Re: Q: Endless loop by dispatching

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On Tue, 22 Jul 2003 15:11:15 +0000 (UTC), Lutz Donnerhacke
<lutz@iks-jena.de> wrote:

>* Dmitry A Kazakov wrote:
>> On Tue, 22 Jul 2003 12:18:57 +0000 (UTC), Lutz Donnerhacke
>>>If publicly defined tagged types are fixed at the private syntax element,
>>>there is no possibility to define dispatching procedures and functions
>>>afterwards. Of course, it's not a choice, because it will break existing
>>>programs.
>>
>> But what if I need to extend the interface by adding some new
>> primitive operations?
>
>Not possible in the private part. Plain and simple.
>
>package X is
>   type T is tagged private;
>   function Foo return T; -- Ok.
>private -- fix T
>   procedure Bar(o : T); -- Fails.
>   type T is new Ada.Finalization.Controlled with null record; -- Fails.
>end X;

But I do need private primitive operations to implement T and note, to
reuse them while implementing the types derived from T. This proposal
in effect would make impossible to have both public and private
operations. This will force a type designer to publish everything
breaking information hiding.

Just ask the question, why

   type T is new Ada.Finalization.Controlled with null record;

was made private? The only answer is: to hide Initialize/Finalize! To
my taste even 3.9.3(10) is too limiting. It should be possible to
declare a private abstract primitive subprogram! For example:

   type T is private abstract tagged private; -- (:-))
private
   type T is abstract tagged ...;
   procedure Write_etc_passwd (This : T) is abstract;

You can derive from T only in a child package. The public declaration
of T shall specify this fact, of course.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de