sub fetch_hook
{
    my $this = shift ;
    my $type = shift ;
    foreach my $layer (reverse @{ $LayerStack{$this} })
    {
        &{ $layer->{$type} }() if defined $layer->{$type} ;
    }
}

sub MyDESTROY
{
    my $this = shift ;
    delete $LayerStack{$this} ;

    # call real DESTROY
    $this =~ /^(.*)=/;
    &{ $origDESTROY{$1} }($this);
}

1;

__END__

=head1 NAME

DBM_Filter -- Filter DBM keys/values 

=head1 SYNOPSIS

    use DBM_Filter ;
    use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File

    $db = tie %hash, ...

    $db->Filter_Push(Fetch => sub {...},
                     Store => sub {...});

    $db->Filter_Push('my_filter1');
    $db->Filter_Push('my_filter2', params...);

    $db->Filter_Key_Push(...) ;
    $db->Filter_Value_Push(...) ;

    $db->Filter_Pop();
    $db->Filtered();

    package DBM_Filter::my_filter1;
    
    sub Store { ... }
    sub Fetch { ... }

    1;

    package DBM_Filter::my_filter2;

    sub Filter
    {
        my @opts = @_;
        ...
        return (
            sub Store { ... },
            sub Fetch { ... } );
    }

    1;

=head1 DESCRIPTION

This module provides an interface that allows filters to be applied
to tied Hashes associated with DBM files. It builds on the DBM Filter
hooks that are present in all the *DB*_File modules included with the
standard Perl source distribution from version 5.6.1 onwards. In addition
to the *DB*_File modules distributed with Perl, the BerkeleyDB module,
available on CPAN, supports the DBM Filter hooks. See L<perldbmfilter>
for more details on the DBM Filter hooks.

=head1 What is a DBM Filter?

A DBM Filter allows the keys and/or values in a tied hash to be modified
by some user-defined code just before it is written to the DBM file and
just after it is read back from the DBM file. For example, this snippet
of code

    $some_hash{"abc"} = 42;

could potentially trigger two filters, one for the writing of the key
"abc" and another for writing the value 42.  Similarly, this snippet

    my ($key, $value) = each %some_hash

will trigger two filters, one for the reading of the key and one for
the reading of the value.

Like the existing DBM Filter functionality, this module arranges for the
C<$_> variable to be populated with the key or value that a filter will
check. This usually means that most DBM filters tend to be very short.

=head2 So what's new?