From: ES <ruby-ml@...>
Date: 2005-03-14T06:08:35+09:00
Subject: Re: how do I convince Hash that two keys are the same?


In data 3/13/2005, "Sam Roberts" <sroberts@uniserve.com> ha scritto:

>(I sent this to ruby-doc by accident, and got a response, but I am
>reposting to ruby-talk, sorry for the cross-post.)
>
>Thanks, thats very informative, and I would not have guessed.
>
>I tried a different approach, I derived from String. This has the
>very odd behaviour of blowing the Stack!
>
>  class Str < String
>    def hash; self.downcase.hash; end # <- this doesn't look recursive
>      # to me, but it blows the stack
>  end
>
>  h = { }
>
>  k = Str.new('a')
>
>  k == 'A'
>  'A' == k
>
>  h[k] = 'lower'
>
>  k = Str.new('A')
>  p k.downcase            # Should return an instance of String, right?
>  p k.downcase.hash       # <--- this line cause stack overflow
>  k.hash == 'a'.hash
>
>  h.has_key? k
>
>
>I think Str#downcase should return the same thing as String#downcase =>
>a String, and not a Str, so I don't see why there is a recursive call,
>here.

When you derive, you establish an is_a relationship. Str is_a String.
All methods are also derived so you're actually calling Str#downcase,
although the code being executed was defined in String.

You'd possibly want to alias hash to old_hash and then call
self.downcase.old_hash, which seems to be what you're trying
to do.

>What am I missing?
>
>Thanks,
>Sam
>
>Quoting g_ogata@optushome.com.au, on Mon, Mar 14, 2005 at 05:41:40AM +1100:
>> Sam Roberts <sroberts@uniserve.com> writes:
>>
>> > I've tried:
>> >
>> >
>> >   h = { }
>> >
>> >   k = 'a'
>> >
>> >   class << k
>> >     def hash; self.downcase.hash; end
>> >     def ==(s); self.downcase == s.downcase; end
>> >     def eql?(s); self == s; end
>> >     def ===(s); self == s; end
>> >   end
>> >
>> >   k = 'A'
>> >
>> >   class << k
>> >     def hash; self.downcase.hash; end
>> >     def ==(s); self.downcase == s.downcase; end
>> >     def eql?(s); self == s; end
>> >     def ===(s); self == s; end
>> >   end
>> >
>> >   k == 'a'
>> >   k.hash == 'a'.hash
>> >
>> >   # I want this to be true!
>> >   h.has_key? k
>> >
>> > But it doesn't work. I'm particularly confused because if I write a
>> > class Str that wraps String and defines those methods things do work
>> > out...
>> >
>> > Any pointers?
>>
>> This looks like a consequence of the fact that when ruby takes the
>> hash of a key, if the key is a String, it uses the default
>> implementation of String#hash directly, even if String#hash has been
>> redefined, or the object has a singleton class.  (The same thing is
>> true with Fixnums and Symbols.)
>>
>> Try:
>>
>>   class String
>>     def hash
>>       puts '!'
>>       super
>>     end
>>   end
>>
>>   {}['a'] = 1
>>
>> Then change the String to an Object.
>>
>> HTH.

E