File: singleton.rb [singleton: Ruby Standard Library Documentation]

Last Modified
  2021-11-24 04:21:17 -0700
Requires

## Description

The `Singleton` module implements the `Singleton` pattern.

## Usage¶ ↑

To use `Singleton`, include the module in your class.

    class Klass
       include Singleton
       # ...
    end

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

    a,b  = Klass.instance, Klass.instance

    a == b
    # => true

    Klass.new
    # => NoMethodError - new is private ...

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

    class OtherKlass
      include Singleton
      # ...
    end

    ObjectSpace.each_object(OtherKlass){}
    # => 0

    OtherKlass.instance
    ObjectSpace.each_object(OtherKlass){}
    # => 1

This behavior is preserved under inheritance and cloning.

## Implementation¶ ↑

This above is achieved by:

- Making Klass.new 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 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)
      end

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

    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