1Class::Singleton(3) User Contributed Perl Documentation Class::Singleton(3)
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6 Class::Singleton - Implementation of a "Singleton" class
7
9 use Class::Singleton;
10 my $one = Class::Singleton->instance(); # returns a new instance
12 my $two = Class::Singleton->instance(); # returns same instance
13
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 For a description and discussion of the Singleton class, see "Design
26 Patterns", Gamma et al, Addison-Wesley, 1995, ISBN 0-201-63361-2.
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29 "Class::Singleton" requires Perl version 5.004 or later. If you have an
30 older version of Perl, please upgrade to latest version, available from
31 your nearest CPAN site (see INSTALLATION below).
32
34 The "Class::Singleton" module is available from CPAN. As the 'perlmod'
35 man page explains:
36 CPAN stands for the Comprehensive Perl Archive Network.
38 This is a globally replicated collection of all known Perl
39 materials, including hundreds of unbunded modules.
40 [...]
42 For an up-to-date listing of CPAN sites, see
44 http://www.perl.com/perl/ or ftp://ftp.perl.com/perl/ .
45 The module is available in the following directories:
47 /modules/by-module/Class/Class-Singleton-<version>.tar.gz
49 /authors/id/ABW/Class-Singleton-<version>.tar.gz
50 "Class::Singleton" is distributed as a single gzipped tar archive file:
52 Class-Singleton-<version>.tar.gz
54 Note that "<version>" represents the current version number, of the
56 form "1.23". See VERSION below to determine the current version number
57 for "Class::Singleton".
58 Unpack the archive to create an installation directory:
60 gunzip Class-Singleton-<version>.tar.gz
62 tar xvf Class-Singleton-<version>.tar
63 'cd' into that directory, make, test and install the module:
65 cd Class-Singleton-<version>
67 perl Makefile.PL
68 make
69 make test
70 make install
71 The '"make install"' will install the module on your system. You may
73 need root access to perform this task. If you install the module in a
74 local directory (for example, by executing ""perl Makefile.PL
75 LIB=~/lib"" in the above - see "perldoc MakeMaker" for full details),
76 you will need to ensure that the "PERL5LIB" environment variable is set
77 to include the location, or add a line to your scripts explicitly
78 naming the library location:
79 use lib '/local/path/to/lib';
81
83 To import and use the "Class::Singleton" module the following line
84 should appear in your Perl program:
85 use Class::Singleton;
87 The instance() method is used to create a new "Class::Singleton"
89 instance, or return a reference to an existing instance. Using this
90 method, it is only possible to have a single instance of the class in
91 any system.
92 my $highlander = Class::Singleton->instance();
94 Assuming that no "Class::Singleton" object currently exists, this first
96 call to instance() will create a new "Class::Singleton" and return a
97 reference to it. Future invocations of instance() will return the same
98 reference.
99 my $macleod = Class::Singleton->instance();
101 In the above example, both $highlander and $macleod contain the same
103 reference to a "Class::Singleton" instance. There can be only one.
104
106 A module class may be derived from "Class::Singleton" and will inherit
107 the instance() method that correctly instantiates only one object.
108 package PrintSpooler;
110 use base 'Class::Singleton';
111 # derived class specific code
113 sub submit_job {
114 ...
115 }
116 sub cancel_job {
118 ...
119 }
120 The "PrintSpooler" class defined above could be used as follows:
122 use PrintSpooler;
124 my $spooler = PrintSpooler->instance();
126 $spooler->submit_job(...);
128 The instance() method calls the _new_instance() constructor method the
130 first and only time a new instance is created. All parameters passed to
131 the instance() method are forwarded to _new_instance(). In the base
132 class the _new_instance() method returns a blessed reference to a hash
133 array containing any arguments passed as either a hash reference or
134 list of named parameters.
135 package MyConfig;
137 use base 'Class::Singleton';
138 sub foo {
140 shift->{ foo };
141 }
142 sub bar {
144 shift->{ bar };
145 }
146 package main;
148 # either: hash reference of named parameters
150 my $config = MyConfig->instance({ foo => 10, bar => 20 });
151 # or: list of named parameters
153 my $config = MyConfig->instance( foo => 10, bar => 20 );
154 print $config->foo(); # 10
156 print $config->bar(); # 20
157 Derived classes may redefine the _new_instance() method to provide more
159 specific object initialisation or change the underlying object type (to
160 a list reference, for example).
161 package MyApp::Database;
163 use base 'Class::Singleton';
164 use DBI;
165 # this only gets called the first time instance() is called
167 sub _new_instance {
168 my $class = shift;
169 my $self = bless { }, $class;
170 my $db = shift || "myappdb";
171 my $host = shift || "localhost";
172 $self->{ DB } = DBI->connect("DBI:mSQL:$db:$host")
174 || die "Cannot connect to database: $DBI::errstr";
175 # any other initialisation...
177 return $self;
179 }
180 The above example might be used as follows:
182 use MyApp::Database;
184 # first use - database gets initialised
186 my $database = MyApp::Database->instance();
187 Some time later on in a module far, far away...
189 package MyApp::FooBar
191 use MyApp::Database;
192 # this FooBar object needs access to the database; the Singleton
194 # approach gives a nice wrapper around global variables.
195 sub new {
197 my $class = shift;
198 bless {
199 database => MyApp::Database->instance(),
200 }, $class;
201 }
202 The "Class::Singleton" instance() method uses a package variable to
204 store a reference to any existing instance of the object. This
205 variable, ""_instance"", is coerced into the derived class package
206 rather than the base class package.
207 Thus, in the "MyApp::Database" example above, the instance variable
209 would be:
210 $MyApp::Database::_instance;
212 This allows different classes to be derived from "Class::Singleton"
214 that can co-exist in the same system, while still allowing only one
215 instance of any one class to exists. For example, it would be possible
216 to derive both '"PrintSpooler"' and '"MyApp::Database"' from
217 "Class::Singleton" and have a single instance of each in a system,
218 rather than a single instance of either.
219 You can use the has_instance() method to find out if a particular class
221 already has an instance defined. A reference to the instance is
222 returned or "undef" if none is currently defined.
223 my $instance = MyApp::Database->has_instance()
225 || warn "No instance is defined yet";
226
228 instance()
229 This method is called to return a current object instance or create a
230 new one by calling _new_instance().
231 has_instance()
233 This method returns a reference to any existing instance or "undef" if
234 none is defined.
235 my $testing = MySingleton1->has_instance()
237 || warn "No instance defined for MySingleton1";
238 _new_instance()
240 This "private" method is called by instance() to create a new object
241 instance if one doesn't already exist. It is not intended to be called
242 directly (although there's nothing to stop you from calling it if
243 you're really determined to do so).
244 It creates a blessed hash reference containing any arguments passed to
246 the method as either a hash reference or list of named parameters.
247 # either: hash reference of named parameters
249 my $example1 = MySingleton1->new({ pi => 3.14, e => 2.718 });
250 # or: list of named parameters
252 my $example2 = MySingleton2->new( pi => 3.14, e => 2.718 );
253 It is important to remember that the instance() method will only call
255 the _new_instance() method once, so any arguments you pass may be
256 silently ignored if an instance already exists. You can use the
257 has_instance() method to determine if an instance is already defined.
258
260 Andy Wardley <abw@wardley.org> <http://wardley.org/>
261 Thanks to Andreas Koenig for providing some significant speedup patches
263 and other ideas.
264
266 This is version 1.4, released September 2007
267
269 Copyright Andy Wardley 1998-2007. All Rights Reserved.
270 This module is free software; you can redistribute it and/or modify it
272 under the same terms as Perl itself.
273perl v5.12.0 2007-09-28 Class::Singleton(3)