From: Robert Klemme <shortcutter@...>
Date: 2007-12-14T17:35:20+09:00
Subject: Re: Is there any way to redefine the `new' method?

2007/12/13, Rick DeNatale <rick.denatale@gmail.com>:
> On 12/13/07, pluskid <pluskid@gmail.com> wrote:
> > Hi! I'm writing a toy interpreter for scheme in Ruby. I created a
> > class to represent scheme Symbol.
> > And as the scheme Symbol, I want only one instance for symbols with
> > the same name. I try to
> > do it like this:
> >
> > class Symbol
> >   alias orig_new new
> >   def new(name)
> >     @@symbols[name] ||= orig_new(name)
> >     @@symbols[name]
> >   end
> > end
> >
> > But I failed. It says `new' is undefined. Is there any way to do the
> > tricky? Thanks!
>
> Well.
>
> First, Ruby has a core class called Symbol, which is the class of
> those funny :abc thingies. You probably don't want to mess with it,
> and it might just already do what your are looking for.  Note that you
> can't do Symbol.new  symbol instances are created either as literals,
> or with conversion methods like "abc".to_sym

Good point.

> Second, Ruby probably does instance instantiation and initialization a
> bit differently than whatever language you might be familiar with.
>
> The new method which is an instance method of the class Class, is
> normally never overridden, and it really isn't intended to be.  It
> effectively calls another method initialize to allocate the space for
> the object, and then calls initialize on the result passing the
> arguments of new.
>
> The way Class#new does this is a bit magical. You can't override
> allocate easily since ruby doesn't use normal method lookup to invoke
> it.

In this case you don't need to override #allocate - #new is perfectly
ok (see below).

>  This works for ruby since, unlike many other OO languages, the
> space needed for an object doesn't depend on things how many instance
> variables it has, since these are acquired dynamically and found via a
> hash.

It's not that difficult:

$ ./scheme-symbols.rb
[134314620]
09:32:48 ~/ruby
$ cat ./scheme-symbols.rb
#!/usr/bin/env ruby

module Scheme
  Symbol = Struct.new :name

  class Symbol
    @names = Hash.new do |h, k|
      k = k.dup.freeze
      h[k] = __new(k)
    end

    class << self
      alias __new new
      def new(name)
        @names[name]
      end
    end
  end

end

syms = (1..2).map { Scheme::Symbol.new "foo" }
p syms.map {|o| o.object_id}.uniq

Cheers

robert

-- 
use.inject do |as, often| as.you_can - without end