When is it wise to use Singleton classes in Ruby? [closed]

Question

I was reading about Singletons in Ruby. I never really had the need to use them before, but I got curious and decided to look them up, to see if I should've used them in the past, or maybe I could use them in the future if I knew what they are used for.

The only times I can think of to use this is:

• ? When I need to have a special object. Example: "smart" Hash object, that acts like a normal Ruby hash with a few additional quirks.
• When I want to make it so only one instance of a class can exist.

However, I am not really sure I had the need for either of the above.

Answer 1

Note that a class mixing in the Singleton module is functionally equivalent to a class or module with 'class' methods and either a guaranteed initialization call or inline initialization. Compare this usage of Singleton:

require 'singleton'
class Bar
  include Singleton
  attr_reader :jam
  def initialize
    @jam = 42
  end
  def double
    @jam *= 2
  end
end

b1 = Bar.instance
b1.double
b2 = Bar.instance
b2.double
p b1.jam          #=> 168

with this no-magic module:

module Foo
  @jam = 42
  def self.double
    @jam *= 2
  end
  def self.jam
    @jam
  end
end

Foo.double
Foo.double
p Foo.jam          #=> 168

In both cases you have a single global object that maintains state. (Because every constant you create in the global scope, including classes and modules, is a global object.)

The only functional difference is that with the Singleton you delay the initialization of the object until the first time you ask for it.

So, if you ever have 'class' methods on a class or module and you use those to change the state of that object (e.g. a class keeping track of all subclasses that inherit from it) you are essentially using a singleton.