deep copy of nested data structure

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

I want to copy a nested data structure. The FAQ tells me:

The Data::Dumper module on CPAN is nice for printing out data structures, and FreezeThaw for copying them. For example:

      use FreezeThaw qw(freeze thaw);
      $new = thaw freeze $old;

Where $old can be (a reference to) any kind of data structure you'd like. It will be deeply copied.

However, the following program:

use FreezeThaw qw(freeze thaw);
use Data::Dumper;

$hashref = { a => {}, b => {} };

print Dumper($hashref);
$copy = thaw freeze $hashref;
print Dumper($copy);
[download]

gives the following output:

$VAR1 = {
          'a' => {},
          'b' => {}
        };
$VAR1 = 1;

What's the problem?

Comment on deep copy of nested data structure Download Code

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Re: deep copy of nested data structure by autark (Friar) on Nov 22, 2000 at 13:54 UTC
For cloning purposes, may I suggest using either the Storable modules dclone method `use Storable qw(dclone); my $copy = dclone($some_ref);` [download] or perhaps the Clone module `use Clone qw(clone); my $copy = clone($some_ref);` [download] Back to your problem at hand, the reason you get 1 as a result of the `thaw` method, is because it is documented to return an array. And an array in scalar context returns the number of elements, which in your case is 1. Try something like this: `@copy = thaw freeze $hashref` Autark.	[reply] [d/l] [select]
Re: deep copy of nested data structure by davorg (Chancellor) on Nov 22, 2000 at 14:12 UTC
The `thaw` function in FreezeThaw returns an array which you are assigning to a scalar. The scalar (`$copy`) therefore ends up getting the number of elements in the array (in this case 1). Try replacing your code with this: `($copy) = thaw freeze $hashref;` [download] to force an array assignment. -- <http://www.dave.org.uk> "Perl makes the fun jobs fun and the boring jobs bearable" - me	[reply] [d/l]
Re: deep copy of nested data structure by arturo (Vicar) on Nov 22, 2000 at 20:50 UTC
Let me also refer you to Adam's highly informative and useful node RE: Data::Dumper (Adam: Sample Usage), which helps explain how to use Data::Dumper to dump deeply nested data structures "as is." Philosophy can be made out of anything. Or less -- Jerry A. Fodor	[reply]
Re: deep copy of nested data structure by little (Curate) on Nov 22, 2000 at 13:45 UTC
1 is just the return value of the function ("thaw" in your case cause it's processed after "freeze") when it succeeded the module is changing %hashref or @hashref, you should use the strict pragma which should throw an error on your use of $hashref as it pretty much looks alike a hash So better print out the scalar, array or hash when you've frozen it and thawed it again to see how it works. Have a nice day All decision is left to your taste Update You might consider to use module Storable as well. And no, don't rely on my first sentence, I was wrong, the 1 is the length of the returned array as described below.	[reply]