singleton.rb

The Singleton module implements the Singleton pattern.

Usage:

   class Klass
      include Singleton
      # ...
   end

• 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 a.new # 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
      # ...
    end
    ObjectSpace.each_object(OtherKlass){} # => 0.
  

• This behavior is preserved under inheritance and cloning.

This is achieved by marking

• Klass.new 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__
     end
  

• 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’’.