Re: Use of abstract tagged types without access types -- again and complete

["Prof. Dr. Carl Weierstrass" <carl.weierstrass@googlemail.com>]

On 9 Jun., 23:06, "Randy Brukardt" <ra...@rrsoftware.com> wrote:
> "Ludovic Brenta" <ludo...@ludovic-brenta.org> wrote in message
>
> news:87hb7yn4fs.fsf@ludovic-brenta.org...
>
> > "Prof. Dr. Carl Weierstrass" writes on comp.lang.ada:
> >>    type Parent_Type is new Ada.Finalization.Controlled with record
> >>          Child : Child_Type'Class;
> >>    end record;
>
> >> This doesn't work, because:
> >> class-wide subtype with unknown discriminants in component declaration
>
> >> Of course I could use an access type to avoid the error but is there a
> >> way to do something like this without access types.
>
> > No.  The reason is that the size of Child is unknown, so you cannot
> > encapsulate a Child into a Parent_Type, which has a fixed size.  In
> > contrast, the size of an access value is fixed at comile time, so you
> > can encapsulate an access value in the Parent_Type.
>
> Not quite accurrate -- an access type is necessary somewhere, but it doesn't
> have to be visible.
>
> Specifically, use the Holder container for this purpose:
>
>      package Child_Holder is new Ada.Containers.Indefinite_Holders
> (Child_Type'Class);
>
>      type Parent_Type is new Ada.Finalization.Controlled with record
>            Child : Child_Holder.Holder;
>      end record;
>
> This is a slight complication when using the component (you have to use
> Element and Replace_Element to access the contents), but it is usually
> better than writing all of this storage management yourself.
>
> The only downside is whether your Ada compiler supports this container (it
> was added after Ada 2005 was finished, early in the Ada 2012 process). It's
> used in the latest ASIS (that's why it was invented), so a compiler
> supporting ASIS for Ada 2005 probably supports the container.
>
> If your compiler doesn't yet support this container, it's easy enough to
> create it yourself rather than using explicit access types.
>
>                                              Randy.

Thank you very much. I think that this could be the solution that I
was looking for.
In case of many children I considered using an
Ada.Containers.Indefinite_Vectors type,
but I was looking for a solution when the is only one child.
Unfortunately I'am using
GNAT GPL 2010 and can't find that container in the library.

Any idea, where I can find an implementation? I hope it will be in the
next GNAT GPL 2011
version.

Regards