From: Farrel Lifson <farrel.lifson@...>
Date: 2006-03-10T01:20:07+09:00
Subject: Re: nuby question: sorting with nils

I don't think you can access the attributes of a Struct from inside it
using object variables (like @a).

C:\Documents and Settings\flifson\Desktop>irb
irb(main):001:0> X = Struct.new :a,:b
=> X
irb(main):002:0> class X
irb(main):003:1>   def print_attributes
irb(main):004:2>     puts "#{@a}:#{b}"
irb(main):005:2>   end
irb(main):006:1> end
=> nil
irb(main):007:0> s = X.new 'hello','world'
=> #<struct X a="hello", b="world">
irb(main):008:0> s.print_attributes
:world

Notice only the value of  the b attribute was printed ('world').
That's because self. was implicitly added to the front of it, so it
became self.b which will return the correct value. Changing it to:

def <=> other
   if not a
     1
   elsif not b
     -1
   else
     if a == other.a
       b <=> other.b
     else
       a <=> other.a
     end
   end
 end

produces:

C:\Documents and Settings\flifson>ruby settest.rb
#<SortedSet: {#<struct X a=1, b=2>, #<struct X a=2, b=1>, #<struct X a=3, b=3>,
#<struct X a=3, b=4>}>
-1
1
0

My guess is that the Struct is really a type of Hash and that it uses
the method_missing system hook to actually retrieve the data from
whatever it uses to store the data. It has a lot of Hash like methods
such as values, each_pair etc etc.

Farrel