Re: [Qemu-devel] [PATCH v2 1/6] qapi: Add qobject_to()

[Eric Blake <eblake@redhat.com>]

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On 01/20/2018 09:44 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 <mreitz@redhat.com>
> ---
> You're more than welcome to convince me to call it QOBJECT_TO()!

I can live with the lower-case version; especially with your good list
of reasons for it ;)

> ---
>  include/qapi/qmp/qobject.h | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h
> index 38ac68845c..1211989ca0 100644
> --- a/include/qapi/qmp/qobject.h
> +++ b/include/qapi/qmp/qobject.h
> @@ -50,6 +50,24 @@ 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
> +
> +#ifdef CONFIG_STATIC_ASSERT
> +_Static_assert(QTYPE__MAX == 7,
> +               "The QTYPE_CAST_TO_* list needs to be extended");
> +#endif

Why not use QEMU_BUILD_BUG_ON() instead of _Static_assert?  That way,
this check will break the build even on older compilers that lack
_Static_assert but where we still know how to trigger a build failure.

> +
> +#define qobject_to(obj, type) \
> +    container_of(qobject_check_type(obj, glue(QTYPE_CAST_TO_, type)) ?: \
> +                     QOBJECT((type *)NULL), \
> +                 type, base)

Slick!

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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