From: Joel VanderWerf <joelvanderwerf@...>
Date: 2010-06-22T09:42:14+09:00
Subject: Re: Got to be a better way to code class variables....

Dave Howell wrote:
> I'm trying to set up an object with overridable defaults, so it will act like this:
> 
> 	>>	 s1 = Smee.new
> 	=> 	#<Smee:0x102007618 @middle=nil, @front=nil, @back=nil>
> 	>> 	s1.middle = 'hi'
> 	=> 	"hi"
> 	>> 	s1
> 	=> 	#<Smee:0x102007618 @middle="hi", @front=nil, @back=nil>
> 	>> 	Smee.front = "O"
> 	=> 	"O"
> 	>>	 s2 = Smee.new
> 	=> 	#<Smee:0x101ffdbb8 @middle=nil, @front="O", @back=nil>
> 	>> 	s1 => #<Smee:0x102007618 @middle="hi", @front="O", @back=nil> 
> 	>> 	Smee.middle = "utu"
> 	=>	 "utu"
> 	>> 	s1.middle
> 	=> 	"hi"
> 	>> 	s2.middle
> 	=> 	"utu"
> 
> So I can set the properties of an instance of Smee, but if I don't set them, or if I set them back to nil, then the corresponding values that I set on the class will take their place.
> 
> This is the closest thing I've gotten so far and it seems ludicrously clunky.
> 
> 	class Smee
> 		attr_writer :front, :middle, :back
> 
> 		class << self
> 			attr_accessor :front, :middle, :back
> 		end
> 	
> 		def front
> 			@front || Smee.front
> 		end
> 
> 		def middle
> 			@middle || Smee.middle
> 		end
> 
> 		def back
> 			@back || Smee.back
> 		end
> 	end
> 
> I'd like to have other methods that worked the same on the class and the instance, but the only way I can figure out how is to type every single method twice. There has *got* to be some terribly clever ruby-esque way of including instance methods as class methods or vice versa, no?
> 
> 


Some metaprogging...


module Overridable
   def overridable(*names)
     names.each do |name|
       attr_writer name
       (class << self; self; end).class_eval do
         attr_accessor name
       end
       class_eval %{
         def #{name}
           @#{name} || self.class.#{name}
         end
       }
     end
   end
end

class Smee
   extend Overridable

   overridable :front
   self.front = 12
end

sm0 = Smee.new

sm1 = Smee.new
sm1.front = 56

p sm0.front, sm1.front