Joshua ben Jore


Class::WeakSingleton - A Singleton that expires when all the references to it expire


This is the Class::WeakSingleton 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, system registry or database connection. A "weak" Singleton is not immortal and expires when all other references to the original instance have expired. This module implements a Singleton class from which other classes can be derived, just like Class::Singleton. By itself, the Class::WeakSingleton module does very little other than manage the instantiation of a single object. In deriving a class from Class::WeakSingleton, 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 Class::Singleton and "Design Patterns", Gamma et al, Addison-Wesley, 1995, ISBN 0-201-63361-2.


  use Class::WeakSingleton;
      my $c = Class::WeakSingleton->instance;
      my $d = Class::WeakSingleton->instance;
      die "Mismatch" if $c != $d;
  }   # Class::WeakSingleton->instance expires
      my $e = Class::WeakSingleton->instance;
          my $f = Class::WeakSingleton->instance;
          die "Mismatch" if $e != $f;
  }   # Class::WeakSingleton->instance expires


$singleton = YourClass->instance(...)

Module constructor. Creates an Class::WeakSingleton (or derivative) instance if one doesn't already exist. A weak reference is stored in the $_instance variable of the parent package. This means that classes derived from Class::WeakSingleton will have the variables defined in *THEIR* package, rather than the Class::WeakSingleton package. Also, because the stored reference is weak it will be deleted when all other references to the returned object have been deleted. The first time the instance is created, the YourClass->_new_instance(...) constructor is called which simply returns a reference to a blessed hash. This can be overloaded for custom constructors. Any additional parameters passed to YourClass->instance(...) are forwarded to YourClass->_new_instance(...).

Returns a normal reference to the existing, or a newly created Class::WeakSingleton object. If the ->_new_instance(...) method returns an undefined value then the constructer is deemed to have failed.

$singleton = YourClass->_new_instance(...)

Simple constructor which returns a hash reference blessed into the current class. May be overloaded to create non-hash objects or handle any specific initialisation required.

Returns a reference to the blessed hash.

USING THE Class::WeakSingleton MODULE

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

  use Class::WeakSingleton;

The Class::WeakSingleton->instance(...) method is used to create a new Class::WeakSingleton 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 at any given time. The instance expires when all references to it also expire.

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

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

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

In the above example, both $highlander and $macleod contain the same reference to a Class::WeakSingleton instance. There can be only one. Except that now that both $highlander and $macleod went out of scope the singleton did also. So MacLeod is now dead. Boo hoo.

DERIVING Class::WeakSingleton CLASSES

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

  package Database;
  use base 'Class::WeakSingleton';

  # derived class specific code
  sub user_name { shift()->{user_name} }
  sub login {
      my $self = shift;
      my ($user_name, $user_pass) = @_;

      return unless $user_name eq 'JJORE'
                and $user_pass eq 'sekret';

        $self->{user_name} = $user_name;

      return 1;

The Database class defined above could be used as follows:

    use Database;


    sub do_somestuff {
        my $db = Database->instance();


The Database->instance() method calls the Database->_new_instance() constructor method the first and only time a new instance is created (until the instance expires and then it starts over). All parameters passed to the Database->instance() method are forwarded to Database->_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 array reference, for example).

  package MyApp::Database;
  use base 'Class::WeakSingleton';
  use DBI;

  # Object is an array ref, here are the names for the values
  use constant DB => 0;

  our $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";

      $self->[ DB ] = DBI->connect("DBI:mSQL:$db:$host")
      if ( not defined $self->[DB] ) {
          $ERROR = "Cannot connect to database: $DBI::errstr\n";
          return undef;

      # any other initialisation...

      # return sucess
      return $self;

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.

      # new instance is returned
      my $database = MyApp::Database->instance();

      # more stuff...
      # call some methods

  sub some_methods {
      # more usual stuff

      # Get the same object that was used in new()
      my $database = MyApp::Database->instance;

The Class::WeakSingleton 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::WeakSingleton 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 'Database' and 'MyApp::Database' from Class::WeakSingleton and have a single instance of each.


Joshua ben Jore <>

Thanks to Andy Wardley for writing Class::Singleton.


Copyright (C) 2006 Joshua ben Jore. All Rights Reserved.

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


Design Patterns

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