require 'singleton'
	class Aardvark
	  include Singleton
	end
That's it. Mixing in the Singleton module makes the "new" method private and creates an "instance" method that returns the sole instance of Aardvark. (So you'd access the Aardvark by saying "Aardvark.instance".) It also takes care of thread safety.

For educational purposes, this is how you would implement it without the use of the standard library:
	class Aardvark
          private_class_method :new
          @@instance = new
          def Aardvark.instance
            @@instance
          end
	end
Or, if you want it to be lazy-loading:
	class Aardvark
          private_class_method :new
          def Aardvark.instance
            @@instance = new unless @@instance
            @@instance
          end
	end
----

The following is an implementation of a module (MixIn) that will turn a class into a thread-safe, lazy-loading singleton.

      module Thread''''''Safe''''''Singleton
        def self.append_features(clazz)
          require 'thread'
          clazz.module_eval { 
            private_class_method :new
            @instance_mutex = Mutex.new
            def self.instance 
              @instance_mutex.synchronize {
                @instance = new unless @instance
                @instance
              }
            end
          }
        end
      end
      
 # For further educational enjoyment, the following provides a test for
 # thread-safeness and a contrasting non-thread-safe implementation

      require 'test/unit'
      
      module Simple''''''Singleton
        def self.append_features(clazz)
          clazz.module_eval { 
            private_class_method :new
            def self.instance 
              @instance = new unless @instance
              @instance
            end
          }
        end
      end
      class Simple''''''Singleton''''''Class
        include Simple''''''Singleton
        def initialize
          sleep 1
        end
      end
      
      class Thread''''''Safe''''''Singleton''''''Class
        include Thread''''''Safe''''''Singleton
        def initialize
          sleep 1
        end
      end
      
      class Test''''''Singleton < Test::Unit::Test''''''Case
        def testSimpleIsNotThreadSafe
          threads = Array.new(101) { Thread.new { Simple''''''Singleton''''''Class.instance }}
          assert(threads.collect { |t| t.value }.uniq.size > 1)
        end
        def testThreadSafeIsThreadSafe
          threads = Array.new(101) { Thread.new { Thread''''''Safe''''''Singleton''''''Class.instance }}
          assert_equal(1, threads.collect { |t| t.value }.uniq.size)
        end
      end



----  
Compare with PerlSingleton and PythonSingleton
----
CategoryRuby