my_struct.rb

class MyStruct
  def self.new(*method_names, &block)
    raise ArgumentError, "wrong number of arguments (0 for 1+)" if method_names.empty?

    Class.new do
      class_eval &block if block

      include Enumerable

      def initialize(*args)
        @attributes = Hash[members.map(&:to_s).zip args]
      end

      def inspect
        "#<struct #{@attributes.map { |key, value| "#{key}=#{value.inspect}"}.join ', '}>"
      end

      def size
        members.size
      end

      def values
        @attributes.values
      end

      def each(&block)
        values.each(&block)
      end

      def select(&block)
        values.select &block
      end

      def [](key)
        key = key.to_s
        raise NameError, "no member '#{key}' in struct" unless @attributes.has_key? key
        @attributes[key]
      end

      def []=(key, value)
        key = key.to_s
        raise NameError, "no member '#{key}' in struct" unless @attributes.has_key? key
        @attributes[key] = value
      end

      define_method :members do
        method_names
      end

      method_names.each do |method_name|
        raise TypeError, "#{method_name} is not a symbol" unless method_name.kind_of? Symbol

        define_method method_name do
          @attributes[method_name.to_s]
        end

        define_method "#{method_name}=" do |arg|
          @attributes[method_name.to_s] = arg
        end
      end
    end
  end
end