How can I implement a singleton class in perl?

Question

What's the best practice for implementing Singletons in Perl?

Answer 1

You can use the Class::Singleton module from CPAN.

For a simple class that's not supposed to be inherited, you can implement singleton using a private variable via my():

package My::Singleton;

{
    my $singleton;

    sub instance { # always returns the same instance
        unless (defined $singleton) {
            my $class = shift;
            my $self = { .. }; # create a new object     
            $singleton = bless $self, $class;
        }
        return $singleton;
    }
}

Answer 2

If you're using Moose, then MooseX::Singleton. Its interface is compatible with Class::Singleton.

Answer 3

Singleton Summary:

• Most of the time a normal object will work.
• Be careful with singletons.
• Localize interaction as much as possible

While singletons are a nice idea, I tend to just implement a normal object and use it. If it is critical that I only have one such object, I'll modify the constructor to throw a fatal exception when the second object is created. The various singleton modules don't seem to do much besides add a dependency.

I do this because it is easy, it works, and when in some weird future I need to work with a second object in my app, the changes are minimized.

I also like to localize interaction with my 'singleton' objects--keep interaction in as few places as possible. So instead of every object having direct access to the singleton, I mediate all interaction through my "Application" object. Whenever possible, the application object gets data from the 'singleton', and passes it as a parameter to the method in the other objects. Responses from other objects may also be munged and passed to the 'singleton'. All this effort helps when I need to make changes in the 'singleton' object, and when I want to reuse other objects in another app that may not need or be able to use the original 'singleton' object.