On 01/21/2013 11:05 AM, Peter Maydell wrote: > On 21 January 2013 17:56, Eric Blake <ebl...@redhat.com> wrote: >> On 01/21/2013 08:48 AM, Peter Maydell wrote: >>> On 14 January 2013 23:08, <fred.kon...@greensocs.com> wrote: >>>> +#define TYPE_VIRTIO_BUS "virtio-bus" >>>> +#define VIRTIO_BUS_GET_CLASS(obj) \ >>>> + OBJECT_GET_CLASS(VirtioBusClass, obj, TYPE_VIRTIO_BUS) >>>> +#define VIRTIO_BUS_CLASS(klass) \ >>>> + OBJECT_CLASS_CHECK(VirtioBusClass, klass, TYPE_VIRTIO_BUS) >>> >>> 'obj' and 'klass' need brackets round them, because they're >>> macro arguments. >> >> Brackets are only necessary if the C parser could misinterpret things >> without the brackets; but in this particular case, there is no ambiguity >> (unless OBJECT_GET_CLASS() and OBJECT_CLASS_CHECK() are improperly >> under-parenthesized). > > Yes, but "we can get away without parens here because we know > that other macro over there has parens in it" is unnecessarily > tricky in my opinion.
The rule of thumb for when to use parentheses in macros should generally be: can the caller ever pass a series of tokens which would cause a different parse than a single token; likewise, can the macro expand to more than one token where the expansion no longer behaves like a single function call. If so, then we are burdening the caller with the need to use parenthesis, and the macro is a potential bug. If not, then the parenthesis are redundant. For something like '#define foo(x) x++', we can demonstrate that passing *a' leads to '*a++', which has a different meaning than '(*a)++'. Thus, that macro has a bug unless written '#define foo(x) ((x)++)'. But for argument lists, there is nothing the user can pass that can cause the parser to get confused on the end of the argument list; even if the user passes more than one token to the parameter 'obj', there is no way that VIRTIO_BUS_GET_CLASS(*function_to_give_obj()) expanding to OBJECT_GET_CLASS(VirtioBusClass, *function_to_give_obj(), TYPE_VIRTIO_BUS) can be parsed differently than OBJECT_GET_CLASS(VirtioBusClass, (*function_to_give_obj()), TYPE_VIRTIO_BUS), so the extra parentheses would be redundant in the definition of VIRTIO_BUS_GET_CLASS. But if you insist on a rule of thumb that all macro arguments are always parenthesized, even when not strictly necessary, because it makes for less thinking when writing the macro, I'm not going to complain. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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