deep_copy objects question

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

I have a routine from Randal Schwartz's site at: http://www.stonehenge.com/merlyn/UnixReview/col30.html called deep copy which copies the contents of a reference to stuff in a deep manner rather than just copying the pointers.
My problem is it loses the object information if it's copying objects.
Any one have any idea of how to modify the routine to handle objects?

my $arrayObjsRef = \@arrayOfObjects;
my $copy = _deep_copy( $arrayObjsRef );


sub _deep_copy
{
   my $this = shift;
 
   my $ref = ref $this;
   
   if ( not ref $this )
   {
      $this;
   }
   elsif ( ref $this eq "ARRAY" )
   {
      [ map _deep_copy( $_ ), @$this ];
   }
   else
   { 
     # this is where the objects would be copied
      +{ map { $_ => _deep_copy( $this->{$_} ) } keys %$this };
   }
}
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Re: deep_copy objects question by GreyGlass (Sexton) on Aug 13, 2004 at 19:57 UTC
Notice that Randal's code supports copying only structures composed of arrays and hashes. You really do not want to break object abstractions by copying their guts - it is bound to lead to problems sooner or later. Anyway, in the most general sense, you cannot always correctly copy objects, handles, code, etc. Code is a problem, since there may be persistent state associated with it - how do you duplicate it? Handles almost always have state that cannot be duplicated to an independent copy. Even if you assume that your structure does not contain anything but pure Perl objects, those object can contain references to code and handles. Moreover, the class implementations for the objects may be keeping class-wide state, which means the only valid way of copying the object implementation (from the class point of view) is to use a class-specific copy constructor, if the class allows this by implementing one. This, of course, does not disagree with the other posters' suggestions - you can always take the risk and just deep-copy and re-bless anything you find. Just watch out for the consequences.	[reply]
•Re^2: deep_copy objects question by merlyn (Sage) on Aug 13, 2004 at 20:54 UTC
In my article on constructing objects, I talk a bit about cloning objects as well, including the issues of deep and shallow cloning and initialization. -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply]
Re^2: deep_copy objects question by JAMBOID (Sexton) on Aug 13, 2004 at 20:16 UTC
Thanks for the wisdom. I recoded to not require copying but hold onto the original references. But if I need to copy objects I'll use Clone or Storeable. Peace and a veritable lack of bugs. --jambo	[reply]
Re: deep_copy objects question by dragonchild (Archbishop) on Aug 13, 2004 at 19:24 UTC
You need to bless the copy with the class the original was blessed into. You might want to take a look at Scalar::Util and the blessed() function, because not all objects are hashrefs (mine tend to be arrays) and, even if all your objects were hashrefs, then you'd assume all hashrefs were objects. ------ We are the carpenters and bricklayers of the Information Age. Then there are Damian modules.... sigh* ... that's not about being less-lazy -- that's about being on some really good drugs -- you know, there is no spoon.* - flyingmoose I shouldn't have to say this, but any code, unless otherwise stated, is untested	[reply]
Re: deep_copy objects question by Fletch (Bishop) on Aug 13, 2004 at 19:34 UTC
There's also always Storable's `dclone` routine which handles blessed references as well.	[reply] [d/l]
Re: deep_copy objects question by ihb (Deacon) on Aug 13, 2004 at 22:27 UTC
One thing you really want to watch out for are recursive data structures. Try your clone code against `my $foo = {}; $foo->{foo} = $foo;` [download] `ihb` Read argumentation in its context!	[reply] [d/l] [select]
Re: deep_copy objects question by johnnywang (Priest) on Aug 13, 2004 at 19:44 UTC
In addition to Storable, there are some CPAN modules for cloning Clone	[reply]