From: matt@... (matt neuburg)
Date: 2008-10-14T04:17:14+09:00
Subject: status of cache in Memoizable

[Backstory, skip if desired:]

In my app I found a couple of places where I could speed things up by
memoizing. I did this manually but since I was doing exactly the same
thing in two different places I looked for a bit of metaprogramming that
would do generalize it. I looked at various "memoize" implementations
and found that James Gray's Memoizable was doing almost exactly the same
thing I was doing.

Recall how it goes:

module Memoizable
  def memoize( name, cache = Hash.new )
    original = "__unmemoized_#{name}__"
    ([Class, Module].include?(self.class) ? self :
self.class).class_eval do
      alias_method original, name
      private original
      define_method(name) { |*args| cache[args] ||= send(original,
*args) }
    end
  end
end

And to use it, you "extend Memoizable" in a class and say "memoize
:my_method". This works exactly the way I want it to; like me, James is
caching at class level, so that various instances of a class share the
cache for this method. Naturally I can think of various concerns that
might arise (what if your cache doesn't return nil to mean "not found",
what if the cached value runs the risk of being modified after being
returned, etc.) but I think I can cope with all that.

[The actual question:]

There's one thing happening here I don't understand. Let's say you don't
supply a value for the "cache" parameter. So the cache is simply
Hash.new. But where does this Hash.new live? It isn't assigned to a
variable name so what keeps it alive? It works, but how?

Thx - m.

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