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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Why does `Unchecked_Deallocation` need the access type?
Date: Tue, 28 Jul 2015 16:25:19 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2015-07-28T16:25:19-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:170ajsv4tazu3.mgrs0or8en9w$.dlg@40tude.net...
> 3. Tagged hierarchies
>
> type S is new T with private;
>
> S'Parent (T'Class) = T (in a public context)
>
> The argument indicates the hierarchy where to search for the ancestor (due
> to MI).

We tried such an attribute sometime during the Ada 2005 work and eventually 
gave up. The problem being that the parent (in a language sense - it's the 
subtype after "new") depends on the visibility on the type, and that is very 
unconfortable (besides causing problems):

  package P is
      type S is new T with private;
      procedure Bar (Obj : in out S'Parent); -- S'Parent is T here.
  private
      type S in new U with private; -- U is descended from T, of course.
  end P;

  package body P is
      procedure Bar (Obj : in out S'Parent) is -- !! - S'Parent is U here.
      ...
  end P;

The body of Bar does not conform with the specification as S'Parent means 
something different in the two locations. That potentially includes having 
different operations available (Imagine if U has a function Foo that T does 
not have.), so there is a potential for a real problem.

Eventually we added a function Parent_Tag in Ada.Tags; since it's runtime it 
ignores privacy and thus it has a unique value.

I think you are using "Parent" in your write-up as what Ada calls a direct 
ancestor - that includes the parent (if any) and any progenitors. That's a 
different problem -- at least hiding interfaces isn't allowed so the 
visibility problem doesn't come up for progentitors. (Of course, if we ever 
did full MI, it would come back with a vengance.)

                                                Randy.