NAME

Class::Null - Implements the Null Class design pattern

SYNOPSIS

use Class::Null;
use Class::MethodMaker
  new_with_init => 'new',
  new_hash_init => 'new_hash',
  get_set       => 'log';

sub init {
  my $self = shift;
  $self->log(Class::Null->new);
  $self->new_hash(@_);
}

sub do_it {
  my $self = shift;
  $self->log->log(level => 'debug', message => 'starting to do it');
  ...
  $self->log->log(level => 'debug', message => 'still doing it');
  ...
  $self->log->log(level => 'debug', message => 'finished doing it');
}

DESCRIPTION

This class implements the Null Class design pattern.

Suppose that methods in your object want to write log messages to a log object. The log object is possibly stored in a slot in your object and can be accessed using an accessor method:

package MyObject;

use Class::MethodMaker
  new_hash_init => 'new',
  get_set       => 'log';

sub do_it {
  my $self = shift;
  $self->log->log(level => 'debug', message => 'starting to do it');
  ...
  $self->log->log(level => 'debug', message => 'still doing it');
  ...
  $self->log->log(level => 'debug', message => 'finished doing it');
}

The log object simply needs to have a log() method that accepts two named parameters. Any class defining such a method will do, and Log::Dispatch fulfils that requirement while providing a lot of flexibility and reusability in handling the logged messages.

You might want to log messages to a file:

use Log::Dispatch;

my $dispatcher = Log::Dispatch->new;

$dispatcher->add(Log::Dispatch::File->new(
  name      => 'file1',
  min_level => 'debug',
  filename  => 'logfile'));

my $obj = MyObject->new(log => $dispatcher);
$obj->do_it;

But what happens if we don't define a log object? Your object's methods would have to check whether a log object is defined before calling the log() method. This leads to lots of unwieldy code like

sub do_it {
  my $self = shift;
  if (defined (my $log = $self->log)) {
    $log->log(level => 'debug', message => 'starting to do it');
  }
  ...
  if (defined (my $log = $self->log)) {
    $log->log(level => 'debug', message => 'still doing it');
  }
  ...
  if (defined (my $log = $self->log)) {
    $log->log(level => 'debug', message => 'finished doing it');
  }
}

The proliferation of if-statements really distracts from the actual call to log() and also distracts from the rest of the method code. There is a better way. We could ensure that there is always a log object that we can call log() on, even if it doesn't do very much (or in fact, anything at all).

This object with null functionality is what is called a null object. The null class implementation is rather simple:

package Class::Null;
sub new { $singleton ||= bless {}, shift }
sub AUTOLOAD { *{$AUTOLOAD} = sub {} }

So we can create the object the usual way, using the new() constructor, and call any method on it, and all methods will do the same - nothing. It's effectively a catch-all object. We can use this class with our own object like this:

package MyObject;

use Class::Null;
use Class::MethodMaker
  new_with_init => 'new',
  new_hash_init => 'new_hash',
  get_set       => 'log';

sub init {
  my $self = shift;
  $self->log(Class::Null->new);
  $self->new_hash(@_);
}

sub do_it {
  my $self = shift;
  $self->log->log(level => 'debug', message => 'starting to do it');
  ...
  $self->log->log(level => 'debug', message => 'still doing it');
  ...
  $self->log->log(level => 'debug', message => 'finished doing it');
}

Note that we define two constructors (new() and new_hash()) since Class::MethodMaker's new_hash_init option doesn't let us define an object initialization method, whereas new_with_init doesn't process named arguments. So we define both and call the constructor that processes named arguments from our init() method.

This is only one example of using a null class, but it can be used whenever you want to make an optional helper object into a mandatory helper object, thereby avoiding unnecessarily complicated checks and preserving the transparency of how your objects are related to each other and how they call each other.

Although Class::Null is exceedingly simple it has been made into a distribution and put on CPAN to avoid further clutter and repetitive definitions.

INSTALLATION

See perlmodinstall for information and options on installing Perl modules.

AVAILABILITY

The latest version of this module is available from the Comprehensive Perl Archive Network (CPAN). Visit <http://www.perl.com/CPAN/> to find a CPAN site near you. Or see <http://www.perl.com/CPAN/authors/id/M/MA/MARCEL/>.

VERSION

This document describes version 1.02 of Class::Null.

AUTHOR

Marcel Grünauer, <marcel@cpan.org>

COPYRIGHT AND LICENSE

Copyright 2002 by Marcel Grünauer

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

cpanm Class::Null

perl -MCPAN -e shell
install Class::Null