Description

Multiton module that ensures only one object to be allocated for a given argument list.

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

Synopsis

  require 'carat/multiton'

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

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

In previous versions the Class.new method was made private, but for Semi Multitons this restriction has been lifted with the following caveat: Class.instance will attempt to retreive a previously cached object, constructing one if needed, but Class.new will always construct a new object - this should allow great flexiblity with multiton classes.

How It Works

We try to use 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

note: 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. Thus, it is the constructor *args exclusively which determine the uniqueness of an object. A workaround is for the class to define the class method multiton_id, eg:

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

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

Author(s)

• Thomas Sawyer