From: Joel VanderWerf <vjoel@...>
Date: 2007-06-12T03:37:30+09:00
Subject: Re: Constants and metaclasses

Andreas Launila wrote:
> Is there a way to define a constant in a metaclass and then have it used
> in methods defined in the original class?
> 
> An example:
> 
> 
> class Foo
>   def foo
>     p BAR
>   end
> end
> 
> f = Foo.new
> class <<f
>   BAR = "Hello"
> 
>   def bar
>     p BAR
>   end
> end
> 
> f.bar
> f.foo
> 
> 
> The line "f.bar" will work fine, but "f.foo" will fail with "NameError:
> uninitialized constant Foo::BAR". Is there some way to have the method
> defined in the original class pick up on the constant in the metaclass
> (without altering "p BAR")?
> 

Yes, with a little wrangling:

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

class Foo
   def foo
     p self.singleton_class::BAR
   end
end

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

   def bar
     p BAR
   end
end

f.bar
f.foo



-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407