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Date: Fri, 19 Mar 2010 01:41:59 +0000
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: [PATCH 0/9] Virtio cleanups
Message-ID: <20100319014159.GB14108@shareable.org>
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To: Juan Quintela <quintela@redhat.com>
Cc: qemu-devel@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>

Juan Quintela wrote:
> vstrucut virtio_common *create_virtio_comon(...., size we really want);
> Again, this implements superclass/subclass as well as you can implemnt
> it in C.

It would be more type-safe for create_virtio_common() to be a macro
taking the subclass *type* rather than sizeof.

And it would make the calls short: No need to cast the result, because
the macro would return the desired type (doing the cast itself).

#define create_virtio_common(...., type) \
   ((type *)_create_virtio_common(...., sizeof(type)))

Once you have that, it's easy to change to add a field name and
container_of:

#define create_virtio_common(...., type, field) \
   (container_of(_create_virtio_common(...., sizeof(type)), type, field))

That gives you malloc in common init, and type-safe callers everywhere
(no possibility for mistaken sizeof).

I think it's a simpler to use API and better at protecting against
caller mistakes; you may disagree.

-- Jamie