2011/6/9 Nico Weber <[email protected]> > On Wed, Jun 1, 2011 at 10:26 AM, Matthieu Monrocq > <[email protected]> wrote: > > > > > > 2011/5/30 Eli Friedman <[email protected]> > >> > >> On Mon, May 30, 2011 at 2:40 PM, Argyrios Kyrtzidis <[email protected]> > >> 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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