From: Vincent Fourmond <vincent.fourmond@9online.fr>
Date: 2006-12-29T20:01:17+09:00
Subject: Re: case bug when comparing classes...?

Tim Becker wrote:
> Hi,
> I'm encountering weird behavior I don't understand when creating a
> case expression that does comparison using classes. Consider the
> following:
> 
> class SomeClass
> end
> 
> def case_test elem
>     case elem.class
>         when Object
>             puts "Object"
>         when String
>             puts "String"
>         when SomeClass
>             puts "SomeClass"
>         else
>             puts "Unknown: #{elem.class}"
>     end
> end
> 
> executing:
> 
> case_test Object.new
> case_test "bla"
> case_test SomeClass.new
> case_test 1
> 
> yields:
> 
> Object
> Object
> Object
> Object
> 
> 
> On the other hand, running:
> 
> def if_test elem
>     if Object == elem.class
>         puts "Object"
>     elsif String == elem.class
>         puts "String"
>     elsif SomeClass == elem.class
>         puts "SomeClass"
>     else
>         puts "Unknown: #{elem.class}"
>     end
> end
> 
> if_test Object.new
> if_test "bla"
> if_test SomeClass.new
> if_test 1
> 
> 
> results in:
> 
> Object
> String
> SomeClass
> Unknown: Fixnum
> 
> I've not been able to find anything that fits searching the bug
> database or doing a rudimentary google search, and it would seem to me
> that someone else would have come across this before.
> 
> Am I missing something?

  Yes ;-).

  Using the case construct implies that you're using the === operator.
And the === operator of a instance of Class is exactly is_a?. Which
means that, since elem.class is a instance of Class, it is an Object,
and your first test will always match.

  You should try this:

def case_test elem
    case elem
        when String
            puts "String"
        when SomeClass
            puts "SomeClass"
        when Object
            puts "Object"
        else
            puts "Unknown: #{elem.class}"
    end
end

You need to put Object at the end, and look only at elem. (and you else
clause will never get executed)

  Cheers,

	Vince

-- 
Vincent Fourmond, PhD student
http://vincent.fourmond.neuf.fr/