tie-ing then tie-ing again

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

I am having some problems with tie and DBI. I have boiled the problem down to this short example. Basically, when you run the code below, you get the following error:

Can't use an undefined value as an ARRAY reference at ./example.pl lin
+e 37.
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But if you comment out the line

$hash{blah} = 1;
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Everything works as expected. FETCH returns an anonymous array and foreach properly iterates through it printing out the following:

$user = {
          'val' => 1
        };
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Can anyone explain why when you tie a hash, then tie another inside a FETCH call, you see these problems? thanks very much, --eric

#!/usr/bin/perl -w
package MyTie1;
 
sub TIEHASH { bless {}, shift; }
 
sub STORE {
  my $self = shift;
 
  my ($key, $value) = @_;
 
  $self->{$key} = $value;
}
 
package MyTie2;
 
sub TIEHASH { bless {}, shift; }
 
 
sub FETCH {
 
  my $self = shift;
  my $key = shift;
 
  my %hash;
  tie %hash, 'MyTie1';
 
  $hash{blah} = 1;
 
 return [{val => 1}];
}
 
package main;
 
my %hash;
tie %hash, 'MyTie2';
use Data::Dumper;
foreach my $user (@{$hash{one}}){
  print Data::Dumper->Dump([$user],['user']);
 
}
[download]

Comment on tie-ing then tie-ing again Select or Download Code

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(tye)Re: tie-ing then tie-ing again by tye (Sage) on Apr 11, 2001 at 19:37 UTC
Looks like a bug in Perl to me. You can't used a tied variable inside of another tied variable's FETCH. I did some experimenting and came up with the following case that also illustrates the problem but with fewer moving parts inside the FETCH call: #!/usr/bin/perl -w package MyTie1; sub TIEHASH { bless {}, shift; } sub FETCH { my $self = shift; my $key = shift; return [{foo=>'bar'}]; } package MyTie2; sub TIEHASH { my $self= {}; my %hash; $self->{hash}= \%hash; tie %hash, 'MyTie1'; bless $self, shift; } sub FETCH { my $self = shift; my $key = shift; return [{val => $self->{hash}->{key}}]; } package main; my %hash; tie %hash, 'MyTie2'; use Data::Dumper; foreach my $user (@{$hash{one}}){ print Data::Dumper->Dump([$user],['user']); } __END__ Can't use an undefined value as an ARRAY reference at refetch.pl line +33. [download] I recommend you report this using the perlbug script that comes with Perl. - tye (but my friends call me "Tye")	[reply] [d/l]

Replies are listed 'Best First'.

(tye)Re: tie-ing then tie-ing again
by tye (Sage) on Apr 11, 2001 at 19:37 UTC

Looks like a bug in Perl to me. You can't used a tied variable inside of another tied variable's FETCH. I did some experimenting and came up with the following case that also illustrates the problem but with fewer moving parts inside the FETCH call:

#!/usr/bin/perl -w
package MyTie1;

sub TIEHASH { bless {}, shift; }

sub FETCH {
  my $self = shift;
  my $key = shift;
  return [{foo=>'bar'}];
}

package MyTie2;

sub TIEHASH {
    my $self= {};
    my %hash;
    $self->{hash}= \%hash;
    tie %hash, 'MyTie1';
    bless $self, shift;
}

sub FETCH {
  my $self = shift;
  my $key = shift;
  return [{val => $self->{hash}->{key}}];
}

package main;

my %hash;
tie %hash, 'MyTie2';
use Data::Dumper;
foreach my $user (@{$hash{one}}){
  print Data::Dumper->Dump([$user],['user']);
}
__END__
Can't use an undefined value as an ARRAY reference at refetch.pl line 
+33.
[download]

tye

[reply]
[d/l]