From: Brian Candler <B.Candler@...>
Date: 2003-02-20T01:04:35+09:00
Subject: Re: How to test for existence of instance variable?

On Thu, Feb 20, 2003 at 12:26:57AM +0900, Paul Brannan wrote:
> 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"]

Yeah, not sure entirely about that, because even the instance variable is
invisible to 'inspect':

irb(main):006:0> obj.d="foo"
"foo"
irb(main):008:0> obj
#<Struct::Mystruct a=nil, b=nil, c=nil>     # where's d?

But it does work.

> 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.

I want a general method to add a member to the class dynamically, so if
there were
    Struct#add_member
I would happily use it.

I don't see what you mean about subclassing though. Creating SuperStruct as
a subclass of Struct would mean that it had two sets of members (the members
created for the initial Struct and then the members I added later) which
would be horrible - better just to rewrite Struct from scratch, which I
don't fancy. OTOH, if I wanted to learn the internals of Struct I could add
a new method 'add_member' without subclassing it at all.

Subclassing *instances* of Struct is not what I want either, since it's not
possible to change existing objects from type A to type B (even if B is a
subclass of A) after they have been created.

Regards,

Brian.