如何在Perl中实现单例类？

Question

在Perl中实现Singletons的最佳实践是什么？

Answer 1

You can use the Class::Singleton module.

A "Singleton" class can also be easily implemented using either my or state variable (the latter is available since Perl 5.10). But see the @Michael's comment below.

package MySingletonClass;
use strict;
use warnings;
use feature 'state';

sub new {
    my ($class) = @_;
    state $instance;

    if (! defined $instance) {
        $instance = bless {}, $class;
    }
    return $instance;
}

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.