Re: [Qemu-devel] [PATCH 1/5] qom/object: Add a new function object_initialize_as_child()

[Peter Maydell]

On 12 July 2018 at 17:55, Thomas Huth <address@hidden> wrote:
> On 12.07.2018 18:52, Eduardo Habkost wrote:
>> On Thu, Jul 12, 2018 at 05:30:59PM +0200, Thomas Huth wrote:
>>> @@ -749,6 +749,25 @@ int object_set_propv(Object *obj,
>>>  void object_initialize(void *obj, size_t size, const char *typename);
>>>
>>>  /**
>>> + * object_initialize_as_child:
>>> + * @parentobj: The parent object to add a property to
>>> + * @propname: The name of the property
>>> + * @childobj: A pointer to the memory to be used for the object.
>>> + * @size: The maximum size available at @obj for the object.
>>> + * @type: The name of the type of the object to instantiate.
>>> + * @errp: If an error occurs, a pointer to an area to store the area
>>> + *
>>> + * This function will initialize an object. The memory for the object 
>>> should
>>> + * have already been allocated. The object will then be added as child 
>>> property
>>> + * to a parent with object_property_add_child() function. The returned 
>>> object
>>> + * has a reference count of 1, and will be finalized when the last 
>>> reference is
>>> + * dropped.
>>> + */
>>> +void object_initialize_as_child(Object *parentobj, const char *propname,
>>> +                                void *childobj, size_t size, const char 
>>> *type,
>>> +                                Error **errp);
>>
>> Why did you use void* instead of Object*?
>
> That's the same what object_initialize() is doing (see above). Otherwise
> all the callers have to cast their pointers with OBJECT() first.

Casting with OBJECT() would not work, because it does a typecheck.
At the point where something is calling object_initialize() or
object_initialize_with_child(), the memory at 'childobj' has not
been initialized, so a typecheck is impossible.

It's only once the object has actually been initialized that
the memory is a valid Object* that you can put through OBJECT().

thanks
-- PMM