marvin at rectangular
Jun 15, 2007, 5:35 PM
Post #3 of 6
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On Jun 15, 2007, at 1:52 PM, Nathan Kurz wrote:
> On 6/15/07, Marvin Humphrey <marvin [at] rectangular> wrote:
>> To subclass OrScorer, create MyOrScorer.c and MyOrScorer.h following
>> the guidelines documented in the POD for devel/lib/BoilerPlater.pm.
>> All you'll need is a constructor and a MyOrScorer_tally function.
>> The rest will inherit.
>
> Got it, and it seems to Build correctly.
Wow, hot damn.
The BoilerPlater stuff came out well, but it wasn't and isn't really
designed to be a public API. It arose out of necessity because the
faked-up inheritance schemes that Dave Balmain was using with Ferret
and I was using with KS 0.15 were messy and scaled poorly. The
design was hashed out last fall on the Lucy developer's list.
> First subclass subquestion: why do I actually need a constructor?
In theory you could write an XS function that "reblessed" the object
by changing the vtable pointer to point at a different one.
MODULE = KinoSearch PACKAGE = KinoSearch::Search::MyOrScorer
kino_MyORScorer*
new(unused, sim, subscorers)
SV *unused;
kino_Similarity *sim;
kino_VArray *subscorers;
CODE:
RETVAL = kino_ORScorer_new(sim, subscorers);
RETVAL->_ = &KINO_MYORSCORER; /* <-------- rebless */
OUTPUT: RETVAL
I've opted never to do that mainly because I want XS code to be
limited to glue whenever possible. XS is powerful, but it's nasty
and esoteric.
Take a look at RichPostingScorer_new() -- it does exactly the same
thing as that XS function above, but within a dedicated C constructor.
> Is the class_name field actually used at runtime?
Yes, absolutely. self->_->class_name is used all over the place,
particularly when crossing the Perl/C boundary.
Note that class_name is a member of the vtable, and not a member of
the object struct. That means we don't have to waste space in every
object with an extra pointer to the class name -- but also that the
class name of an object is fixed.
> And if I do need a
> constructor, is it possible to piggyback on top of the constructor for
> the class I'm inheriting from? Something like:
>
> MyORScorer *self = STEAL(MyORScorer, ORScorer_new(sim,
> subscorers));
> (or BLESS, or RECREATE, RECLASS, whatever)
Cool idea, but it would have to look slightly different, because of
the limitations of C syntax. It would have to be either a function,
or a multi-line macro like this:
#define KINO_RECLASS(var, obj, type, vtable) \
type* var = (type*)(obj); \
var->_ = &(vtable)
> I don't understand the innards of your vtable implementation well
> enough to know quite what this would need to do, but if it is possible
> it seems likely you've already done it somewhere but I just haven't
> found where.
Yes. Dynamic subclassing is supported via DynVirtualTable and the
CREATE_SUBCLASS macro defined in MemManager.h.
The implementation is fairly complex, which is unfortunate, because
it doesn't accomplish very much -- it just allows the object to be
associated with an arbitrary class name. :\ The feature is used by
Schema, FieldSpec and Similarity to allow users to subclass via Perl
without knowing anything about the underlying C objects.
Ideally, our discussion will result in an improvement upon that
scheme that will allow you to write your ORScorer subclass without
touching BoilerPlater. Something like this:
package MyORScorer;
use base qw( KinoSearch::Search::ORScorer );
__PACKAGE__->register_c_method( tally => 'my_tally' );
use Inline => C << 'END_C';
kino_Tally*
my_tally(kino_OrScorer *self) {
/* ... */
}
END_C
Marvin Humphrey
Rectangular Research
http://www.rectangular.com/
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