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From: "Vinzent Hoefler" <nntp-2010-10@t-domaingrabbing.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada is getting more popular!
Date: Thu, 14 Oct 2010 00:34:04 +0200
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Date: 2010-10-14T00:34:04+02:00
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On Wed, 13 Oct 2010 23:53:14 +0200, ramon_garcia <ramon.garcia.f@gmail.com> wrote:

> I have a record with an array member constrained. I want to obtain an
> unconstrained accesss, because I want to return it in a procedure. I
> want to return an access, because returning an array implies a copy.

Huh? Since when do "out" parameters imply a copy?

> And I want the caller to get the bounds of the returned access at
> runtime.

I don't understand that. Either you are creating the array
dynamically, then getting an access to it should not be a problem
or the caller already knows the bounds.

> type buffer is array(Natural range<>) of Unsigned_8;
> type buffer_access is access all buffer;
>
> type t is record
>    data: buffer(1..4);

There an "aliased" would be missing.

> end record;
>
> a_t: t := .....;
>
> abuf_access := a_t.data'Unrestricted_Access;
>
> Note that this has two dependencies with GNAT:
>  - Access to arrays use double pointers. So in the obtained access one
> can obtain the bounds correctly, with abuf_access'First,
> abuf_access'Last.

This is not GNAT specific. An access type is guaranteed to work
correctly here. An *address* OTOH may not, but that's different
anyway.

>  - The attribute Unrestricted_Access.
>
> I cannot understand why a procedure that expects an unconstrained
> array can be passed a constrained one,

Because every array is constrained at the point of the call. There
are no unconstrained array objects.

> and it is not possible to get
> an access to an unconstrained array from a constrained one.

It is. But not if it's explicitly constrained (by index constraints).
Unfortunately, this is the only way to do it inside a record.


Vinzent.

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