Re: The Foundation of OO (XDb)

"Thomas Beale" <thomas_at_deepthought.com.au> wrote in message news:3D0EFBDF.2070606_at_deepthought.com.au...
>
> Bob Badour wrote:
>
> >>which again raises the notion of type inheritance.
> >>
> >Doing so only confuses logical and physical. The logical model does not
care
> >how the dbms physically represents the data. A video is a video. AVI,
MPEG
> >and QuickTime are all just representations of Video.
> >
> but not all subtypes have the semantics of implementation format of
> something (and depending on what you are trying to do, formats are not
> just a representation detail either, they do have different observable,
> functional characteristics). In another model, subtypes may exist to
> describe e.g. types of investment fund, or types of organisational unit.
> You cannot just hide the subtypes if that is the whole point of the model.

Subtypes and representations are different things. Different encodings of the same value are representations. Subtypes are proper subsets of a type with important distinguishing properties and operations.

A Video has operations to extract a sequence of images and a stream of sound with any necessary data to synchronize the two. It may also have additional operations such as a sequence of scenes with time encodings etc. or a set of contributors with roles etc. If a specific representation of the images and sound of Video does not include operations such as Scenes or Contributors, the possible representation of Video using that representation of images and sound will have to include specify the others separately.

This is similar to representing a Complex number with both a RealPart and an ImaginaryPart or with both a Magnitude and a Phase. Each of these representations are equally valid representations of the same type.

If one chooses an appropriate physical representation for the actual representation of Video, one can derive the observable, functional characteristics of all the other representations.

Different types of investment funds are different types. Different representations of the same investment fund type are different representations. Received on Tue Jun 18 2002 - 18:59:27 CEST