From: Joel VanderWerf <vjoel@...>
Date: 2004-06-04T03:48:18+09:00
Subject: Re: Extending Objects Without Inheritance

Alex Gutteridge wrote:
> Hi,
> 
> I'm no great OO expert (or Ruby expert) so maybe the answer to this is
> obvious or maybe it's impossible.
> 
> I currently have a bunch of classes which I use to parse a data file.
> Essentially there are fve layers of containers:
> 
> PDB -> Model -> Chain -> Residue -> Atom
> 
> A PDB object contains one or more Model objects each of which contain
> one or more Chain objects each of which contain... etc down to Atoms.
> Note that Model doesn't inherit from PDB or Chain from Model - each
> class is just a container for the next one down.
> 
> The problem I have is that sometimes I have extra data associated with
> the Atom and/or Residue objects. This extra data always comes later
> (from a different data file) and usually won't be present. when I need
> to, I'd like to add this data to the (already created data structure)
> with the minimum amount of work/fuss.
> 
> As far as I can see I have the following options:
> 
> 1. Modify the Atom and Residue classes to include the extra data, set
> to nil originally and then filled in when needed. I'd rather not do
> this because the code is part of a bigger project which I don't have
> direct control over, and it seems rather inelegant to have these extra
> fields which hardly ever get used.

Probably the caffeine hasn't hit me yet, but I'll suggest this anyway: 
why not define the extra accessors you might need and assign to them 
only when needed? No extra memory is used until you assign, as the 
following shows:

class Atom
   attr_accessor :x, :y, :extra1, :extra2
   def initialize x, y
     @x, @y = x, y
   end
end

atom = Atom.new(1,2)

p atom
p atom.instance_variables

atom.extra1 = "extra stuff"

p atom
p atom.instance_variables

__END__

Output:

#<Atom:0x401c68e0 @y=2, @x=1>
["@y", "@x"]
#<Atom:0x401c68e0 @y=2, @x=1, @extra1="extra stuff">
["@y", "@x", "@extra1"]