From: Ross Bamford <rossrt@...>
Date: 2006-10-30T02:02:37+09:00
Subject: Re: Interesting trival example of why open classes are good?

Hi,

On Sun, 2006-10-29 at 16:45 +0900, Peter Michaux wrote:
> Does anyone have an interesting, standalone example
> showing the utility of open classes in under 30 lines? under 20?

The example below probably isn't what you're after, but as David said
it's pretty tricky to justify open classes in a self-contained example
this short.
It's just over 30 lines.

One other good example of their usefulness springs to mind though - I've
seen a few projects (I think RCov is one, though I could be wrong) that
have an extension replace some of their 'default' (ruby-implemented)
methods with ones implemented in C, for better performance or
integration. Obviously the benefit being that, if you can support the
native extension you get the improvements, but if not you can still use
the pure-ruby implementation, without changing your code at all.

Anyway, here's the (contrived) example:

#!/usr/bin/env ruby
class User
  def initialize(name, email)
    @name, @email = name, email
  end
  attr_accessor :name, :email
end

class Instrument
  class << self
    def on_call(clz, imethods = clz.instance_methods)
      clz.class_eval do
        imethods.each do |m|
          unless m =~ /^__/
            alias_method "__uninst_#{m}", m
            define_method(m) do |*args|
              r = __send__("__uninst_#{m}", *args)
              yield(self, m, args, r)
              r
            end
          end
        end
      end
    end
  end
end

Instrument.on_call(User, [:initialize, :name, :email]) do |slf, name,
args, r|
  puts "#{slf}.#{name} (#{args.inspect}) => #{r.inspect}"
end

u = User.new("billy", "billy@example.com")
puts "Username is: #{u.name}"
puts "Email is   : #{u.email}"

puts "Uninst name: #{u.__uninst_name}"


-- 
Ross Bamford - rosco@roscopeco.REMOVE.co.uk