Class::StrongSingleton is a stronger and more secure Singleton base class.

SYNOPSIS

package My::Singleton::Class;

use base qw(Class::StrongSingleton);

sub new {
my ($class, %my_params) = @_;
# create our object instance
my $instance = { %my_params };
bless($instance, $class);
# and initialize it as a singleton
$instance->_init_StrongSingleton();
return $instance;
}

1;

# later in your code ...

# create the first instance of our class
my $instance = My::Singleton::Class->new(param => "value");

# try to create a new one again, and
# you end up with the same instance, not
# a new one
my $instance2 = My::Singleton::Class->new(param => "other value");

# calling instance returns the singleton
# instance expected
my $instance3 = My::Singleton::Class->instance();

# although rarely needed, if you have to
# you can destroy the singleton

# either through the instance
$instance->DESTROY();
# or through the class
My::Singleton::Class->DESTROY();

# of course, this is assuming you
# did not override DESTORY yourself

# Also calling instance before calling new
# will returns a new singleton instance
my $instance = My::Singleton::Class->instance();

This module is an alternative to Class::Singleton and Class::WeakSingleton, and provides a more secure Singleton class in that it takes steps to prevent the possibility of accidental creation of multiple instances and/or the overwriting of existsing Singleton instances. For a detailed comparison please see the "SEE ALSO" section.

Here is a description of how it all works. First, the user creates the first Singleton instance of the class in the normal way.

my $obj = My::Singleton::Class->new("variable", "parameter");

This instance is then stored inside a lexically scoped variable within the Class::StrongSingleton package. This prevents the variable from being accessed by anything but methods from the Class::StrongSingleton package. At this point as well, the new method to the class is overridden so that it will always return the Singleton instance. This prevents any accidental overwriting of the Singleton instance. This means that any of the follow lines of code all produce the same instance:

my $instance = $obj->instance();
my $instance = My::Singleton::Class->instance();
my $instance = $obj->new();
my $instance = My::Singleton::Class->new();

Personally, I see this an an improvement over the usual Gang of Four style Singletons which discourages the use of the new method entirely. Through this method, a user can be able to use the Singleton class in a normal way, not having to know its actually a Singleton. This can be handy if your design changes and you no longer need the class as a Singleton.