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

["Andreas Färber" <afaerber@suse.de>]

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 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

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