From: Rick DeNatale <rick.denatale@...>
Date: 2009-04-27T04:55:29+09:00
Subject: Re: Using method missing to create getters and setters

On Sun, Apr 26, 2009 at 3:30 PM, Robert Klemme
<shortcutter@googlemail.com> wrote:
> On 26.04.2009 19:40, Tim Conner wrote:

>> My code currently looks like this:
>> class BatchExternalBooking
>>  def method_missing(method_sym, *args)
>>    if method_sym.to_s =~ /^message_thread_([0-9]*)=?(\w*)?$/
>>      BatchExternalBooking.instance_eval "attr_accessor
>> :message_thread_#{$1}"
>
> You might rather want to use class_eval here.

No instance_eval works just as well for sending attr_accessor to a
class/module. The only difference with class_eval is when using def to
define a method.

Both do the evaluation in the context of the receive but class_eval
sets the current class to the receiver.

Class.class_eval("def foo;end") defines an instance method while
Class.instance_eval("def foo;end") defines a class method.

This seems a bit less odd when you consider that a class method is
just a singleton method on the class (albeit one which can be
inherited by subclasses).

>
> Also, as Robert pointed out already, using $1 only once and storing the
> value in a local variable is safer because it can be changed by any method
> you invoke.
>
>>      self.send("message_thread_#{$1}=", $2)

That's not true either. The $n variables are frame local, in a given
invocation frame they will only change when another regex match is
done in the same invocation.  The bug here is that this line should
have been

       self.send("message_thread_#{$1}=", *args)  # or args.first if you must.

Since the regexp only had a single capture, $2 is always nil, so even
though the newly created setter is geting called, it's setting the
instance variable to nil.

-- 
Rick DeNatale

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