From: Paul Brannan <pbrannan@...>
Date: 2003-02-20T00:26:57+09:00
Subject: Re: How to test for existence of instance variable?

On Thu, Feb 20, 2003 at 12:02:21AM +0900, Brian Candler wrote:
>   #!/usr/local/bin/ruby -w
>   mystruct = Struct.new("Mystruct","a","b","c")
>   obj = mystruct.new
> 
>   mystruct.instance_eval { attr_accessor :d }     # add a new member

I don't think this is a good idea, since d isn't recognized as a member:

  irb(main):001:0> mystruct = Struct.new("Mystruct","a","b","c")
  => Struct::Mystruct
  irb(main):002:0> obj = mystruct.new
  => #<Struct::Mystruct a=nil, b=nil, c=nil>
  irb(main):003:0> mystruct.instance_eval { attr_accessor :d }
  => nil
  irb(main):004:0> obj.members
  => ["a", "b", "c"]

It would be better, I think, to inherit from Struct and write a new
method to dynamically add a member.  You can then define members() in
the derived class to include "d" as a member, and you can add a new
method to easily lookup whether a given member is already defined.

Paul