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   Re: Derivation by Restriction

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  • From: Stefan Haustein <stefan.haustein@trantor.de>
  • To: "Fabio Arciniegas A." <l-arcini@uniandes.edu.co>
  • Date: Sun, 23 Jan 2000 13:11:14 +0100

> During the writing of some C++ classes for the representation of data
> conforming with a particular schema I began to wonder about the
> "translation" of an arbitrary schema to domain specific objects
> (pretty much in the way exactml or some oracle products do nowadays
> with DTDs) and I started to wonder about the implications of
> derivation by restriction.
> What I'm looking for is whether the impedance of Der. by Rest. on OO
> apps/representations has been considered by anyone before...(not a
> basic description of how derivation works,Nikita.But thanks).

I think the main point here is that derivation in 
object orientation means specialization. So if B is 
a subclass of A, you can replace any occurence of A 
by B. If you specialize B by restriction, that does 
not hurt object orientation as long a any instance of 
B is still a valid instance of A, too.

This principle would be violated if you "restrict" B 
that a subelement required in A is not required in B. 
In fact, that would not be a specialization or 
restriction but a generalization. If I read the specs 
right, you are not allowed to widen the [minOccurs...
maxOccurs] interval, so the problem you address seems 
not exist.

Best regards


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