Singleton

Ruby Standard Library has a Singleton module which implements the Singleton pattern. The first step in creating a Singleton class is to require and include the Singleton module in a class:

require 'singleton'

class Logger
  include Singleton
end

If you try to instantiate this class as you normally would a regular class, a NoMethodError exception is raised. The constructor is made private to prevent other instances from being accidentally created:

Logger.new

#=> NoMethodError: private method `new' called for AppConfig:Class

To access the instance of this class, we need to use the instance():

first, second = Logger.instance, Logger.instance
first == second

#=> true

Logger example

require 'singleton'

class Logger
  include Singleton

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

  def log(msg)
    @log.puts(msg)
  end
end

In order to use Logger object:

Logger.instance.log('message 2')

Without Singleton include

The above singleton implementations can also be done without the inclusion of the Singleton module. This can be achieved with the following:

class Logger
  def self.instance
    @instance ||= new
  end
end

which is a shorthand notation for the following:

class Logger
  def self.instance
    @instance = @instance || Logger.new
  end
end

However, keep in mind that the Singleton module is tested and optimized, therefore being the better option to implement your singleton with.