A summary of how to implement the Singleton pattern in Ruby.

What is the Singleton Pattern

The Singleton pattern is a design pattern that ensures only one instance exists.

How to Create a Singleton Module

Ruby provides a Singleton module that allows you to implement the Singleton pattern.

require 'singleton'

class Config
  include Singleton

  attr_accessor :setting

  def initialize
    @setting = "default"
  end
end

config1 = Config.instance
config2 = Config.instance

puts config1 == config2 # true 同じインスタンス

config1.setting = "new"
puts config2.setting # new

Since the new method is private, you cannot create an instance with Config.new. Therefore, you can obtain the only instance with Config.instance.

Using Class Methods

Another way is to make new private and manage the instance with a class method.

class Config
  @instance = nil

  private_class_method :new

  def self.instance
    @instance ||= new # 自己代入演算子はnilまたはfalseの場合のみに代入を行う
  end
end

config1 = Config.instance
config2 = Config.instance

puts config1 == config2 # true

Since new is private, you cannot create an instance with Config.new. Therefore, you can obtain the only instance with Config.instance.

Using Constants

You can also implement the Singleton pattern using constants instead of class variables.

class Config
  INSTANCE = new.freeze # 定数自体の変更を禁止

  private_class_method :new
end

config1 = Config::INSTANCE
config2 = Config::INSTANCE

puts config1 == config2 # => true

By assigning a new instance to a constant and making the new method private, you can obtain the only instance.

Summary

There are several ways to implement the Singleton pattern in Ruby.

Among them, using the Singleton module is standard and recommended because it provides a thread-safe implementation. (Probably)