Description

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”.