------------------------------------------------------- Class: Singleton
      The Singleton module implements the Singleton pattern.
         class Klass
            include Singleton
            # ...
      *   this ensures that only one instance of Klass lets call it ``the
          instance'' can be created.
      a,b = Klass.instance, Klass.instance a == b # => true #
      NoMethodError - new is private ...
      *   ``The instance'' is created at instantiation time, in other
          words the first call of Klass.instance(), thus
        class OtherKlass
          include Singleton
          # ...
        ObjectSpace.each_object(OtherKlass){} # => 0.
      *   This behavior is preserved under inheritance and cloning.
      This is achieved by marking
      * and Klass.allocate - as private
      Providing (or modifying) the class methods
      *   Klass.inherited(sub_klass) and Klass.clone() - to ensure that
          the Singleton pattern is properly inherited and cloned.
      *   Klass.instance() - returning ``the instance''. After a
          successful self modifying (normally the first) call the method
          body is a simple:
         def Klass.instance()
           return @instance
      *   Klass._load(str) - calling Klass.instance()
      *   Klass._instantiate?() - returning ``the instance'' or nil. This
          hook method puts a second (or nth) thread calling
          Klass.instance() on a waiting loop. The return value signifies
          the successful completion or premature termination of the
          first, or more generally, current "instantiation thread".
      The instance method of Singleton are
      *   clone and dup - raising TypeErrors to prevent cloning or duping
      *   _dump(depth) - returning the empty string. Marshalling strips
          by default all state information, e.g. instance variables and
          taint state, from ``the instance''. Providing custom _load(str)
          and _dump(depth) hooks allows the (partially) resurrections of
          a previous state of ``the instance''.
 Instance methods:
      _dump, clone, dup


 ==== Singleton ====
 ---- Singleton methods ----
 ---- Instance methods ----
 ---- Singleton methods (inherited) ----
 ---- Instance methods (inherited) ----

* はてなダイアリーキーワード:Singleton