Re: [Qemu-devel] [PATCH qom-next] qom: make object cast assert if NULL object is passed as argument

[Igor Mammedov <imammedo@redhat.com>]

On 06/01/2012 03:04 PM, Andreas Färber wrote:
> Am 01.06.2012 13:18, schrieb Markus Armbruster:
>> Andreas Färber<afaerber@suse.de>  writes:
>>
>>> Am 31.05.2012 13:17, schrieb Igor Mammedov:
>>>> On 05/31/2012 12:16 PM, Paolo Bonzini wrote:
>>>>> Il 31/05/2012 10:30, Markus Armbruster ha scritto:
>>>>>>>> Makes much sense, but maybe it should be done in OBJECT() cast? Assert
>>>>>>>> when we do OBJECT(NULL).
>>>>>> In my opinion, OBJECT(p) where p is a null pointer is perfectly valid
>>>>>> and should yield a null pointer.
>>>>>
>>>>> Perhaps object_dynamic_cast and object_dynamic_cast_assert should do the
>>>>> same?
>>>>>
>>>>
>>>> or better object_dynamic_cast should return NULL if obj is NULL,
>>>> after all it's expected that it may return NULL
>>>
>>> That's what I was suggesting: I think that we should define "NULL is not
>>> of type TYPE_FOO" and thus have the ..._is_... functions return false,
>>> and have the ..._cast_assert assert.
>>
>> Is it?
>
> See http://www.mail-archive.com/qemu-devel@nongnu.org/msg113922.html
>
>> Igor: object_dynamic_cast should return NULL if obj is NULL,
>>
>> You: have the ..._cast_assert assert [on null argument, I presume]
>>
>> Doesn't sound like the same suggestion to me :)
>
> I'll let you to your opinion. :) However, my opinion is that
> object_dynamic_cast_assert() should assert (its name should be program),
> not segfault, and that
> object_dynamic_cast()/object_is_type()/type_is_ancestor() should not
> assert but return false / NULL. So as to the effects and usability that
> pretty much aligns with Igor M., no?
If we decide that object_dynamic_cast() should not assert but rather return NULL
the this block in it will be incorrect in to places:

     if (object_is_type(obj, type_interface)) {
         assert(!obj->interfaces);  <== could be replaced with return NULL
         obj = INTERFACE(obj)->obj; <== calls OBJECT_CHECK() -> object_dynamic_cast_assert ()
     ...
     [snip]

maybe there should be INTERFACE_CHECK and INTERFACE macros calling ..._assert and non assert variants respectively?

>
>> If I understood you correctly: what do such assertions buy us other than
>> silliness like
>>
>>      p ? some_cast(p) : NULL
>>
>> ?
>
> Nack. The point is that currently deployed MY_TYPE(x) should assert
> (because nobody expects it to return NULL) and he who does want to
> handle NULL can use object_dynamic_cast(p). There's no real change to
> what we have except that an error case that was unhandled now is handled.
>
>>> So I still think this patch is correct. It could be accompanied by
>>> further patches adding error handling in the remaining functions.
>>
>> I'm not convinced.
>
> Shed any light?
>
> Andreas
>

-- 
-----
  Igor