[Qemu-devel] Re: [PATCH 2/7] net: Add VLAN client cleanup handler

[Mark McLoughlin]

On Wed, 2009-04-15 at 16:04 +0200, Jan Kiszka wrote:
> Mark McLoughlin wrote:
>  
> >  - Rather than adding yet another param to new_vlan_client(), I just 
> >    initialize vc->cleanup after creating the client; another patch in
> >    my queue removes all callbacks to new_vlan_client() because as more
> >    are added it just gets terribly unwieldy.
> 
> Personally, I prefer a function-based API over, well, hacking the
> structures directly. The driver should not have to poke into its device
> descriptor.

I see this as similar to filing out a vtable. Perhaps it makes sense to
split the function pointers out into a separate struct.

>  Yes, this breaks existing code each time you add another
> callback, but this has the effect that you
>  a) normally think twice if you really need it before doing this,
>  b) likely enhance all users, and
>  c) that new users will find prominent information about this
>     (hopefully) useful callback.

The way I see it is that often when you're adding a new callback, you
don't want to require all users to implement it. Instead you make sure
there is a reasonable default behaviour for when the callback is NULL,
and only set it to non-NULL for the users that really need it.

Cheers,
Mark.