Handling cases where there can be only one

In cases where an application using your library should only have a single instance of the object, you usually would reach for the singleton design pattern. Ruby actually has a standard library for the singleton pattern, appropriately named singleton. This library defines the Singleton module, which you can include in other classes to turn those classes into singletons. A class that includes Singleton no longer has a public new method, since you should not be creating multiple instances. Instead, it provides a class method named instance, which returns the only instance of the class:

require 'singleton'
class OnlyOne
  include Singleton
  def foo
    :foo
  end
end
  
only1 = OnlyOne.instance
only2 = OnlyOne.instance
only1.equal?(only2)
# => true

The singleton library does implement the singleton pattern. So why wasn't it discussed in the previous section,...