Dumping Tie Objects

bichonfrise74 has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I have been playing around with the 'tie' operator and basically I want be able to 'dump' the object using Data::Dumper.

In the code specifically in the line,

print Dumper \%test;
[download]

Nothing is dump at all. I am not sure why. I read somewhere that I should treat variables binded by the tie operator as normal variables. So, normally the above syntax should show me the content of the hash. Any thoughts?

Below is the code.

#!/usr/bin/perl

use strict;
use Data::Dumper;

tie( my %test, 'Sample_Tie_Class' );

$test{'hello'} = 'hi';
print "$test{'hello'}\n";

print Dumper \%test;


package Sample_Tie_Class;

use strict;

sub TIEHASH {
    my $class = shift;
    bless { 'value' => {}, }, $class;
}

sub STORE {
    my ($self, $key, $value) = @_;
    $self->{'value'}->{$key} = $value;
}

sub FETCH {
    my ($self, $key) = @_;
    return $self->{'value'}->{$key};
}

sub FIRSTKEY {
}
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Comment on Dumping Tie Objects Select or Download Code

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Re: Dumping Tie Objects by bichonfrise74 (Vicar) on Nov 20, 2009 at 21:42 UTC
Nevermind. The reason why the Dumper line didn't work because I have not defined the FIRSTKEY and NEXTKEY functions. `sub FIRSTKEY { my ($self) = @_; each %{ $self->{'value'} }; } sub NEXTKEY { my ($self) = @_; each %{ $self->{'value'} }; }` [download] Once I added the two functions, then the Dumper worked. But interestingly, how come Perl didn't tell me that the two functions need to be defined at all?	[reply] [d/l]
Re^2: Dumping Tie Objects by AnomalousMonk (Archbishop) on Nov 21, 2009 at 00:15 UTC
I think (perhaps more knowledgeable monks can confirm) that the `FIRSTKEY` function needs to be defined such that it always returns the first key of the hash. As it is, it's the same as `NEXTKEY` and only returns the first key upon the first iteration or when iteration wraps around to the beginning again. Try (untested): `sub FIRSTKEY { my ($self) = @_; scalar keys %$Self; return each %{ $self->{'value'} }; }` [download] Update: keys resets the internal iterator of a hash. In void context, this happens with no other overhead, so the use of scalar above is only for the purpose of self-documentation. Also: On second thought, I would be inclined to implement `FIRSTKEY` in terms of `NEXTKEY` (also untested): `sub FIRSTKEY { my ($self) = @_; keys %$Self; return $self->NEXTKEY; }` [download]	[reply] [d/l] [select]