use 5.008;
use strict;
use warnings;
package Class::Null;
BEGIN {
$Class::Null::VERSION = '2.110730';
}
# ABSTRACT: Implements the Null Class design pattern
use overload
'bool' => sub { 0 },
'""' => sub { '' },
'0+' => sub { 0 },
fallback => 1;
our $null = bless {}, __PACKAGE__;
sub AUTOLOAD { $null }
1;
__END__
=pod
=head1 NAME
Class::Null - Implements the Null Class design pattern
=head1 VERSION
version 2.110730
=head1 SYNOPSIS
use Class::Null;
# some class constructor and accessor declaration here
sub init {
my $self = shift;
# ...
$self->log(Class::Null->new);
# ...
}
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');
}
=head1 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 base 'Class::Accessor';
__PACKAGE__->mk_accessors(qw(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 C<log()> method that accepts
two named parameters. Any class defining such a method will do, and
C<Log::Dispatch> fulfills 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
C<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 C<log()> and also distracts from the rest of the method code. There
is a better way. We can ensure that there is always a log object that we can
call C<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. We can
create the object the usual way, using the C<new()> constructor, and call any
method on it, and all methods will do the same - nothing. (Actually, it
always returns the same C<Class::Null> singleton object, enabling method
chaining.) It's effectively a catch-all object. We can use this class with our
own object like this:
package MyObject;
use Class::Null;
# some class constructor and accessor declaration here
sub init {
my $self = shift;
...
$self->log(Class::Null->new);
...
}
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');
}
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 C<Class::Null> is exceedingly simple it has been made into a
distribution and put on CPAN to avoid further clutter and repetitive
definitions.
=head1 METHODS
=head2 new
Returns the singleton null object.
=head2 Any other method
Returns another singleton null object so method chaining works.
=head1 OVERLOADS
=over 4
=item Boolean context
In boolean context, a null object always evaluates to false.
=item Numeric context
When used as a number, a null object always evaluates to 0.
=item String context
When stringified, a null object always evaluates to the empty string.
=back
=head1 INSTALLATION
See perlmodinstall for information and options on installing Perl modules.
=head1 BUGS AND LIMITATIONS
No bugs have been reported.
Please report any bugs or feature requests through the web interface at
L<http://rt.cpan.org/Public/Dist/Display.html?Name=Class-Null>.
=head1 AVAILABILITY
The latest version of this module is available from the Comprehensive Perl
Archive Network (CPAN). Visit L<http://www.perl.com/CPAN/> to find a CPAN
site near you, or see L<http://search.cpan.org/dist/Class-Null/>.
The development version lives at L<http://github.com/hanekomu/Class-Null>
and may be cloned from L<git://github.com/hanekomu/Class-Null.git>.
Instead of sending patches, please fork this project using the standard
git and github infrastructure.
=head1 AUTHOR
Marcel Gruenauer <marcel@cpan.org>
=head1 COPYRIGHT AND LICENSE
This software is copyright (c) 2005 by Marcel Gruenauer.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.
=cut