NAME
    Class::Null - Implements the Null Class design pattern

VERSION
    This document describes version 1.04 of "Class::Null".

SYNOPSIS
      use Class::Null;
      use Class::MethodMaker::Util
        new_with_init => 'new',
        new_hash_init => 'new_hash',
        get_set_std   => '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::Util
        new_hash_init => 'new',
        get_set_std   => '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. 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.
    (Actually, it returns another "Class::Null" 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;
      use Class::MethodMaker::Util
        new_with_init => 'new',
        new_hash_init => 'new_hash',
        get_set_std   => '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::Util"'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.

METHODS
    new()
        Returns the singleton null object.

    any other method
        Returns another singleton null object so method chaining works.

DIAGNOSTICS
    There are no diagnostics for this module.

INCOMPATIBILITIES
    None reported.

BUGS AND LIMITATIONS
    No bugs have been reported.

    Please report any bugs or feature requests to
    "bug-class-null@rt.cpan.org", or through the web interface at
    <http://rt.cpan.org>.

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/>.

AUTHOR
    Marcel Grnauer, "<marcel@cpan.org>"

COPYRIGHT AND LICENSE
    Copyright 2004-2005 by Marcel Grnauer

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

DISCLAIMER OF WARRANTY
    BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
    FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
    OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
    PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER
    EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
    WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
    ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
    YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
    NECESSARY SERVICING, REPAIR, OR CORRECTION.

    IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
    WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
    REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE
    TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR
    CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
    SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
    RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
    FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
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    DAMAGES.

