[Ruby]How to create singleton class ?

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()

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