Ruby Meta Programming Explained

meta.rb

class Class
  def where
    "in class Class"
  end
end

person = Class.new
puts "person.where: #{person.where}" # => "in class Class"

class << person
  def where
    "in class person's metaclass"
  end
end
puts "person.where: #{person.where}" # => "in class person's metaclass"

def person.where
  "in class person's metaclass, redefined"
end
puts "person.where: #{person.where}" # => "in class person's metaclass, redefined"

meta_person = class << person; self; end
meta_person.class_eval do
  def where
    "in class person's metaclass, redefined again"
  end
end
puts "person.where: #{person.where}" # => "in class person's metaclass, redefined again"

person.class_eval do
  def where
    "in class person"
  end
end

daqing = person.new
puts "daqing.where: #{daqing.where}" # => "in class person"

def daqing.where
  "in daqing's metaclass"
end
puts "daqing.where: #{daqing.where}" # => "in daqing's metaclass"

meta_daqing = class << daqing; self; end
meta_daqing.class_eval do
  def where
    "in daqing's metaclass, redefined"
  end
end
puts "daqing.where: #{daqing.where}" # => in daqing's metaclass, redefined