Ruby.Metaprogramming. class_eval

Question

There seem to be a mistake in my code. However I just can\'t find it out.

class Class
def attr_accessor_with_history(attr_name)
  attr_name = attr_name.to_s
         


        

Answer 1

@Sergio Tulentsev's answer works, but it promotes a problematic practice of using string eval which is in general fraught with security risks and other surprises when the inputs aren't what you expect. For example, what happens to Sergio's version if one calls (no don't try it):

attr_accessor_with_history %q{foo; end; system "rm -rf /"; def foo}

It is often possible to do ruby meta-programming more carefully without string eval. In this case, using simple interpolation and define_method of closures with instance_variable_[get|set], and send:

module History

  def attr_accessor_with_history(attr_name)
    getter_sym  = :"#{attr_name}"
    setter_sym  = :"#{attr_name}="
    history_sym = :"#{attr_name}_history"
    iv_sym      = :"@#{attr_name}"
    iv_hist     = :"@#{attr_name}_history"

    define_method getter_sym do
      instance_variable_get(iv_sym)
    end

    define_method setter_sym do |val|
      instance_variable_set( iv_hist, [] ) unless send(history_sym)
      send(history_sym).send( :'<<', send(getter_sym) )
      instance_variable_set( iv_sym, val @)
    end

    define_method history_sym do
      instance_variable_get(iv_hist)
    end

  end
end