Class::Singleton(3pm) User Contributed Perl DocumentationClass::Singleton(3pm)
NAME
Class::Singleton - Implementation of a "Singleton" class
SYNOPSIS
use Class::Singleton;
my $one = Class::Singleton->instance(); # returns a new instance
my $two = Class::Singleton->instance(); # returns same instance
DESCRIPTION
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.
Using the Class::Singleton Module
To import and use the "Class::Singleton" module the following line
should appear in your Perl program:
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 $highlander and $macleod contain the same
reference to a "Class::Singleton" instance. There can be only one.
Deriving Singleton Classes
A module class may be derived from "Class::Singleton" and will inherit
the instance() method that correctly instantiates only one object.
package PrintSpooler;
use base '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 the _new_instance() method returns a blessed reference to a hash
array containing any arguments passed as either a hash reference or
list of named parameters.
package MyConfig;
use base 'Class::Singleton';
sub foo {
shift->{ foo };
}
sub bar {
shift->{ bar };
}
package main;
# either: hash reference of named parameters
my $config = MyConfig->instance({ foo => 10, bar => 20 });
# or: list of named parameters
my $config = MyConfig->instance( foo => 10, bar => 20 );
print $config->foo(); # 10
print $config->bar(); # 20
Derived classes may redefine the _new_instance() method to provide more
specific object initialisation or change the underlying object type (to
a list reference, for example).
package MyApp::Database;
use base 'Class::Singleton';
use DBI;
# 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")
|| die "Cannot connect to database: $DBI::errstr";
# any other initialisation...
return $self;
}
The above example might be used as follows:
use MyApp::Database;
# first use - database gets initialised
my $database = MyApp::Database->instance();
Some time later on in a module far, far away...
package MyApp::FooBar
use MyApp::Database;
# this FooBar object needs access to the database; the Singleton
# approach gives a nice wrapper around global variables.
sub new {
my $class = shift;
bless {
database => MyApp::Database->instance(),
}, $class;
}
The "Class::Singleton" instance() method uses a private hash to store a
reference to any existing instance of the object, keyed against the
derived class package name.
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 exist. 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.
You can use the has_instance() method to find out if a particular class
already has an instance defined. A reference to the instance is
returned or "undef" if none is currently defined.
my $instance = MyApp::Database->has_instance()
|| warn "No instance is defined yet";
Methods
instance()
This method is called to return a current object instance or create
a new one by calling _new_instance().
has_instance()
This method returns a reference to any existing instance or "undef"
if none is defined.
my $testing = MySingleton1->has_instance()
|| warn "No instance defined for MySingleton1";
_new_instance()
This "private" method is called by instance() to create a new
object instance if one doesn't already exist. It is not intended to
be called directly (although there's nothing to stop you from
calling it if you're really determined to do so).
It creates a blessed hash reference containing any arguments passed
to the method as either a hash reference or list of named
parameters.
# either: hash reference of named parameters
my $example1 = MySingleton1->new({ pi => 3.14, e => 2.718 });
# or: list of named parameters
my $example2 = MySingleton2->new( pi => 3.14, e => 2.718 );
It is important to remember that the instance() method will only
call the _new_instance() method once, so any arguments you pass may
be silently ignored if an instance already exists. You can use the
has_instance() method to determine if an instance is already
defined.
EXPORTS
None.
KNOWN BUGS
None.
FEEDBACK
Patches, bug reports, suggestions or any other feedback is welcome.
Patches can be sent as GitHub pull requests at
<https://github.com/steve-m-hay/Class-Singleton/pulls>.
Bug reports and suggestions can be made on the CPAN Request Tracker at
<https://rt.cpan.org/Public/Bug/Report.html?Queue=Class-Singleton>.
Currently active requests on the CPAN Request Tracker can be viewed at
<https://rt.cpan.org/Public/Dist/Display.html?Status=Active;Queue=Class-Singleton>.
Please test this distribution. See CPAN Testers Reports at
<https://www.cpantesters.org/> for details of how to get involved.
Previous test results on CPAN Testers Reports can be viewed at
<https://www.cpantesters.org/distro/C/Class-Singleton.html>.
Please rate this distribution on CPAN Ratings at
<https://cpanratings.perl.org/rate/?distribution=Class-Singleton>.
AVAILABILITY
The latest version of this module is available from CPAN (see "CPAN" in
perlmodlib for details) at
<https://metacpan.org/release/Class-Singleton> or
<https://www.cpan.org/authors/id/S/SH/SHAY/> or
<https://www.cpan.org/modules/by-module/Class/>.
The latest source code is available from GitHub at
<https://github.com/steve-m-hay/Class-Singleton>.
INSTALLATION
See the INSTALL file.
AUTHOR
Andy Wardley <abw@wardley.org <mailto:abw@wardley.org>>
<http://wardley.org/>.
Thanks to Andreas Koenig for providing some significant speedup patches
and other ideas.
Steve Hay <shay@cpan.org <mailto:shay@cpan.org>> is now maintaining
Class::Singleton as of version 1.5.
COPYRIGHT
Copyright (C) 1998 Canon Research Centre Europe Ltd.
Copyright (C) 1998-2008 Andy Wardley. All rights reserved.
Copyright (C) 2014, 2020 Steve Hay. All rights reserved.
LICENCE
This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself, i.e. under the terms of either the
GNU General Public License or the Artistic License, as specified in the
LICENCE file.
VERSION
Version 1.6
DATE
02 Dec 2020
HISTORY
See the Changes file.
perl v5.34.0 2022-10-22 Class::Singleton(3pm)
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