Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors

[Paolo Bonzini]

On 19/05/2015 17:55, Daniel P. Berrange wrote:
> Paolo told me on previous posting that object_property_add_child()
> holds a reference on 'obj' for as long as it is registered in the
> object hierarchy composition. So it sufficient to rely on that long
> term reference, and let the caller dispose of the object by calling
> object_unparent(obj) when finally done.

For an example of the same pattern:

DeviceState *qdev_try_create(BusState *bus, const char *type)
{
    DeviceState *dev;

    if (object_class_by_name(type) == NULL) {
        return NULL;
    }
    dev = DEVICE(object_new(type));
    if (!dev) {
        return NULL;
    }

    if (!bus) {
        bus = sysbus_get_default();
    }

    qdev_set_parent_bus(dev, bus);
    object_unref(OBJECT(dev));
    return dev;
}

Effectively this is idea as GObject's "floating reference".
qdev_set_parent_bus (in qdev_try_create) and object_property_add_child
(in Daniel's patches) "sink" the floating reference by doing
object_unref.  If we had floating references, the object would be
returned to the caller unref'ed anyway.

Of course, the reference can go away via QMP.  But that will only happen
after the caller would have called object_unref itself.

Paolo