Re: [Qemu-block] [PATCH v3 2/7] qapi: Add qobject_to()

[Max Reitz]

On 2018-02-24 21:57, Eric Blake wrote:
> On 02/24/2018 09:40 AM, Max Reitz wrote:
>> This is a dynamic casting macro that, given a QObject type, returns an
>> object as that type or NULL if the object is of a different type (or
>> NULL itself).
>>
>> The macro uses lower-case letters because:
>> 1. There does not seem to be a hard rule on whether qemu macros have to
>>     be upper-cased,
>> 2. The current situation in qapi/qmp is inconsistent (compare e.g.
>>     QINCREF() vs. qdict_put()),
>> 3. qobject_to() will evaluate its @obj parameter only once, thus it is
>>     generally not important to the caller whether it is a macro or not,
>> 4. I prefer it aesthetically.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>   include/qapi/qmp/qobject.h | 30 ++++++++++++++++++++++++++++++
>>   1 file changed, 30 insertions(+)
>>
> 
>> +++ b/include/qapi/qmp/qobject.h
>> @@ -50,6 +50,22 @@ struct QObject {
>>   #define QDECREF(obj)              \
>>       qobject_decref(obj ? QOBJECT(obj) : NULL)
>>   +/* Required for qobject_to() */
>> +#define QTYPE_CAST_TO_QNull     QTYPE_QNULL
>> +#define QTYPE_CAST_TO_QNum      QTYPE_QNUM
>> +#define QTYPE_CAST_TO_QString   QTYPE_QSTRING
>> +#define QTYPE_CAST_TO_QDict     QTYPE_QDICT
>> +#define QTYPE_CAST_TO_QList     QTYPE_QLIST
>> +#define QTYPE_CAST_TO_QBool     QTYPE_QBOOL
>> +
>> +QEMU_BUILD_BUG_MSG(QTYPE__MAX != 7,
>> +                   "The QTYPE_CAST_TO_* list needs to be extended");
>> +
>> +#define qobject_to(obj, type) \
>> +    container_of(qobject_check_type(obj, glue(QTYPE_CAST_TO_, type))
>> ?: \
>> +                     QOBJECT((type *)NULL), \
> 
> I guess the third (second?) branch of the ternary is written this way,
> rather than the simpler 'NULL', to ensure that 'type' is still something
> that can have the QOBJECT() macro applied to it?  Should be okay.

It's written this way because of the container_of() around it.  We want
the whole expression to return NULL then, and without the QOBJECT()
around it, it would only return NULL if offsetof(type, base) == 0 (which
it is not necessarily).

OTOH, container_of(&((type *)NULL)->base, type, base) is by definition NULL.

(QOBJECT(x) is &(x)->base)

Max

> 
>> +                 type, base)
>> +
> 
> Reviewed-by: Eric Blake <address@hidden>
>

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