Singleton patterns, private methods and singleton module

Question

I am fighting with singleton patterns in Ruby.

I know that singleton implements a single instance of an object but I don't quite understand if we can replicate it without the singleton module.

Then there is the issue with private methods; Right now I have to do something like this:

class MyTestClass
  private_class_method :new
  class << self
    def test
      puts hello
    end

    private
    def hello
      'hello world'
    end
  end
end

MyTestClass.test

So my questions are:

1. Is the above a good Singleton pattern?
2. Would this make sure there is only a single instance?
3. Is there a way to have private methods using the singleton module?

Answer 1

1 . Is the above a good Singleton pattern

Probably not. Using only class methods you don't get the benefit of having an initialize function executed for your single "instance", so it is missing some pieces you would normally find in a Singleton. Ruby is flexible enough so you can bolt on any missing features to the "class" object as needed, but it starts to look kind of ugly.

2 . Would this make sure there is only a single instance?

Yes. You are modifying the object which represents the class, and there is only one.

3 . Is there a way to have private methods using the singleton module?

Yes. Have you tried? Its just as you would expect.

class Test
  include Singleton
  def public_test
    "foo"
  end
  private
  def private_test
    "bar"
  end
end

Test.instance.public_test  # => "foo"
Test.instance.private_test # => throws exception
Test.instance.send(:private_test) # => "bar"