From: Tim Becker <a2800276@...>
Date: 2007-02-24T21:52:06+09:00
Subject: Extending object instances with <<

Hi,

I'm trying to write a class with a method to extend instances of
itself to contain additional accessors. I thought using `class <<
self` would be the most elegant way to go about it, but I'm running
into some problems. To illustrate:

class Test
  # takes an array of symbols to add to the instance.
  def add syms
    syms.each { |sym|
      @@__tmp = sym
      $__tmp = sym
      class << self
        #attr_accessor sym  # this would be my preferance, but sym
isn't in scope here
        #attr_accessor $__tmp  # this works, but uses globals
        attr_accessor @@__tmp   # this is nearly as bad as using globals
      end # <<
    } # each
  end # add
end # Test

t = Test.new
t.add [:thingie, :thingie2]

t.thingie="whatever"
t.thingie2="bla"
puts t.thingie
puts t.thingie2



I don't like the idea of using globals to transport the symbol
information and the class members approach is nearly as bad
(synchronization issues mainly, apart from elegance). But I can't
think of another way to transport dynamic data into the `class<<self`
block.

Alternatives would be to handle this using `method_missing` though
that wouldn't just affect a single instance or using `eval` which
would involve executing strings I'm banging together.

Another thing I tried was:

...
self.class.attr_accessor sym
...

but that doesn't work because `attr_accessor` is private (contrary to
what it says in the documentation...)

Any ideas? Am I missing something?
   -tim