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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Equivalent of dynamic_cast (downcast) for tagged types
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Date: Fri, 28 Jan 2011 15:13:08 +0100
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Date: 2011-01-28T15:13:08+01:00
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On Fri, 28 Jan 2011 05:11:16 -0800 (PST), AdaMagica wrote:

> On 28 Jan., 10:16, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>> On Thu, 27 Jan 2011 14:35:28 -0800 (PST), Maciej Sobczak wrote:
>>> I have submitted this bug report more than one year ago. Looks like
>>> nobody even bothered to check it.
>>
>> Well, I am not a language lawyer to tell if the behavior (public overriding
>> of a private primitive operation) is legal.
>>
>>> The bug submission states that Adjust is wrongly recognized as
>>> overriding, which it shouldn't be - but now I have hit something more
>>> interesting.
>>
>> It must be overriding because Adjust is a primitive operation. Whether the
>> compiler should allow it to become publicly declared as an overriding is
>> another question.
> 
> The public specification of Ada.Finalization.Limited_Controlled has no
> Adjust operation, so there is no primitive operation with that name
> which you can override, full stop.

It has no *visible* primitive operation.

> It's irrelevant if there is some
> internal one deeply burrowed in the compiler's implementation.

Unfortunately it leaks out. Consider this:

package P is
   type T is tagged private;
private
   type T is tagged null record;
   procedure Foo (X : in out T);
end P;

with P;
package Q is
   type S is new P.T with null record;
   procedure Foo (X : in out S);
end Q;

with Q;
package P.R is
   type U is new Q.S with null record;
   procedure Bar (Object : in out U'Class);
end P.R;

package body P.R is
   procedure Bar (Object : in out U'Class) is
   begin
      Foo (Object); -- Which Foo is called here? Is there one or two?
   end Bar;
end P.R;

This problem can also be observed with components. E.g. P.T may have a
component Baz, declared privately. Q.S may have "another" Baz declared
publicly. P.R.U faces a name conflict which must be resolved in some of two
possible ways: "overloading" vs. "overriding."

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