Re: [Qemu-devel] [PATCH v2 2/3] qobject: introduce QObjectCommon

[Eric Blake]

On 03/28/2018 08:07 AM, Marc-André Lureau wrote:
> By moving the common fields to a QObjectCommon, QObject can be a type
> which also has a 'base' QObjectCommon field. This allows to write a
> generic QOBJECT() macro that will work with any QObject type,
> including QObject itself. The container_of() macro ensures that the
> object to cast has a QObjectCommon base field, give me some type
> safety guarantees. However, for it to work properly, all QObject types
> must have 'base' at offset 0 (which is ensured by static checking from
> previous patch)
>
> Signed-off-by: Marc-André Lureau <address@hidden>
> ---

> +++ b/include/qapi/qmp/qbool.h
> @@ -17,7 +17,7 @@
>   #include "qapi/qmp/qobject.h"
>   
>   struct QBool {
> -    QObject base;
> +    struct QObjectCommon base;

Why no 'typedef struct QObjectCommon QObjectCommon' in scope to make 
this simpler?

> -/* Get the 'base' part of an object */
> -#define QOBJECT(obj) (&(obj)->base)
> +struct QObject {
> +    struct QObjectCommon base;
> +};
> +
> +#define QOBJECT(x) \
> +   container_of(&(x)->base, QObject, base)

If I understand correctly, this still causes clang complaints when 
called as QOBJECT(NULL).  As long as we are touching this, should we 
improve this macro to be friendly to NULL conversion?

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