From: "David A. Black" <dblack@...>
Date: 2005-07-21T01:46:16+09:00
Subject: Re: Variable class (newb question)

Hi --

On Thu, 21 Jul 2005, Zach Dennis wrote:

> The only thing I don't like about the above solution is potential insecurity 
> if someone passes in "<code here> ; B" , the <code here> would execute. This 
> could changed to be more secure if you were always using toplevel classes...
>
>
> def get_class_for_string( class_name )
>    eval("#{class_name}.new") if Object.constants.include?( class_name )
> end
>
> Which the above code makes sure that the passed in class_name has been 
> defined on the top-level Object otherwise it will return nil. This won't work 
> for things like "MyModule::B" or "MyClass::InnerClass:B", although you could 
> change it to work. And here is the modified version to make it work across 
> the board:
>
> def get_class_for_string( class_name )
>    last_constant = Object
>    class_name.split( /::/ ).each do |cons_str|
>       if last_constant.constants.include?( cons_str )
>       	last_constant = eval( "last_constant::#{cons_str}" )
>       else
>         return nil
>       end
>    end
>    eval("#{last_constant}.new")
> end

A more concise and eval-free way to do that is:

   class Module
     def deep_const_get(str)
       str.split("::").inject(Object) {|a,b| a.const_get(b) }
     end
   end

"Traditional", as they say for folk songs :-)  I've written it and
others have too -- I don't know who first.

Note that there's no need to test for failure; it will fail if any
call to const_get fails.


David

-- 
David A. Black
dblack@wobblini.net