From: Sean O'Halpin <sean.ohalpin@...>
Date: 2008-08-24T10:37:53+09:00
Subject: Re: Passing parameters into a singleton class def?

On Sun, Aug 24, 2008 at 2:09 AM, Doug Glidden <41mortimer@gmail.com> wrote:
> Hi,
>
> I'm a little new to Ruby, so please feel free to suggest a completely
> different and better way to do this, if you have one.  I'm trying to
> figure out a way to access a variable within a singleton class
> definition that is defined outside the singleton class definition.
> Basically, I want to be able to add new, user-defined instance variables
> and readers and writers for the instance variables to an object.  I can
> do it fine if I construct the list of instance variables inline.  Given
> a definition of MyClass, of course, the following code works
> beautifully:
>
> var = MyClass.new
> class << var
>  attr_accessor :field1, :field2, :field3
> end
>
> However, I need to be able to build the list prior to doing the
> singleton class definition (ideally, I want to be able to pass the list
> of instance variables into an instance method and have it take care of
> the singleton class definition on itself).  This is the way code is
> written right now:
>
> class MyClass
>  ...
>
>  def add_fields(*field_list)
>    class << self
>      attr accessor *field_list
>    end
>  end
> end
>
> I've also tried the following in irb (identical to the above code that
> works, except that the list of fields is assigned to a variable instead
> of being constructed inline):
>
> var = MyClass.new
> list = :field1, :field2, :field3
> class << var
>  attr_accessor list
> end
>
> Both throw an error because field_list is not defined within the
> singleton class definition?  Am I making a simple mistake, or should I
> be doing this differently?
>
> Thanks,
> Doug G.
> --
> Posted via http://www.ruby-forum.com/.
>
>
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