On Fri, Sep 30, 2016 at 09:33:20AM +1000, David Gibson wrote: > 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.
Yeah, that method seems dubious - my mistake in just copying all the existing property methods. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://entangle-photo.org -o- http://search.cpan.org/~danberr/ :|