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.
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.
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";
- 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.
None.
None.
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>.
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>.
See the
INSTALL file.
Andy Wardley <
[email protected] <mailto:
[email protected]>>
<
http://wardley.org/>.
Thanks to Andreas Koenig for providing some significant speedup patches and
other ideas.
Steve Hay <
[email protected] <mailto:
[email protected]>> is now maintaining
Class::Singleton as of version 1.5.
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.
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 1.6
02 Dec 2020
See the
Changes file.