From: Florian Gross <flgr@...>
Date: 2004-06-26T03:03:05+09:00
Subject: Re: Is it considered Harmful?

Jim Weirich wrote:

> The "standard" application for the self.class= method (aka become(class))
> is use it to do demand on load style proxies.  Suppose you have a largish
> object in external storage (database, file, whatever).  You create a
> small, efficient proxy item for the large object until you really need the
> true data in the object.  Then you load the object and let the proxy
> "become" the newly loaded object.
> 
> Without class=/become methods,
> (a) the proxy remains and forwards all messages to the real object, or
> (b) you replace every reference in the program to the proxy with          
>   a reference to the real object.
> 
> (a) is a small, but anoying constant cost, and (b) may be difficult,
> depending on how wide spread the proxy was used.

There's another small, but annoying problem with the proxy-approach: 
There are operations which can't be forwarded. I can only think of the 
truth value of an Object right now.

It's a problem in this case:

irb(main):001:0> [obj = false, ref = WeakRef.new(obj)]
=> [false, false]
# We should now be able to treat obj and ref as
# if they were actually the same thing
irb(main):002:0> [obj.to_s, ref.to_s]
=> ["false", "false"] # This is okay, methods get forwarded
irb(main):003:0> [if not obj then "foo" end, if not ref then "foo" end]
=> ["foo", nil] # truth value decisions can't be forwarded
irb(main):004:0> exit

I'm not sure how other languages handle -- as far as I know Perl has a 
way of overloading how Objects will be converted into Boolean values.

Python also seems to have this via __nonzero__. (And __len__ == 0)

I have no idea however if such a method (let's call it #to_bool) could 
be do in a way such that it doesn't disturb the performance of all Ruby 
scripts, but it looks more possible every day.

Regards,
Florian Gross