From: Zach Dennis <zdennis@...>
Date: 2005-07-21T02:12:19+09:00
Subject: Re: Variable class (newb question)

David A. Black wrote:
> 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.
> 

Very beautiful! And thank you for the more concise and better 
implemented code, I will have to add this to my repertoire.

Zach