lapointd has asked for the wisdom of the Perl Monks concerning the following question:

Learned monks, I am writing a very abstract and challenging object-oriented application in Perl and I've hit the same problem a few times. It deals with the copying of objects...

I find in a number of classes, it is necessary to add a copy function like this (in the object package)... 


sub copy {
  my ($old)=@_;
  my $new_step=pathstep->new();
  foreach my $key (keys %{$old}) {
    # copy all attributes and values
    $new_step->{$key}=$old->{$key};
  }
  return $new_step;
}

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This is obviously a bit awkward and non-intuitive. What I would really like to do is this, at the application level: 
$copy=$original;  # DOES NOT WORK

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...except all that does is copy the reference, not the underlying data structure. Changes to $copy show up in $original. Another try on the problem is: 
%copy=%original;  # DOES NOT WORK

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...this complains in %copy being an unblessed reference. If you bless %copy in other than the proper module, it thinks it is the wrong kind of object (which can cause strange problems if the access functions are similar!).

It isn't a show stopper or anything - I essentially have it working - it just seems like there should be better way for future endeavors. It gets a bit more complicated when the copy operation is multi-layer (deep copies of objects within objects). The extra code just seems like an invitation to error.

Thanks in advance for your meditations.

(Hmmm, if I could overload operators, I would be in heaven!)

Replies are listed 'Best First'.
•Re: Copying Objects
by merlyn (Sage) on Jul 16, 2003 at 18:08 UTC
    You probably don't need to be able to copy any arbitrary object: just objects in one or more of your object hierarchies. Simply add the proper clone method to the top of each object tree, overriding the needed sub-parts when you specialize, and you should do fine. Then, you have access to the different modalities: 
    my $b = $a; # copy the reference, to pass a single object around
my $c = $a->copy; # create a copy, to have a new object like the old o
+ne

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    That way, the code will "do the right thing" depending on the object type as well. For a simple shallow copy of a hash object, this will work: 
    sub copy {
  my $self = shift;
  bless { %$self }, ref $self;
}

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    -- Randal L. Schwartz, Perl hacker
    Be sure to read my standard disclaimer if this is a reply.

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[d/l]
[select]
      Thanks - I hadn't thought of the aspect of reuse thru object hierarchies. It's true what I'm really looking for is potentially a few operations (not just copy/clone) that would be applicable to every object.

      For this particular app (its a multi-language code generator for process control modeling) I don't really have a lot of hierachies unfortunately. But, about the third time I cut and paste my generic copy() function I figured there must be a better way.

      I like your copy verion - it is much more compact. I think next time around I would abstract everything from a 'mother of all objects' class.

      It's still too bad the language doesn't build something in.

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        It's still too bad the language doesn't build something in.
        The problem is that the "right thing" to do is very different depending on the object. Sometimes you want no copy (for a singleton object, for example). Sometimes you want a shallow copy. Sometimes you want a deep copy. Only the author of a class can really know that.

        For example, suppose you had an object that had a link to the database from which it came. When you "copied" that object, you surely wouldn't want an entire clone of the database!

        Smalltalk solves this by having a "copy" method that "does the normal thing" for each class, which defaults to a deep copy, and a "shallowCopy" and "deepCopy" method that defaults to "copy". If a need to have something other than a deepcopy arises for the default (like the link to a database example), then the class author changes copy to be shallowCopy or some shallowCopy plus a copy of some of the deeper memebers. It works rather nicely, but does require some thinking for each class.

        -- Randal L. Schwartz, Perl hacker
        Be sure to read my standard disclaimer if this is a reply.

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Re: Copying Objects
by edan (Curate) on Jul 17, 2003 at 10:43 UTC
    (Hmmm, if I could overload operators, I would be in heaven!)

    If you are interested, look here: overload

    ... and be sure to send a postcard ;)

    --
    3dan
[reply]
      Yes - thank you - that rocks!
[reply]

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