singleton_cvar.rb

class SingletonCvar

  def self.instance
    @@instance ||= new
  end
  private_class_method :new

end

singleton = SingletonCvar.instance
singleton_again = SingletonCvar.instance
p singleton.object_id # => 70235617244200
p singleton_again.object_id # => 70235617244200

SingletonCvar.new
# singleton_cvar.rb:15:in `<main>': private method `new' called
# for SingletonCvar:Class (NoMethodError)

singleton_ivar.rb

module Singleton
  def self.included(base)
    base.class_eval do
      def self.instance
        @instance ||= new
      end
      private_class_method :new
    end
  end
end
class SingletonIvar
  include Singleton
end

singletion = SingletonIvar.instance
p singletion.object_id # => 70242076750240

singleton_again = SingletonIvar.instance
p singleton_again.object_id # => 70242076750240

SingletonIvar.new
# singleton_ivar.rb:21:in `<main>': private method `new'
# called for SingletonIvar:Class (NoMethodError)

singleton_ivar_wo_private_class_method.rb

# No usage of private_class_method here
module Singleton
  def self.included(base)
    base.send(:extend, ClassMethods)
  end

  module ClassMethods

    def instance
      singleton_class.send(:private, :new) unless is_private_method?(:new)
      @instance ||= new
    end

    private
    def is_private_method?(meth_name)
      self.private_methods.include?(meth_name)
    end
  end

end
class SingletonIvarWoPrivateClassMethods
  include Singleton

  def foo
    12
  end
end

singletion = SingletonIvarWoPrivateClassMethods.instance
p singletion.object_id #=> 70174947355600

singleton_again = SingletonIvarWoPrivateClassMethods.instance
p singleton_again.object_id #=> 70174947355600

SingletonIvarWoPrivateClassMethods.new
#=> singleton_wo_private_class_method.rb:35:in `<main>': private method `new'
#   called for SingletonIvarWoPrivateClassMethods:Class (NoMethodError)

singleton_lib.rb

require 'singleton' # use Ruby implementation

class SingletonImp
  include Singleton
end

singletion = SingletonImp.instance
p singletion.object_id #=> 70127047740700

singleton_again = SingletonImp.instance
p singleton_again.object_id #=> 70127047740700

SingletonImp.new