On Thu, Sep 29, 2016 at 09:14:16AM +0100, Daniel P. Berrange wrote: > On Thu, Sep 29, 2016 at 10:16:41AM +1000, David Gibson wrote: > > QOM has the concept of both "object class" properties and "object > > instance" properties. > > > > The accessor functions installed for the rarely-used class properties > > still take an Object *, so the *value* of such properties is still > > per-instance; it's just the *existence* (and type) of the property > > that is per-class. > > Yes, of course. This is the whole point of class properties. It avoids > allocating the same ObjectProperty struct against every object instance > which wastes massive amounts of memory in scenarios where there are lots > of instances created.
Ah, that makes sense.
> > Of course, that's also true in practice for the great majority of
> > "instance" properties, because they're created identically and
> > unconditionally for every instance from the per-class instance_init
> > hook.
> >
> > This also means that the (unused) object_class_property_add_*_ptr()
> > functions don't make a lot of sense, since they require a fixed
> > pointer which means the value of such a property would only be
> > per-class.
> >
> > Given that, is there really any value to supporting the "class"
> > properties in addition to the "instance" properties? This series is
> > an RFC which removes all support for class properties, changing the
> > few existing users to instance properties instead.
> >
> > Alternatively, if we *don't* want to remove class properties, should
> > we instead be trying to convert the many, many "instance" properties
> > whose existence is actually per-class to be class properties?
>
> Practically all instances properties should become class properties
> as its going to save wasting memory once most are converted.
Heh, ok. Well, I'll keep that in mind when I'm adding properties in
future. I wonder if there's a way we can better get the word out that
this is how properties should usually be done.
That said.. I'm still thinking we should remove
object_class_property_add_*_ptr(). Those take an actual pointer to
the value, meaning that it can't have different values per-instance.
These only create read-only properties, so they're not actually
dangerous, but they really don't seem very useful.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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