# multiton.rb

Path: | lib/supplemental/facets/multiton.rb
Last Update: | Mon Dec 05 22:58:33 +0000 2011

# Multiton

## Synopsis

Multiton design pattern ensures only one object is allocated for a given state.

The ‘multiton’ pattern is similar to a singleton, but instead of only one instance, there are several similar instances. It is useful when you want to avoid constructing objects many times because of some huge expense (connecting to a database for example), require a set of similar but not identical objects, and cannot easily control how many times a contructor may be called.

      class SomeMultitonClass
        include Multiton
        attr :arg
        def initialize(arg)
          @arg = arg
        end
      end

      a = SomeMultitonClass.new(4)
      b = SomeMultitonClass.new(4)   # a and b are same object
      c = SomeMultitonClass.new(2)   # c is a different object

## Previous Behavior

In previous versions of Multiton the new method was made private and instance had to be used in its stay —just like Singleton. But this is less desirable for Multiton since Multitions can have multiple instances, not just one.

So instead Multiton now defines create as a private alias of the original new method (just in case it is needed) and then defines new to handle the multiton; instance is provided as an alias for it.

## How It Works

A pool of objects is searched for a previously cached object, if one is not found we construct one and cache it in the pool based on class and the args given to the contructor.

A limitation of this approach is that it is impossible to detect if different blocks were given to a contructor (if it takes a block). So it is the constructor arguments only which determine the uniqueness of an object. To workaround this, define the class method ::multiton_id.

      def SomeMultitonClass.multiton_id(*args, &block)
        # ...
      end

Which should return a hash key used to identify the object being constructed as (not) unique.

## Authors

- Christoph Rippel
- Thomas Sawyer

# Copying

Copyright (c) 2007 Christoph Rippel, Thomas Sawyer

Ruby License

This module is free software. You may use, modify, and/or redistribute this software under the same terms as Ruby.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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