The Singleton module implements the Singleton pattern.


To use Singleton, include the module in your class.

class Klass
   include Singleton
   # ...

This ensures that only one instance of Klass can be created.

a,b  = Klass.instance, Klass.instance

a == b
# => true
# => NoMethodError - new is private ...

The instance is created at upon the first call of Klass.instance().

class OtherKlass
  include Singleton
  # ...

# => 0

# => 1

This behavior is preserved under inheritance and cloning.


This above is achieved by:

  • Making and Klass.allocate private.

  • Overriding Klass.inherited(sub_klass) and Klass.clone() to ensure that the Singleton properties are kept when inherited and cloned.

  • Providing the Klass.instance() method that returns the same object each time it is called.

  • Overriding Klass._load(str) to call Klass.instance().

  • Overriding Klass#clone and Klass#dup to raise TypeErrors to prevent cloning or duping.

Singleton and Marshal

By default Singleton’s _dump(depth) returns the empty string. Marshalling by default will strip state information, e.g. instance variables and taint state, from the instance. Classes using Singleton can provide custom _load(str) and _dump(depth) methods to retain some of the previous state of the instance.

require 'singleton'

class Example
  include Singleton
  attr_accessor :keep, :strip
  def _dump(depth)
    # this strips the @strip information from the instance
    Marshal.dump(@keep, depth)

  def self._load(str)
    instance.keep = Marshal.load(str)

a = Example.instance
a.keep = "keep this"
a.strip = "get rid of this"

stored_state = Marshal.dump(a)

a.keep = nil
a.strip = nil
b = Marshal.load(stored_state)
p a == b  #  => true
p a.keep  #  => "keep this"
p a.strip #  => nil
Class Methods

By default calls instance(). Override to retain singleton state.

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Instance Methods

By default, do not retain any state when marshalling.

Raises a TypeError to prevent cloning.

Raises a TypeError to prevent duping.