[Vm-dev] #cannotInterpet: use last class rather than receiver's class?

Eliot Miranda eliot.miranda at gmail.com
Thu Dec 8 16:58:37 UTC 2011

Hi Marian,

On Thu, Dec 8, 2011 at 7:50 AM, Mariano Martinez Peck <marianopeck at gmail.com
> wrote:

> Hi guys. When the finds a methodDict in nil during the method lookup, it
> sends #cannotInterpret: to the receiver, but starting the methodLookup in
> the superclass of the class whose method dict was nil.
> Example, if I have A subclass from B and B has the method dict in nil, if
> I sent #foo to an instance of A, it will send #cannotInterpret: to such
> instance but starting the method lookup (of the #cannotInterpret:) in B
> (the superclass).
> Now, when sending #cannotInterpret:  aMessage, it sends as parameter an
> instance of Message. The thing is that in the instVar "lookupClass" it puts
> the receiver's class. In this example, A. But when you are implementing
> #cannotInterpret:  such info (the lookupClass) is already there, because
> you DO have the receiver (in self), so you could always do a "self class".
> In my case, for some proxies I am doing, I would need not the lookupClass,
> but the class whose methodDict was nil, in this case, B.

Not quite.  It puts the lookup class in the lookupClass inst var.  So if a
super send invokes cannotInterpret: lookupClass will hold the superclass of
the method in which the send occurred.

> So, questions:
> 1) Do I break something if I do this?  Am I limiting the solution somehow?
> it is not always true that I can get the lookupClass while doing "self
> class" ?  is there someone using this hook of #cannotInterpret:?

I have no idea.  But given the lookupClass it is a simple matter to walk
the hierarchy starting at lookupClass, looking for the first nil
methodDictionary.  This will be the class at which cannotInterpret: was
initiated.  Isn't this enough?

> 2) Ok, I know that I should send an instance of Message with something it
> is not the lookupClass in the instVar 'lookupClass', but it is a
> lookupClass in some form ;)
> Thanks in advance,
> --
> Mariano
> http://marianopeck.wordpress.com

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