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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: tagged record child: override constructor?
Newsgroups: comp.lang.ada
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Date: Tue, 13 Sep 2005 09:32:58 +0200
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Date: 2005-09-13T09:32:54+02:00
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On 12 Sep 2005 22:58:54 -0700, sean.gilbertson@gmail.com wrote:

> I have a tagged record that is declared private, along with a
> constructor function which returns an instance.  I do this to enforce
> the assignment of several required fields in the record, so if you know
> how to do this another way, please let me know!

Ada.Finalization?

> But Ada is telling me I have to override this function in the child.

I presume you have something like:

   type X is private;
   function Create (...) return X;
private
   type X is tagged ...;

> This doesn't make a lot of sense.

On the contrary, Create above is covariant, that means that a derived type
cannot inherit it, because otherwise the result would be of the parent type
X. Thus it has to be overridden in each derived type. This is a very
important feature that saves a lot of debugging.

>  How can I deal with this?

Just make Create contravariant, provided that you know how to create
derived instances from there. In Ada contravariant subroutines are
class-wide:

   type X is tagged private;
      -- Publicly tagged to have an ability to declare class-wides
   function Create (...) return X'Class;
      -- This is same for all derived types
private
   type X is tagged ...;

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