[cfe-commits] r131989 - in /cfe/trunk: include/clang/Basic/DiagnosticGroups.td include/clang/Basic/DiagnosticSemaKinds.td lib/Sema/SemaExprCXX.cpp test/SemaCXX/destructor.cpp

[Matthieu Monrocq]

2011/6/9 Nico Weber <thakis at chromium.org>

> On Wed, Jun 1, 2011 at 10:26 AM, Matthieu Monrocq
> <matthieu.monrocq at gmail.com> wrote:
> >
> >
> > 2011/5/30 Eli Friedman <eli.friedman at gmail.com>
> >>
> >> On Mon, May 30, 2011 at 2:40 PM, Argyrios Kyrtzidis <akyrtzi at gmail.com>
> >> wrote:
> >> > On May 30, 2011, at 2:36 PM, Argyrios Kyrtzidis wrote:
> >> >
> >> > Hi Nico,
> >> > On May 30, 2011, at 12:19 PM, Nico Weber wrote:
> >> >
> >> > Hi Argyrios and Matthieu,
> >> > this warning found a few problems in chromium – thanks! However, it
> also
> >> > finds a few false positives, so I don't think I can turn it on by
> >> > default,
> >> > which is a bummer.
> >> > All of the false positives are of this form:
> >> > class SomeInterface {
> >> >  public:
> >> >   virtual void interfaceMethod() {}  // or = 0;
> >> >  protected:
> >> >   ~SomeInterface() {}
> >> > }
> >> > class WorkerClass : public SomeInterface {
> >> >  public:
> >> >   // many non-virtual functions, but also:
> >> >   virtual void interfaceMethod() override { /* do actual work */ }
> >> > };
> >> > void f() {
> >> >   scoped_ptr<WorkerClass> c(new WorkerClass);  // simplified example
> >> > }
> >> > This is a somewhat standard pattern (see
> >> > e.g. http://www.gotw.ca/publications/mill18.htm, "Virtual Question
> #2").
> >> > Do you have any good suggestions how to deal with this case?
> >> >
> >> > If WorkerClass gets subclassed in the future, deletion in "f()" will
> be
> >> > undefined behavior. Ideally WorkerClass should be marked "final" and
> >> > then
> >> > there will also be no warning; does this sound reasonable ?
> >> >
> >> > Um, to be exact, undefined behavior if you delete a WorkerClass *
> >> > pointer.
> >>
> >> Sure, except that there isn't any way to do that outside of C++0x mode.
> :)
> >>
> >> -Eli
> >
> > Hi Nico,
> >
> > first of all, thanks for field-testing the warning on the chromium code
> > base, and I am glad it helped you spot some bugs :)
> >
> > I've tried to make the warning as tight as possible, to prune as much
> > false-positives as possible however there is one difficulty: I only
> reason
> > about the static type. I do not know if Clang has information about the
> > dynamic type in the AST. In general it's a hard problem (and requires
> > Inter-Procedural Analysis) and out of reach for a "simple" warning. It
> could
> > perhaps be added to the Static Analyzer.
> >
> > If you can, I would suggest using the "final" attribute:   class
> WorkerClass
> > final: public SomeInterface
> > (this could be activated only on the Clang build)
> >
> > If you cannot, unfortunately I cannot see anything (yet) to prune out
> this
> > case. Perhaps that someone will have a genial idea ?
>
> Follow-up: We decided that we don't want to add "final" to chromium
> code, with the argument that it would be used only in comparatively
> few cases, so most people wouldn't know "final" exists. And we felt
> C++ is complicated enough as is already.
>
> Instead, we're just making the destructor of WorkerClass virtual.
> While that's not needed, it makes clang happy. I did this for all
> files we build with -Werror (most notably, this includes all chromium
> code and all webkit code) and updated our builders to a clang version
> that has this warning enabled. I'll keep an eye on how often it
> complains about useful things and how often it doesn't.
>
> In my attempt to enable this warning, I fixed 1 real bug (a destructor
> should've been virtual but wasn't, and the subclass destructor did
> real work), 5 latent bugs (a destructor that should've been virtual
> but wasn't, but the subclass didn't have a destructor and no non-POD
> members), and added "virtual" to 47 destructors just to make the
> warning happy. That's a signal-noise ratio of 12%. I will keep an eye
> on how this warning does in practice (i.e. when it turns our build
> red, was it for good or bogus reasons?).
>
> For reference, when I initially turned on -Woverride-virtual, I fixed
> 18 bugs (methods that once were overrides but where the overrides
> silently broke due to the superclass changing), and renamed 25 methods
> that triggered the warning but weren't actually buggy (but still
> confusing, so these changes were still useful). This is a signal-noise
> ratio of 72%. That seemed low to me at the time, but the warning has
> been proven extremely useful in practice.
>
> I hope this is useful feedback :-)
>
> Nico
>
> ps: Tracking bug for -Wdelete-non-virtual-dtor was
> http://crbug.com/84424 , -Woverride-virtual was http://crbug.com/72205
>

Thank you very much!

It is very useful indeed, based on your experience I guess we will not
activate this warning by default any time soon. "final" would probably have
allowed a number of optimizations too (devirtualizations of functions calls)
but I can understand the reluctance to introduce supplementary keywords in
an already convoluted language :)

-- Matthieu
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