From: James Britt <james_b@...>
Date: 2006-06-08T12:19:14+09:00
Subject: Re: Using a string to find a class

Dave Burt wrote:
> Pat Maddox wrote:
> 
>> I've got a string, and I want to find out if there's a class with that
>> name.  So something like
>>
>> class_exists? "Foo"
>>
>> and then I can eval it if it does, I guess.  Or is there a better way
>> than eval in that case?  At any rate, I just want to be able to take a
>> string and find out if there's a class with that name or not.
> 
> 
> module.const_get "Foo"
> # or just
> const_get "Foo"

You'll get an exception if Foo isn't defined.

And not every constant refers to a class you can instantiate:

   class Bar ; end
   kls = Object.const_get "Bar"
   c  = kls.new() # OK

   Foo = 12
   kls = Object.const_get "Foo"
   c  = kls.new() # Not so OK


Quick version:

  def class_exists? kls
   begin
     kls = Object.const_get kls
     kls.kind_of? Class
   rescue Exception
     false
   end
  end


  def new_if_class kls, default = nil
   class_exists?( kls ) ?  Object.const_get( kls ).new : default
  end


  b = new_if_class "Bar"
  p b

  c = new_if_class "Foo"
  p c

-- 
James Britt

"Take eloquence and wring its neck."
  - Paul Verlaine