From: ludovic.courtes@laas.fr (Ludovic Courtès)
Subject: Re: GOOPS: Customizing class instantiation
Date: Wed, 27 Sep 2006 15:16:28 +0200 [thread overview]
Message-ID: <87irj9witv.fsf@laas.fr> (raw)
In-Reply-To: <87psdjnig5.fsf@laas.fr> (Ludovic Courtès's message of "Tue, 26 Sep 2006 10:24:58 +0200")
Hi,
Nobody won the prize, because nobody gave the correct answer. For those
of you who played, I'm giving the correct answer below. ;-)
ludovic.courtes@laas.fr (Ludovic Courtès) writes:
> (use-modules (oop goops))
> (read-set! keywords 'prefix)
>
> (define-class <my-class> (<class>)
> (the-slot :init-value #t))
>
>
> (define-method (compute-cpl (c <my-class>))
> (format (current-error-port) "CPL (~a)~%" c)
> (list c <top>))
>
> (define-method (make-instance (c <my-class>))
> (format (current-error-port) "make-instance (~a)~%" c)
> (next-method))
>
>
> (define (make-a-class name)
> (let ((c (make <my-class>)))
> (slot-set! c 'name name)
> c))
>
> The issue is that when instantiating an instance of `<my-class>' (read
> that twice ;-)), I'm getting the following error:
>
> ;; Create a class (instance of `<my-class>').
> guile> (define c (make-a-class 'paf))
> CPL (#<<my-class> ??? 301f8930>)
> guile> c
> #<<my-class> paf 301f8930>
>
> ;; Instantiate it.
> guile> (make c)
> make-instance (#<<my-class> paf 301f8930>)
>
> <unnamed port>: In expression (let* (#) (format # "make-instance (~a)~%" ...) ...):
> <unnamed port>: No applicable method for #<<generic> initialize (10)> in call (initialize #<struct 301f8930:301f7320> ())
> ABORT: (goops-error)
The whole issue here is that `%allocate-instance'
(aka. `scm_sys_allocate_instance') returns an apparently improperly
initialized object (a raw struct) instead of a regular GOOPS object that
has a class, etc. Since there are no `initialize' methods for raw
structs, we get this no-applicable-method error.
But why is it so? Looking at `goops.c', it's actually `wrap_init ()'
that returns the offending struct. However, `wrap_init ()' _does_
specify class information as part of the type tag. So perhaps it is the
class information that is not properly initialized in the case of an
instance of `<my-class>'?
The correct way to instantiate a class is found in `make-class' in
`goops.scm'. This function passes several keyword arguments to `make'
that my `make-a-class' function did not provide. In particular, the
`:dsupers' argument is the one that must not be forgotten:
guile> (make (make <my-class>))
[...]
<unnamed port>: No applicable method for #<<generic> initialize (11)> in call (initialize #<struct 104ac060:104ae270> ())
guile> (make (make <my-class> :dsupers (list <top>)))
#<??? 104321b0>
Note that the `compute-cpl' method specialized for `<my-class>' is still
needed and it must contain a class for which an `initialize' method
exists (i.e., not `<top>'):
(define-method (compute-cpl (c <my-class>))
(list c <object> <top>))
Notice: `compute-cpl' and `:dsupers' don't even have to agree...
Now, this does not really explain why an instance of such an
improperly-initialized class would show up as a raw struct. Well,
clarifying is left as an exercise to the reader. :-)
In short, the lesson is: one should use `make-class' (although
it's undocumented) rather than `(make <class> ...)' when instantiating a
new class.
Thanks,
Ludovic.
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next prev parent reply other threads:[~2006-09-27 13:16 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-09-26 8:24 GOOPS: Customizing class instantiation Ludovic Courtès
2006-09-26 11:47 ` Ludovic Courtès
2006-09-26 15:26 ` Pat Lasswell
2006-09-27 8:20 ` Ludovic Courtès
2006-09-27 13:16 ` Ludovic Courtès [this message]
2006-09-28 8:50 ` Neil Jerram
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