From: Stefano Crocco <stefano.crocco@...>
Date: 2007-08-16T20:00:55+09:00
Subject: Re: Relationship operator for classes

Alle gioved� 16 agosto 2007, Ronald Fischer ha scritto:
> I wanted to distinguish between cases, where a variable
> is of type Symbol, or Hash, or something other. My first
> attempt failed:
>
>   case v.class
>   when Symbol
>     ...
>   when Hash
>     ...
>   else
>     ...
>   end
>
> Even if v was a Hash, processing endet up in the "else" part.
> Indeed, a quick check with irb showed:
>
>     Hash === Hash # => false
>
> I solved my problem by changing my code to
>
>   case v.class.to_s
>   when 'Symbol'
>     ...
>   when 'Hash'
>     ...
>   else
>     ...
>   end
>
> But now I wonder why === is defined for classes that way - it seems to
> *always* return false.
>
> Ronald

You need
  
  case v
  when Symbol
    ...
  when Hash
    ...
  else
    ...
  end

The reason is that Class#=== returns true if the argument is an instance of 
the class or of its descendents. In other words, it works like 
Object#instance_of?:

  a = []
  a.instance_of? Array
  => true
  a.instance_of? Object
  => true
  a.instance_of? String
  => false

  Array === a
  => true
  Object === a
  => true
  String === a
  => false

 Notice that you need to be careful if you need to check whether the class of 
an object is *exactly* a given class. If you need to distinguish between a 
class and a descendent, you have to put them in the correct order in the case 
statement: first the derived class, then the base class:

class A;end

class B < A; end

case obj
when B
  ...
when A
  ...
end

I hope this helps

Stefano