From: Phil Stone <lists@...>
Date: 2012-04-11T08:33:55+09:00
Subject: A Beginner's Question about Metaprogramming

Hey guys, have a question about Ruby programming and found this forum
per Google.

I was recently reading chapter 6 of Why's Poignant Guide to Ruby
(http://mislav.uniqpath.com/poignant-guide/book/chapter-6.html), when I
stumbled upon the following code that was supposed to demonstrate
metaprogramming:


--------------------------------------------------------------
 # The guts of life force within Dwemthy's Array
 class Creature

   # Get a metaclass for this class
   def self.metaclass; class << self; self; end; end

   # Advanced metaprogramming code for nice, clean traits
   def self.traits( *arr )
     return @traits if arr.empty?

     # 1. Set up accessors for each variable
     attr_accessor *arr

     # 2. Add a new class method to for each trait.
     arr.each do |a|
       metaclass.instance_eval do
         define_method( a ) do |val|
           @traits ||= {}
           @traits[a] = val
         end
       end
     end

     # 3. For each monster, the `initialize' method
     #    should use the default number for each trait.
     class_eval do
       define_method( :initialize ) do
         self.class.traits.each do |k,v|
           instance_variable_set("@#{k}", v)
         end
       end
     end

   end

   # Creature attributes are read-only
   traits :life, :strength, :charisma, :weapon
 end

--------------------------------------------------------------

 class Dragon < Creature
   life 1340     # tough scales
   strength 451  # bristling veins
   charisma 1020 # toothy smile
   weapon 939    # fire breath
 end

--------------------------------------------------------------

I don't understand this code at all, especially the line "def
self.metaclass; class << self; self; end; end". Where do all the names
such as metaclass.instance_eval or class_eval come from? I'm really
confused about this piece of code, and would appreciate some
explanation. Sorry if these questions are rather basic.

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