From: Zach Dennis <zdennis@...>
Date: 2005-07-21T01:19:25+09:00
Subject: Re: Variable class (newb question)

Shaun Fanning wrote:
> I'm trying to figure out how to use Ruby to implement a strategy type
> pattern that I used in PHP. Basically I took a set of class names passed in
> as variables and instantiated the right class depending on the value of the
> variable. It was roughly something like:
> 
> Class SurveyQuestion
>   drawQuestion
>   storeResponse
>   reportResponse
>   ...
> 
> Class SurveyQuestionMultiChoice extends SurveyQuestion
> Class SurveyQuestionCheckBox extends SurveyQuestion
> ....etc.

One way using a base class and subclassing similar to your approach in PHP:

class A
    def talk
       "talking"
    end
end

class B < A
end

defget_class_for_string( class_name )
    eval( "#{class_name}.new" )
end

obj = get_class_for_string( "B" )
obj.talk


> 
> //build a list of question types based on what the user just submitted
> $these_survey_questions = array('MultiChoice', 'MultiChoice', 'CheckBox')
> 
> For each $these_survey_questions as $index=$question_type
>   $class_name = "SurveyQuestion".$question_type
>   $q = new $class_name()
>   $q.storeResponse($response_from_this_user)
> 
> 
> So I'm struggling to figure out how this type of thing would typically be
> done using Ruby. Or maybe it's a bad approach to this type of situation to
> begin with so feel free to offer an alternative. 

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

Then you could do stuff like...

get_class_for_string( "A" )
get_class_for_string( "M::C" )
get_class_for_string( "M::C::D::E::F" )

HTH,

Zach