An example of using WeakRef for caching data

transient_cache.rb

require 'weakref'

class TransientCache < Hash
  class AmbivalentRef < WeakRef
    def __getobj__
      super rescue nil
    end
  end
  
  def []= key, object
    super(key, AmbivalentRef.new(object))
  end
  
  def [] key
    ref = super(key)
    self.delete(key) if !ref.weakref_alive?
    ref
  end
  
end

# Here is a silly little example:
if __FILE__ == $0
  ## Sample usage
  c = TransientCache.new

  x = "important"
  c[:a] = "Hello"
  c[:b] = Object.new
  c[:x] = x
  
  # Lets see what was in the cache
  puts c[:a].inspect
  puts c[:b].inspect
  puts c[:x].inspect
  
  # Force garbage collection. 
  # This will collect the weak-refs that are no longer within scope.
  # This doesn't guarantee they are removed though.
  puts "*** Garbage Collection ***"
  ObjectSpace.garbage_collect
  puts c[:a].inspect
  puts c[:b].inspect
  puts c[:x].inspect
end