gistfile1.rb

class Hoge
  attr_accessor :a
  attr_accessor :b

  def initialize(initialize_options = {})
    defaults = {
      :a => "aaaa",
      :b => "bbbb"
    }
    defaults.merge!(initialize_options)
    defaults.each do |k ,v|
      send((k.to_s + "="), v)
    end
  end
end

hoge = Hoge.new(:a=>"AAAA")
hoge.a #=> AAAA
hoge.b #=> bbbb