[armedbear-devel] class-prototype

[Alessio Stalla]

On Wed, Mar 10, 2010 at 6:43 PM, Alan Ruttenberg
<alanruttenberg at gmail.com> wrote:
> Hi,
>
> Thanks for adding class-prototype. Unfortunately I realize that the
> specification doesn't guarantee the behavior I was looking for, so I
> though I would solicit advise.
>,
> Basically I want to be able to access class-allocated slot values
> before I create any real instances. I thought I could grab the class
> prototype and do slot value on them, but according to the doc below
> initialize is not called, which I guess is when class-allocated slots
> are being set up.
>
> Is there a portable way of asking for the values of class slots absent
> the creation of at least one initialized instance?

I'm afraid there isn't, because all slots (both instance- and
class-allocated) are initialized by shared-initialize, as part of the
instance initialization protocol[1]. Also note that, as you posted,
according to AMOP "The results are undefined if a portable program
modifies the binding of any slot of prototype instance", so
initializing its slots in any way, even class-allocated ones, is not
portable. You should use mop::allocate-instance instead of
class-prototype. For the record, in ABCL the two are equivalent, but
e.g. in SBCL they aren't (the class prototype is cached).

AFAIK, class-prototype is only useful when you have to call a method
specialized on a class C, but which does not use the object of class C
in any way except for actually selecting the correct method. With
class-prototype, you have an option to call such a method even without
an instance of C available.

Alessio

[1] http://www.lispworks.com/documentation/HyperSpec/Body/f_shared.htm

> -Alan
>
>
> http://www.franz.com/support/documentation/8.2/doc/mop/dictionary.html#class-prototype
>
> Generic Function class-prototype class
>
> Returns a prototype instance of class. Whether the instance is
> initialized is not specified. The results are undefined if a portable
> program modifies the binding of any slot of prototype instance.
>
> This generic function signals an error if class has not been finalized.
>
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