Class::Singleton - Implementation of a "Singleton" class

    use Class::Singleton;

    my $one = Class::Singleton->instance();   # returns a new instance
    my $two = Class::Singleton->instance();   # returns same instance

This is the Class::Singleton module. A Singleton describes an object class that can have only one instance in any system. An example of a Singleton might be a print spooler or system registry. This module implements a Singleton class from which other classes can be derived. By itself, the Class::Singleton module does very little other than manage the instantiation of a single object. In deriving a class from Class::Singleton, your module will inherit the Singleton instantiation method and can implement whatever specific functionality is required.

For a description and discussion of the Singleton class, see Design Patterns, Gamma et al, Addison-Wesley, 1995, ISBN 0-201-63361-2.

Class::Singleton requires Perl version 5.004 or later. If you have an older version of Perl, please upgrade to latest version. Perl 5.004 is known to be stable and includes new features and bug fixes over previous versions. Perl itself is available from your nearest CPAN site (see INSTALLATION below).

The Class::Singleton module is available from CPAN. As the 'perlmod' man page explains:

    CPAN stands for the Comprehensive Perl Archive Network.
    This is a globally replicated collection of all known Perl
    materials, including hundreds of unbunded modules.

    [...]

    For an up-to-date listing of CPAN sites, see
    http://www.perl.com/perl/ or ftp://ftp.perl.com/perl/ .

The module is available in the following directories:

    /modules/by-module/Class/Class-Singleton-<version>.tar.gz
    /authors/id/ABW/Class-Singleton-<version>.tar.gz

For the latest information on Class-Singleton or to download the latest pre-release/beta version of the module, consult the definitive reference:

    http://www.kfs.org/~abw/perl/

Class::Singleton is distributed as a single gzipped tar archive file:

    Class-Singleton-<version>.tar.gz

Note that <version> represents the current version number, of the form 1.23. See REVISION below to determine the current version number for Class::Singleton.

Unpack the archive to create an installation directory:

    gunzip Class-Singleton-<version>.tar.gz
    tar xvf Class-Singleton-<version>.tar

'cd' into that directory, make, test and install the module:

    cd Class-Singleton-<version>
    perl Makefile.PL
    make
    make test
    make install

The 'make install' will install the module on your system. You may need root access to perform this task. If you install the module in a local directory (for example, by executing perl Makefile.PL LIB=~/lib in the above - see CWperldoc MakeMaker for full details), you will need to ensure that the PERL5LIB environment variable is set to include the location, or add a line to your scripts explicitly naming the library location:

    use lib '/local/path/to/lib';

To import and use the Class::Singleton module the following line should appear in your Perl script:

    use Class::Singleton;

The instance() method is used to create a new Class::Singleton instance, or return a reference to an existing instance. Using this method, it is only possible to have a single instance of the class in any system.

    my $highlander = Class::Singleton->instance();

Assuming that no Class::Singleton object currently exists, this first call to instance() will create a new Class::Singleton and return a reference to it. Future invocations of instance() will return the same reference.

    my $macleod    = Class::Singleton->instance();

In the above example, both CW$highlander and CW$macleod contain the same reference to a Class::Singleton instance. There can be only one.

A module class may be derived from Class::Singleton and will inherit the instance() method that correctly instantiates only one object.

    package PrintSpooler;
    use vars qw(@ISA);
    @ISA = qw(Class::Singleton);

    # derived class specific code
    sub submit_job {
        ...
    }

    sub cancel_job {
        ...
    }

The PrintSpooler class defined above could be used as follows:

    use PrintSpooler;

    my $spooler = PrintSpooler->instance();

    $spooler->submit_job(...);

The instance() method calls the _new_instance() constructor method the first and only time a new instance is created. All parameters passed to the instance() method are forwarded to _new_instance(). In the base class this method returns a blessed reference to an empty hash array. Derived classes may redefine it to provide specific object initialisation or change the underlying object type (to a list reference, for example).

    package MyApp::Database;
    use vars qw( $ERROR );
    use base qw( Class::Singleton );
    use DBI;

    $ERROR = '';

    # this only gets called the first time instance() is called
    sub _new_instance {
        my $class = shift;
        my $self  = bless { }, $class;
        my $db    = shift || "myappdb";    
        my $host  = shift || "localhost";

        unless (defined ($self->{ DB } 
                         = DBI->connect("DBI:mSQL:$db:$host"))) {
            $ERROR = "Cannot connect to database: $DBI::errstr\n";
            # return failure;
            return undef;
        }

        # any other initialisation...

        # return sucess
        $self;
    }

The above example might be used as follows:

    use MyApp::Database;

    # first use - database gets initialised
    my $database = MyApp::Database->instance();
    die $MyApp::Database::ERROR unless defined $database;

Some time later on in a module far, far away...

    package MyApp::FooBar
    use MyApp::Database;

    sub new {
        # usual stuff...

        # this FooBar object needs access to the database; the Singleton
        # approach gives a nice wrapper around global variables.

        # subsequent use - existing instance gets returned
        my $database = MyApp::Database->instance();

        # the new() isn't called if an instance already exists,
        # so the above constructor shouldn't fail, but we check
        # anyway.  One day things might change and this could be the
        # first call to instance()...  
        die $MyAppDatabase::ERROR unless defined $database;

        # more stuff...
    }

The Class::Singleton instance() method uses a package variable to store a reference to any existing instance of the object. This variable, _instance, is coerced into the derived class package rather than the base class package.

Thus, in the MyApp::Database example above, the instance variable would be:

    $MyApp::Database::_instance;

This allows different classes to be derived from Class::Singleton that can co-exist in the same system, while still allowing only one instance of any one class to exists. For example, it would be possible to derive both 'PrintSpooler' and 'MyApp::Database' from Class::Singleton and have a single instance of each in a system, rather than a single instance of either.

Andy Wardley, CW<abw@cre.canon.co.uk>

Web Technology Group, Canon Research Centre Europe Ltd.

Thanks to Andreas Koenig CW<andreas.koenig@anima.de> for providing some significant speedup patches and other ideas.

$Revision: 1.3 $

Copyright (C) 1998 Canon Research Centre Europe Ltd. All Rights Reserved.

This module is free software; you can redistribute it and/or modify it under the term of the Perl Artistic License.

Canon Research Centre Europe Perl Pages

http://www.cre.canon.co.uk/perl/

The Author's Home Page

http://www.kfs.org/~abw/

Design Patterns

Class::Singleton is an implementation of the Singleton class described in Design Patterns, Gamma et al, Addison-Wesley, 1995, ISBN 0-201-63361-2