From: Andreas Launila <ruby-talk@...>
Date: 2007-06-12T06:01:26+09:00
Subject: Re: Constants and metaclasses

Joel VanderWerf wrote:
> It doesn't seem possible to use const_missing to do this, because
> const_missing is called on Foo (not on the instance or the singleton
> class) and there is no way to identify the instance of Foo in the
> const_missing method.
> 

I didn't know that const_missing existed, that might be just enough to
not have to redefine constants if one accepts some dirtiness. Here is an
adaptation of your code with that thrown in.


class Object
  def singleton_class
    class << self; self; end
  end
end

class Foo
  def foo
    p BAR
  end

  private

  def self.const_missing(name)
    if !@caller.nil? and @caller.singleton_class.const_defined? name
      @caller.singleton_class.const_get(name)
    else
      super
    end
  end

  def self.next_caller(caller)
    @caller = caller
  end
end

f = Foo.new
class <<f
  BAR = "Hello"

  alias_method :old_foo, :foo
  def foo
    self.class.next_caller(self)
    old_foo
    self.class.next_caller(nil)
  end
end

g = Foo.new
class <<g
  BAR = "Goodbye"

  alias_method :old_foo, :foo
  def foo
    self.class.next_caller(self)
    old_foo
    self.class.next_caller(nil)
  end
end

f.foo
g.foo
f.foo


Outputs
"Hello"
"Goodbye"
"Hello"

It isn't pretty, it isn't thread safe, it will not work if Foo is called
 directly, it could be shortened/generalized/altered/improved depending
on the purpose.

Thank you :)

-- 
Andreas Launila