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References: <20180321134005.8822-1-marcandre.lureau@redhat.com>
	<20180321134005.8822-3-marcandre.lureau@redhat.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <3ec274aa-0dea-0932-e560-f388c1cbac41@redhat.com>
Date: Wed, 21 Mar 2018 15:01:39 +0100
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Subject: Re: [Qemu-devel] [PATCH 2/3] qobject: assume base of a qobject is
 at offset 0
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To: =?UTF-8?Q?Marc-Andr=c3=a9_Lureau?= <marcandre.lureau@redhat.com>, qemu-devel@nongnu.org
Cc: berrange@redhat.com, eblake@redhat.com, armbru@redhat.com

On 21/03/2018 14:40, Marc-Andr=C3=A9 Lureau wrote:
> +/* A typecast, checking for the type of arguments */
> +/* QObject is at offset 0, for all QObject-derived types */
> +#define QOBJECT(x) QEMU_GENERIC(x,              \
> +    (QNull *, (QObject *) x),                   \
> +    (const QNull *, (const QObject *) x),       \
> +    (QNum *, (QObject *) x),                    \
> +    (const QNum *, (const QObject *) x),        \
> +    (QString *, (QObject *) x),                 \
> +    (const QString *, (const QObject *) x),     \
> +    (QDict *, (QObject *) x),                   \
> +    (const QDict *, (const QObject *) x),       \
> +    (QList *, (QObject *) x),                   \
> +    (const QList *, (const QObject *) x),       \
> +    (QBool *, (QObject *) x),                   \
> +    (const QBool *, (const QObject *) x),       \
> +    (QObject *, x),                             \
> +    (const QObject *, x),                       \
> +    qobject_unknown_type(x))

Why not just

	QEMU_GENERIC(x,
		(QObject *, x),
		(const QObject *, x),
		({ \
			QEMU_BUILD_BUG_ON(offsetof(typeof(*x), base));
			&(x)->base;
		}))

That is just an extension of what was being done before, and it is
resilient against people putting a random "QObject base" in the middle
of a struct.

Paolo