodes have the type the original object as-is. Same 
approach.

P.S. It is kind of multiple inheritance for poor. Reference counts or 
links are "inherited" into the object's representation in the pool.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de