The main enhancements over the standard DBM Filter hooks are:

=over 4

=item *

A cleaner interface.

=item *

The ability to easily apply multiple filters to a single DBM file.

=item *

The ability to create "canned" filters. These allow commonly used filters
to be packaged into a stand-alone module.

=back

=head1 METHODS

This module will arrange for the following methods to be available via
the object returned from the C<tie> call.

=head2 $db->Filter_Push()

=head2 $db->Filter_Key_Push()

=head2 $db->Filter_Value_Push()

Add a filter to filter stack for the database, C<$db>. The three formats
vary only in whether they apply to the DBM key, the DBM value or both.

=over 5

=item Filter_Push

The filter is applied to I<both> keys and values.

=item Filter_Key_Push

The filter is applied to the key I<only>.

=item Filter_Value_Push

The filter is applied to the value I<only>.

=back


=head2 $db->Filter_Pop()

Removes the last filter that was applied to the DBM file associated with
C<$db>, if present.

=head2 $db->Filtered()

Returns TRUE if there are any filters applied to the DBM associated
with C<$db>.  Otherwise returns FALSE.



=head1 Writing a Filter

Filters can be created in two main ways

=head2 Immediate Filters

An immediate filter allows you to specify the filter code to be used
at the point where the filter is applied to a dbm. In this mode the
Filter_*_Push methods expects to receive exactly two parameters.

    my $db = tie %hash, 'SDBM_File', ...
    $db->Filter_Push( Store => sub { },
                      Fetch => sub { });

The code reference associated with C<Store> will be called before any
key/value is written to the database and the code reference associated
with C<Fetch> will be called after any key/value is read from the
database.

For example, here is a sample filter that adds a trailing NULL character
to all strings before they are written to the DBM file, and removes the
trailing NULL when they are read from the DBM file

    my $db = tie %hash, 'SDBM_File', ...
    $db->Filter_Push( Store => sub { $_ .= "\x00" ; },
                      Fetch => sub { s/\x00$// ;    });


Points to note:

=over 5

=item 1.

Both the Store and Fetch filters manipulate C<$_>.

=back