Re: [BLACKBOX] Storing an extended object

From: [at]} <Robert>
Date: Tue, 11 Nov 2008 18:13:15 +0000

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Doug

1 -
In your Base type declare a NEW EMPTY PROCEDURE Externalize2-.
Call it in your Externalize PROCEDURE.

In your extended type implement Externalize2 for the additional fields.

If further extensions are possible, ectend the chain to Externalize3- etc.


2 - For extensive examples of this technique look at Escher, eg Escher.EdgeFigures.

3 - YES - DO store a version byte - it will get you out of trouble every time.

Cheers

Robert


PS: Using 'Super' calls is 'Deprecated' "because of the so-called semantic
fragile base class problem" - See the 'Whats New' Docu. I don't understand any
of this, but just trust the computer scientists to have got it right!


Douglas G. Danforth wrote:
> I am not quite sure how to implement the Externalize and Internalize
> procedures
> for objects that are an extension of a base type. If I write an
> Externalize procedure
> (one for the base and one for the extension) then BB complains. So must
> I do something
> like the following?
>
> TYPE
> Base = POINTER TO EXTENSIBLE RECORD(Stores.Store)
> data: INTEGER
> END;
> Sub = POINTER TO RECORD(Base)
> moreData: INTEGER;
> END;
>
> PROCEDURE (base: Base) Externalize- (VAR wr: Stores.Writer);
> BEGIN
> wr.WriteInt(base.data);
> WITH base: Sub DO
> wr.WriteInt(base.moreData);
> ELSE
> END
> END Externalize;
>
> But now when I internalize what must I do?
>
> This seems cumbersome and not very elegant.
> -Doug Danforth
>
>
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Received on Tue Nov 11 2008 - 19:13:15 UTC