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Re: Clean Object Class Design -- Circle/Ellipse

From: Jan Hidders <hidders_at_REMOVE.THIS.win.tue.nl>
Date: 1 Sep 2001 22:23:49 GMT
Message-ID: <9mrn5l$drk$1@news.tue.nl>


Dmitry A. Kazakov wrote:
> On Fri, 31 Aug 2001 21:34:59 -0400, "Bob Badour" <bbadour_at_golden.net>
> wrote:
>
> >>>The size will depend on the representation and not on the type.
> >>>Physical independence requires us to keep them separate.
> >>
> >>Yes, the size depends on the representation, so what? You know, the
> >>implementation of practically every method depends on the
> >>representation.
> >
> >Again, physical independence may require the DBMS to use multiple
> >implementations of some methods. The allocator will have to allow
> >the DBMS to store multiple different representations of the same
> >type each with possibly a different size -- this means that size
> >itself cannot be a property of the type.
> >
> >The DBMS internally knows the size of the physical representation it
> >is using without making that a property of the type.
>
> So there be no user-written allocators.

I have the impression that you two are talking at different abstraction levels here. Bob is talking at the level of the conceptual database schema. At that level you are abstracting from the implementation details such as the size of the physical representation. But Dmitry is talking at the level of implementation, viz., the programming of the class that actually implements the abstract class that is used at the level of the conceptual database schema. At that level it is very possible that you have to define a method 'size'. This does not violate the principle of physical independence because you are not defining this method at the level of the conceptual database schema.

-- 
  Jan Hidders
Received on Sat Sep 01 2001 - 17:23:49 CDT

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