Path: news.daimi.aau.dk!datpete
From: datpete-nospam@daimi.aau.dk (Peter Andersen)
Newsgroups: comp.lang.beta
Subject: Re: copy (clone) of objects with references ?
Date: 25 Oct 1995 10:00:14 GMT
Organization: DAIMI, Computer Science Dept. at Aarhus University
Lines: 95
Message-ID: <46l1re$hi0@krone.daimi.aau.dk>
References: <46g2cq$401@fbi-news.Informatik.Uni-Dortmund.DE>
Reply-To: datpete@mjolner.dk (Peter Andersen)
NNTP-Posting-Host: fraxinus.daimi.aau.dk
Keywords: copy, clone, object, references

In <46g2cq$401@fbi-news.Informatik.Uni-Dortmund.DE> Thus spake rosans00@marvin.informatik.uni-dortmund.de (Thomas Rosanski):

>I have tried to use the copy (clone) pattern, as described in the Beta FAQ.
>But I have problems to get a copy, that has references to identic objects,
>which should also be generated new. (if you do not understand me english,
>just take a look of me program code:)

>(# mat:
>     (# rows: 
>          (# next: ^rows;   
>             pointer: ^cols; 
>             index: @integer;
>           #);
>         cols: 
>           (# next: ^cols;    
>              ele: @integer;     
>              index: @integer;   
>            #);
>        begin: ^rows;
>        (* some attributes like init, add, mult, ... *)
>        copy: (# (* as suggested in the BETA faq *) #);
>      #); (* end of mat *)

>    A: @ mat;
>    B: ^ mat;
>    do
>    (* some inits, filling A with references to rows, cols and
>       setting integer values *)

>    A.copy -> B[];   (* trying to clone the object A *)

>#)

>After A.copy -> B[]; B has the same pattern strukture like A, but
>the references next[] and pointer[] are NONE. So me question: Is there
>an easy way to copy (clone) an object, that contains refernce attributes?
>(I think, it is possible via the persistent store - storing A to
>a persistent store and restore the content of the persistent store to B -
>but this seems to use much time).

The FAQ explains it like this:

the following trick does the job: 

   P: (# (* internal P structures *)
         copy:< (* generic copy operation *)
           (# copyType: ##P;
              theCopy: ^P;
           do this(P)## -> copyType##;
              &copyType[] -> theCopy[];
              (* copying internal P structures into theCopy *) <<<<-----
              INNER copy;
              (* possible finalization of the copying process *)
           exit theCopy[]
           #)
      #);

   Q: P(# (* internal Q structures *)
          copy::<
            (# Qcopy: ^Q 
            do theCopy[] -> Qcopy[];
              (* copying internal Q structures into Qcopy *) <<<<-----
              INNER copy;
              (* possible finalization of the Q copying process *)
            #)
       #);   
   
   R: Q(# (* internal R structures *)
          copy::<
            (# Rcopy: ^R 
            do theCopy[] -> Rcopy[];
              (* copying internal R structures into Rcopy *)   <<<<-----
              INNER copy;
              (* possible finalization of the R copying process *)
            #)
       #);

Here the comments 

   (* copying internal R structures into Rcopy *)

denotes code not shown, i.e. at this place you should add code that copies
*internal* parts of the object, e.g. next[] and pointer[] ind your example.

I aggree, that the FAQ example should probably be a little more detailed.

Peter Andersen
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