RE: [BLACKBOX] SYSTEM.VAL, WITH, IS, and type casting

From: null <">
Date: Tue, 23 Aug 2011 07:25:08 +0000

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Hi Doug,

> -----Original Message-----
[...]
> PROCEDURE ReadMyObject (VAR rd: Reader, OUT x: MyObject);
> BEGIN
> rd.ReadStore(SYSTEM.VAL(Stores.Store, x))
> END;
>
> PROCEDURE WriteMyObject (VAR wr: Writer, x: MyObject);
> BEGIN
> wr.WriteStore(SYSTEM.VAL(Stores.Store, x))
> END;
>
> Although WriteMyObject is superfluous since one can already
> just write.
> wr.WriteStore(x)
> as long as x is an extension of Stores.Store.
>

although I am a bit uncertain, I still fear that you have to resort
to low level stuff at this stage (and we are back at the problem
outlined by Hanspeter M. about storing and retrieving objects) since
you normally don't know what kind of extension you're reading and
you can't decide which one it is. If you write your MyObject (which
I asume to be an extension of Stores.Store) as a Stores.Store you
loose all information of the extended object. When retrieving it
you have a Stores.Store with all additional information lost.

If you have an object of static type Store, you can test for its
dynamic type when writing, but you cannot when reading ...
I think here the asymmetry stems from.

hope this helps ...

--
  Bernhard 
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Received on Tue Aug 23 2011 - 09:25:08 UTC