How to restrict the number of objects of a class to a given number in ruby?

Question

how can i extend the singleton pattern to a number of objects for a class i.e. how can i have only 5 objects of a class at max and not more than that in ruby

Answer 1

Multiton with size constraints.

Here is a naive implementation of a Multiton module that you can include in any class. The default number of objects it can create is 5, and you can override this limit by defining a class method called max_instances that returns a number (maximum instances allowed) in your class.

module Multiton

  MAX_INSTANCES = 5
  COUNT_HOOK = :max_instances

  module MultitonClassMethods
    def instance
      size = @instances.size
      max = respond_to?(COUNT_HOOK) ? self.send(COUNT_HOOK) : MAX_INSTANCES
      @instances << new if size < max
      @instances[rand(size)]
    end
  end

  def self.included(klass)
    klass.class_eval {
      @instances = []
    }
    klass.private_class_method :new
    klass.extend(MultitonClassMethods)
  end

end

Include the module in a class to make it a multiton.

# Falls back to Multiton::MAX_INSTANCES 
class Person
  include Multiton
end

# Overrides the number of allowed instances in max_instances
class Resource
  include Multiton

  def self.max_instances
    58
  end
end

Since the objects are returned randomly from a pool in this multiton, you may not get all the objects back over a short run. But as more objects are requested, it should even out. You can change this behavior in the Multitonmodule by cycling through objects instead of randomly picking one.

people = []
1000.times do
  people << Person.instance
end
# should print 5, but may print a smaller number
p people.uniq.size

resources = []
1000.times do
  resources << Resource.instance
end
# should print 58, but may print a smaller number
p resources.uniq.size

Answer 2

Example code:

# Example class which can be instanciated at most five times
# Naive approach with Class variable

class FiveAtMost
  @@instances = 0

  def initialize()
    if @@instances >= 5
      raise "No more than five instances allowed."
    else
      @@instances += 1
    end
    p "Initialized instance #{@@instances}"
  end
end

one = FiveAtMost.new
two = FiveAtMost.new
three = FiveAtMost.new
four = FiveAtMost.new
five = FiveAtMost.new

# will raise RuntimeError: No more than five instances allowed.
six = FiveAtMost.new

Since the moment when an object gets garbage collected is not predictable, you'll need some kind of workaround for the that event. Maybe you find this useful: http://pleac.sourceforge.net/pleac_ruby/classesetc.html#AEN710