1Class::Singleton(3)   User Contributed Perl Documentation  Class::Singleton(3)
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3
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NAME

6       Class::Singleton - Implementation of a "Singleton" class
7

SYNOPSIS

9           use Class::Singleton;
10
11           my $one = Class::Singleton->instance();   # returns a new instance
12           my $two = Class::Singleton->instance();   # returns same instance
13

DESCRIPTION

15       This is the "Class::Singleton" module.  A Singleton describes an object
16       class that can have only one instance in any system.  An example of a
17       Singleton might be a print spooler or system registry.  This module
18       implements a Singleton class from which other classes can be derived.
19       By itself, the "Class::Singleton" module does very little other than
20       manage the instantiation of a single object.  In deriving a class from
21       "Class::Singleton", your module will inherit the Singleton
22       instantiation method and can implement whatever specific functionality
23       is required.
24
25       For a description and discussion of the Singleton class, see "Design
26       Patterns", Gamma et al, Addison-Wesley, 1995, ISBN 0-201-63361-2.
27
28   Using the Class::Singleton Module
29       To import and use the "Class::Singleton" module the following line
30       should appear in your Perl program:
31
32           use Class::Singleton;
33
34       The instance() method is used to create a new "Class::Singleton"
35       instance, or return a reference to an existing instance. Using this
36       method, it is only possible to have a single instance of the class in
37       any system.
38
39           my $highlander = Class::Singleton->instance();
40
41       Assuming that no "Class::Singleton" object currently exists, this first
42       call to instance() will create a new "Class::Singleton" and return a
43       reference to it. Future invocations of instance() will return the same
44       reference.
45
46           my $macleod    = Class::Singleton->instance();
47
48       In the above example, both $highlander and $macleod contain the same
49       reference to a "Class::Singleton" instance.  There can be only one.
50
51   Deriving Singleton Classes
52       A module class may be derived from "Class::Singleton" and will inherit
53       the instance() method that correctly instantiates only one object.
54
55           package PrintSpooler;
56           use base 'Class::Singleton';
57
58           # derived class specific code
59           sub submit_job {
60               ...
61           }
62
63           sub cancel_job {
64               ...
65           }
66
67       The "PrintSpooler" class defined above could be used as follows:
68
69           use PrintSpooler;
70
71           my $spooler = PrintSpooler->instance();
72
73           $spooler->submit_job(...);
74
75       The instance() method calls the _new_instance() constructor method the
76       first and only time a new instance is created. All parameters passed to
77       the instance() method are forwarded to _new_instance(). In the base
78       class the _new_instance() method returns a blessed reference to a hash
79       array containing any arguments passed as either a hash reference or
80       list of named parameters.
81
82           package MyConfig;
83           use base 'Class::Singleton';
84
85           sub foo {
86               shift->{ foo };
87           }
88
89           sub bar {
90               shift->{ bar };
91           }
92
93           package main;
94
95           # either: hash reference of named parameters
96           my $config = MyConfig->instance({ foo => 10, bar => 20 });
97
98           # or: list of named parameters
99           my $config = MyConfig->instance( foo => 10, bar => 20 );
100
101           print $config->foo();   # 10
102           print $config->bar();   # 20
103
104       Derived classes may redefine the _new_instance() method to provide more
105       specific object initialisation or change the underlying object type (to
106       a list reference, for example).
107
108           package MyApp::Database;
109           use base 'Class::Singleton';
110           use DBI;
111
112           # this only gets called the first time instance() is called
113           sub _new_instance {
114               my $class = shift;
115               my $self  = bless { }, $class;
116               my $db    = shift || "myappdb";
117               my $host  = shift || "localhost";
118
119               $self->{ DB } = DBI->connect("DBI:mSQL:$db:$host")
120                   || die "Cannot connect to database: $DBI::errstr";
121
122               # any other initialisation...
123
124               return $self;
125           }
126
127       The above example might be used as follows:
128
129           use MyApp::Database;
130
131           # first use - database gets initialised
132           my $database = MyApp::Database->instance();
133
134       Some time later on in a module far, far away...
135
136           package MyApp::FooBar
137           use MyApp::Database;
138
139           # this FooBar object needs access to the database; the Singleton
140           # approach gives a nice wrapper around global variables.
141
142           sub new {
143               my $class = shift;
144               bless {
145                   database => MyApp::Database->instance(),
146               }, $class;
147           }
148
149       The "Class::Singleton" instance() method uses a private hash to store a
150       reference to any existing instance of the object, keyed against the
151       derived class package name.
152
153       This allows different classes to be derived from "Class::Singleton"
154       that can co-exist in the same system, while still allowing only one
155       instance of any one class to exist. For example, it would be possible
156       to derive both '"PrintSpooler"' and '"MyApp::Database"' from
157       "Class::Singleton" and have a single instance of each in a system,
158       rather than a single instance of either.
159
160       You can use the has_instance() method to find out if a particular class
161       already has an instance defined.  A reference to the instance is
162       returned or "undef" if none is currently defined.
163
164           my $instance = MyApp::Database->has_instance()
165               || warn "No instance is defined yet";
166
167   Methods
168       instance()
169           This method is called to return a current object instance or create
170           a new one by calling _new_instance().
171
172       has_instance()
173           This method returns a reference to any existing instance or "undef"
174           if none is defined.
175
176               my $testing = MySingleton1->has_instance()
177                   || warn "No instance defined for MySingleton1";
178
179       _new_instance()
180           This "private" method is called by instance() to create a new
181           object instance if one doesn't already exist. It is not intended to
182           be called directly (although there's nothing to stop you from
183           calling it if you're really determined to do so).
184
185           It creates a blessed hash reference containing any arguments passed
186           to the method as either a hash reference or list of named
187           parameters.
188
189               # either: hash reference of named parameters
190               my $example1 = MySingleton1->new({ pi => 3.14, e => 2.718 });
191
192               # or: list of named parameters
193               my $example2 = MySingleton2->new( pi => 3.14, e => 2.718 );
194
195           It is important to remember that the instance() method will only
196           call the _new_instance() method once, so any arguments you pass may
197           be silently ignored if an instance already exists. You can use the
198           has_instance() method to determine if an instance is already
199           defined.
200

EXPORTS

202       None.
203

KNOWN BUGS

205       None.
206

FEEDBACK

208       Patches, bug reports, suggestions or any other feedback is welcome.
209
210       Patches can be sent as GitHub pull requests at
211       <https://github.com/steve-m-hay/Class-Singleton/pulls>.
212
213       Bug reports and suggestions can be made on the CPAN Request Tracker at
214       <https://rt.cpan.org/Public/Bug/Report.html?Queue=Class-Singleton>.
215
216       Currently active requests on the CPAN Request Tracker can be viewed at
217       <https://rt.cpan.org/Public/Dist/Display.html?Status=Active;Queue=Class-Singleton>.
218
219       Please test this distribution.  See CPAN Testers Reports at
220       <https://www.cpantesters.org/> for details of how to get involved.
221
222       Previous test results on CPAN Testers Reports can be viewed at
223       <https://www.cpantesters.org/distro/C/Class-Singleton.html>.
224
225       Please rate this distribution on CPAN Ratings at
226       <https://cpanratings.perl.org/rate/?distribution=Class-Singleton>.
227

AVAILABILITY

229       The latest version of this module is available from CPAN (see "CPAN" in
230       perlmodlib for details) at
231
232       <https://metacpan.org/release/Class-Singleton> or
233
234       <https://www.cpan.org/authors/id/S/SH/SHAY/> or
235
236       <https://www.cpan.org/modules/by-module/Class/>.
237
238       The latest source code is available from GitHub at
239       <https://github.com/steve-m-hay/Class-Singleton>.
240

INSTALLATION

242       See the INSTALL file.
243

AUTHOR

245       Andy Wardley <abw@wardley.org <mailto:abw@wardley.org>>
246       <http://wardley.org/>.
247
248       Thanks to Andreas Koenig for providing some significant speedup patches
249       and other ideas.
250
251       Steve Hay <shay@cpan.org <mailto:shay@cpan.org>> is now maintaining
252       Class::Singleton as of version 1.5.
253

COPYRIGHT

255       Copyright (C) 1998 Canon Research Centre Europe Ltd.
256
257       Copyright (C) 1998-2008 Andy Wardley.  All rights reserved.
258
259       Copyright (C) 2014, 2020 Steve Hay.  All rights reserved.
260

LICENCE

262       This module is free software; you can redistribute it and/or modify it
263       under the same terms as Perl itself, i.e. under the terms of either the
264       GNU General Public License or the Artistic License, as specified in the
265       LICENCE file.
266

VERSION

268       Version 1.6
269

DATE

271       02 Dec 2020
272

HISTORY

274       See the Changes file.
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278perl v5.34.0                      2022-01-21               Class::Singleton(3)