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Re: [Qemu-devel] [PATCH qom-next] qom: make object cast assert if NULL o

From: Igor Mammedov
Subject: Re: [Qemu-devel] [PATCH qom-next] qom: make object cast assert if NULL object is passed as argument
Date: Mon, 04 Jun 2012 15:14:58 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1

On 06/01/2012 03:04 PM, Andreas Färber wrote:
Am 01.06.2012 13:18, schrieb Markus Armbruster:
Andreas Färber<address@hidden>  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

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/address@hidden/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 ()

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?



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