sattr.rb

module SafeAttr
  def SafeAttr.included(other)
    other.send(:extend, SafeAttr::ClassMethods)
    super
  end

  module ClassMethods
    def code_for(name, options = {})
      code = []

      unless options[:read] == false
        code.push(<<-__
            def #{ name }
              (class << self; self; end).instance_variable_get("@#{ name }")
            end
          __
        )
      end

      unless options[:write] == false
        code.push(<<-__
            def #{ name }=(value)
              (class << self; self; end).instance_variable_set("@#{ name }", value)
            end
          __
        )
      end

      code.join("\n\n")
    end

    def sattr_reader(*names)
      names.flatten.compact.map{|name| module_eval(code_for(name, :write => false))}
    end

    def sattr_writer(*names)
      names.flatten.compact.map{|name| module_eval(code_for(name, :read => false))}
    end

    def sattr_accessor(*names)
      names.flatten.compact.map{|name| module_eval(code_for(name))}
    end

    alias_method :sattr, :sattr_accessor
  end
end

class Doc 
  include SafeAttr

  attr_accessor :a
  sattr :b
end

doc = Doc.new

doc.a = 40
doc.b = 2

p doc                    #=> #<Doc:0x007fc1da8b94f0 @a=40>
p doc.instance_variables #=> [:@a]
p(doc.a + doc.b)         #=> 42