Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mitel!sce!cognos!keithc
From: keithc@cognos.UUCP (Keith Campbell)
Newsgroups: comp.lang.eiffel
Subject: Re: How to use STORABLE?
Message-ID: <7738@cognos.UUCP>
Date: 15 Dec 89 21:54:53 GMT
References: <1989Dec11.170552.8455@isy.liu.se>
Reply-To: keithc@cognos.UUCP (Keith Campbell)
Distribution: comp
Organization: Cognos Inc., Ottawa, Canada
Lines: 46

In article <1989Dec11.170552.8455@isy.liu.se>
    nicwi@isy.liu.se (Niclas Wiberg) writes:

>An other approach would be to have an "dummy" object,
>to make the retrieval. But problem arises if you have
>class invariants. You don't want to create a big object
>structure, just to be able to retrieve old objects!

Dr. Meyer posted an article some time ago which suggested that
he was considering the addition of a feature which would allow
the Storable class to be more useful.  That feature would allow
the invocation of features on a void instance.  Retrieve could
then be changed so operate in this way avoiding the necessity
of creating your large object.

For the mean-time, I would suggest that you simply write a dummy
class like the one below.

    CLASS
        Retriever[ T -> Storable ] -- The constraint isn't really necessary.

    EXPORT
        retrieve

    FEATURE

        retrieve( fileName : String ) : T IS
            -- This is essentially a copy of Storable.retrieve
            -- except that c_retrieve returns an object of type "T"
            -- instead of "LIKE CURRENT".
            EXTERNAL
                c_retrieve : T LANGUAGE "C";
            DO
                Result := c_retrieve( fileName.to_c );
            END; -- retrieve

    END -- CLASS Retriever[ T -> Storable ]

You can inherit or create a local Retriever[ BigObject ] and use it
to retrieve the structure created by BigObject.store.

-- 
Keith Campbell               Cognos Incorporated   S-mail: P.O. Box 9707
Voice: (613) 738-1338 x5222                                3755 Riverside Drive
  FAX: (613) 738-0002                                      Ottawa, Ontario
 UUCP: uunet!mitel!sce!cognos!keithc                       CANADA  K1G 3Z4