From: Stefano Crocco <stefano.crocco@...>
Date: 2012-10-24T00:52:45+09:00
Subject: Re: Name/symbol/object type clash? What is happening here?

On Wednesday 24 October 2012 Todd Benson wrote
> It's nonsense code, but I'm curious as to what is going on behind the
> scenes...
> 
> irb(main):001:0> S = Struct.new(:num)
> => S
> irb(main):002:0> class S; def change_to n; @num = n; end; end
> => nil
> irb(main):003:0> s = S.new(3)
> #<struct S num=3>
> irb(main):004:0> s.num = 4
> => 4
> irb(main):005:0> s.num
> => 4
> irb(main):006:0> s.change_to(5)
> => 5
> irb(main):007:0> s.num
> => 4
> irb(main):008:0> s.instance_variables
> => [:@num]
> irb(main):009:0> s.instance_variable_set(:@num, 5)
> => 5
> irb(main):010:0> s.num
> => 4
> 
> Same thing happens if I don't define class S, but just define a method
> on the instance... def s.change_to n; @num = n; end

Apparently, classes created using Struct don't store values in instance 
variables (you can see this by calling #instance_variables on s immediately 
after creating it: it returns an empty array); rather they are stored 
internally in C-level variables which can't be accessed from ruby. Because of 
this, setting the @num instance variable doesn't have any effect on the value 
returned by #num. If you know C, you can see how Struct works internally 
looking at the file struct.c in the ruby source code.

I hope this helps

Stefano