Singleton is one design pattern in the software engineering. Ruby has its own special feature to declare singleton class. I will demonstrate two examples as below:

class Logger

  def initialize

    @log = File.open("log.txt", "a")

  end

  @@instance = Logger.new

  def self.instance

    return @@instance

  end

  def log(msg)

    @log.puts(msg)

  end

  private_class_method :new

end

Logger.instance.log('message 1')

require "singleton"

class Test

  include Singleton

  def idea

    puts "this is test"

  end

  def self.good

    puts "good idea"

  end

end

puts Test.good()

puts Test.instance.idea()