From: Tim Becker <a2800276@...>
Date: 2006-12-29T20:09:47+09:00
Subject: Re: case bug when comparing classes...?

That would explain it :) Thanks!

On 12/29/06, Vincent Fourmond <vincent.fourmond@9online.fr> wrote:
> 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/
>
>