Tying hashes with undef keys

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

Hello monks! I was curious about ties recently, so I looked up the docs and started reading. However, when reading about NEXTKEY, I was wondering how to create a tied hash that has undef as a key. Here's my experimental code:

# TestHash.pm

use strict;
use warnings;

package TestHash;

sub TIEHASH {
  return bless [], $_[0];
}

sub FIRSTKEY {
  return undef;
}

sub NEXTKEY {
  my ($this, $last) = @_;
  return $last >= 10 ? undef : $last + 1;
}
[download]

So when I call keys on a tied hash of this variety, it returns (), as expected. But if I wanted to, how would I return an undef value without ending the keys list?

Thanks, Rob

Comment on Tying hashes with undef keys Download Code

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Re: Tying hashes with undef keys by ysth (Canon) on Jul 09, 2007 at 01:43 UTC
Don't have time to try it now, but based on the perl source, returning an overloaded object should work. It also looks like NEXTKEY gets passed the previous key - something I don't remember seeing in the doc. Ah, it is documented. Learning something new every day++.	[reply]
Re: Tying hashes with undef keys by wind (Priest) on Jul 09, 2007 at 04:19 UTC
Read perltie#Tying hashes I have not spent much time working with tied variables. However, it appears that the FIRSTKEY and NEXTKEY functions are supposed to return a key, value pair. The end of the list would therefore be denoted by (). Although your undef appears to work as well. Note that normally hash keys are as a rule defined strings. However, I would guess that you might be able to return an undefined key using the following `sub FIRSTKEY { return (undef, 'foobar'); }` [download] - Miller	[reply] [d/l]