class_instance_eval.rb

# How to build dynamic instance methods for classes.
# First define a method creator (i.e. attribute_accessor below) that 
# loops through your params to create new methods.
# Via: http://codeshooter.wordpress.com/2009/06/04/understanding-class_eval-module_eval-and-instance_eval/

Object.class_eval do
  class < < self
    def attribute_accessor( *attribute_names )
      attribute_names.each do |attribute_name|
        class_eval %Q?
          def #{attribute_name}
              @#{attribute_name}
          end

          def #{attribute_name}=( new_value )
              @#{attribute_name} = new_value
          end
        ?
      end
    end
  end
end

class Dog
  attribute_accessor :name
end

dog = Dog.new
dog.name = 'Fido'

other_dog = Dog.new
other_dog.name = 'Dido'

puts dog.name
puts other_dog.name

# You can also do it with define_method, which appears to be a fancier way 
# of doing class_eval
# via: http://www.vitarara.org/cms/ruby_metaprogamming_declaratively_adding_methods_to_a_class
class Talker
  def self.say(*args)
    puts "Inside self.say"
    puts "self = #{self}"
    args.each do |arg|
      method_name = ("say_" + arg.to_s).to_sym
      send :define_method, method_name do
        puts arg
      end
    end
  end
end

class MyTalker < Talker
  say :hello
end

m = MyTalker.new
m.say_hello

# The difference between class_eval and instance_eval
# via http://blog.jayfields.com/2007/03/ruby-instanceeval-and-classeval-method.html
Foo = Class.new
Foo.class_eval do
  def bar
    "bar"
  end
end
Foo.instance_eval do
  def baz
    "baz"
  end
end
Foo.bar       #=> undefined method ‘bar’ for Foo:Class
Foo.new.bar   #=> "bar"
Foo.baz       #=> "baz"
Foo.new.baz   #=> undefined method ‘baz’ for #<Foo:0x7dce8>

# Also here's a set of guidelines
# * instance_eval with a proc for class, instance or singleton evaluation
# * instance_eval for changing proc bindings
# * define_method for converting procs into methods
# * def for everything else
# * eval when you really know you need it
# via http://www.mathewabonyi.com/articles/2007/01/11/surprise-10min-benchmark-eval-class_eval-instance_eval-define_method-bind