Re: [BLACKBOX] Storing an extended object

From: Douglas G. Danforth <"Douglas>
Date: Tue, 11 Nov 2008 15:52:45 -0800

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Robert, Josef, and Gerhard

Thank you all for your suggestions.
I will now mull over which is the best way.

-Doug Danforth


Robert wrote:
> 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 Wed Nov 12 2008 - 00:52:45 UTC