From: Doug Glidden <41mortimer@...>
Date: 2008-08-24T11:04:52+09:00
Subject: Re: Passing parameters into a singleton class def?

Sean O'halpin wrote:
> Something like this?
> 
> class A
> end
> 
> a = A.new
> fields = [:a, :b]
> (class << a; self; end).class_eval do
>   attr_accessor *fields
> end
> a.a = 1
> a.b = 2
> p a
> b = A.new
> b.a = 3
> __END__
> #<A:0x25094 @a=1, @b=2>
> rt.rb:13: undefined method `a=' for #<A:0x24f40> (NoMethodError)
> 
> Regards,
> Sean

David Masover wrote:
> Well, if you actually have fields called :field1, :field2, and :field3, 
I
> might suggest an array... Assuming that's not the case.

> I don't even remember the actual Ruby way to do this, but _why's metaid 
gem
> would let you do it like this:

> def add_fields(*field_list)
>   meta_eval do
>     attr_accessor *field_list
>   end
> end

> The key, however, is that it's some sort of _eval method -- that it's 
taking a
> block, which means you get to inherit the parent scope. In fact, if it's
> actually a singleton (as in, only one instance of this class will be
> defined), you can do this:

> def add_fields(*field_list)
>   self.class.class_eval do
>     attr_accessor *field_list
>   end
> end

> In this case, self.class refers to the actual class that the current 
object is
> an instance of. On the other hand, self.metaclass or meta_eval (both 
from
> metaid) refer to the current object's metaclass, or eigenclass.

> Given that you were doing "class << foo" stuff, I'm assuming you wanted 
the
> metaclass.

> One more hack before I'm through:

> def add_fields(*field_list)
>   self.metaclass.send :attr_accessor, *field_list
> end

> I haven't tested any of this. Let me know how it works for you.

Thanks very much to both of you!  I'm not using why's metaid gem, but I 
am familiar with the way he does things.  I had tried using class_eval, 
as Sean suggested, but I wasn't using it quite correctly.  Sean's code 
worked perfectly, but I wanted to make it a little more 
compartmentalized, so I used some of why's strategy.  Here's the final 
working example code:

class MyClass
  def metaclass
    class << self
      self
    end
  end
  ...

  def add_fields(*field_list)
    metaclass.class_eval do
      attr_accessor *field_list
    end
  end
end

Of course, it would be better to move the metaclass method into a mixin, 
or just use why's metaid gem.

Thanks again,
Doug
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