Different class definitions in Ruby

classes.rb

class Logger
  def self.add_logging
    def log(msg)
      STDERR.puts Time.now.strftime("%H:%M:%S: ") + "#{self} (#{msg})"
    end 
  end
end

class Example < Logger
  add_logging
end

ex = Example.new
ex.log("hello")


class Logger
  def self.add_logging(id_string)
    define_method(:log) do |msg|
      now = Time.now.strftime("%H:%M:%S")
      STDERR.puts "#{now}-#{id_string}: #{self} (#{msg})"
    end 
  end
end

class Song < Logger
  add_logging "Tune" 
end

class Album < Logger 
  add_logging "CD"
end
song = Song.new song.log("rock on")

class AttrLogger
  def self.attr_logger(name)
    attr_reader name

    define_method("#{name}=") do |val|
      puts "Assigning #{val.inspect} to #{name}"
      instance_variable_set("@#{name}", val)
    end
  end
end

class Example < AttrLogger
  attr_logger :value
end

ex = Example.new
ex.value = 123
puts "Value is #{ex.value}"
ex.value = "cat"
puts "Value is now #{ex.value}"