On Mon, Apr 16, 2018 at 02:30:40PM +0200, Erik Skultety wrote:
> On Fri, Apr 13, 2018 at 04:47:15PM +0200, Michal Privoznik wrote:
> > Our virObject code relies heavily on the fact that the first
> > member of the class struct is type of virObject (or some
> > derivation of if). Let's check for that.
>
> If a class is missing 'parent' memeber, it's a bug in the definition of the
> struct/class, therefore there should be a static assertion rather than a
> runtime check.
Agreed, my suggestion was for a static assert too.
>
> >
> > Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx>
> > ---
> > src/util/virobject.c | 31 +++++++++++++++++++++----------
> > src/util/virobject.h | 5 ++++-
> > 2 files changed, 25 insertions(+), 11 deletions(-)
> >
> > diff --git a/src/util/virobject.c b/src/util/virobject.c
> > index c5a98d21cc..e184f5349e 100644
> > --- a/src/util/virobject.c
> > +++ b/src/util/virobject.c
> > @@ -77,6 +77,7 @@ virObjectOnceInit(void)
> > {
> > if (!(virObjectClass = virClassNew(NULL,
> > "virObject",
> > + 0,
> > sizeof(virObject),
> > NULL)))
>
> Also, I don't like this extra parameter, which really shouldn't be needed; you
> created a macro which hides this parameter, but that doesn't mean that
> design-wise it makes sense to have it there, think of it as a constructor, you
> don't pass a constructor an offset of the class' member, because it shouldn't
> have need for it, but you do, solely for the purpose of checking whether we have
> a particular member in place.
> So, to start a discussion about this (I also think Dan posted something related
> to this recently, but I don't seem to be able to find it in the archives - do I
> even archive?!!!), I came up with my first compile-time hack ever, it seems to
> work like expected, but I'd like to hear your opinions both the macro itself
> and the approach we're going to take, so here's my replacement patch:
https://www.redhat.com/archives/libvir-list/2018-April/msg00984.html
I had suggested something in the virObjectNew function:
#define virObjectNew(typ) \
(typ *)(&((typ *)virObjectNewImpl(typ # Class)).parent)
catching it with virClassNew works fine too, as it would be a compile
time check too
> diff --git a/src/util/virobject.h b/src/util/virobject.h
> index 92dd51239..2a973d401 100644
> --- a/src/util/virobject.h
> +++ b/src/util/virobject.h
> @@ -75,8 +75,12 @@ virClassPtr virClassForObjectRWLockable(void);
> # define VIR_PARENT_REQUIRED ATTRIBUTE_NONNULL(1)
> # endif
>
> +# define VIR_CLASS_HAS_PARENT(name) \
> + !sizeof(char[0-offsetof(name, parent)])
> +
> # define VIR_CLASS_NEW(prnt, name) \
> - virClassNew(prnt, #name, sizeof(name), name##Dispose)
> + VIR_CLASS_HAS_PARENT(name) ? \
> + virClassNew(prnt, #name, sizeof(name), name##Dispose) : NULL
So we're relying on the fact the the ': NULL" will never execute
because VIR_CLASS_HAS_PARENT will trigger a compile time error.
>
> Notes:
> - I suppose mingw would handle this hack the same way it handles
> VIR_TYPEMATCH, IOW it works...
>
> - it also doesn't need to be a ternary, I suggested extending VIR_CLASS_NEW to
> do the complete assignment in [Patch 7/9], like this:
>
> # define VIR_CLASS_NEW(prnt, name) \
> if (!(name##Class) = virClassNew(prnt, #name, sizeof(name), name##Dispose))
> return -1;
This has the added benefit of enforcing class variable naming scheme
which removes another source of developer error, and is in keeping
with VIR_ALLOC() style.
Regards,
Daniel
--
|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org -o- https://fstop138.berrange.com :|
|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
--
libvir-list mailing list
libvir-list@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/libvir-list
