http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53596
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-07 08:46:09 UTC --- (In reply to comment #3) > Hi, thanks for your answer but I still think it is a bug. Here are my reasons: > > 1) You mention that Clang also warns for my example but this seems to be a bug > in Clang, see > http://lists.cs.uiuc.edu/pipermail/llvmbugs/2012-March/022451.html I expect that bug will get closed, Clang behaves as designed. > 2) Herb Sutter wrote a Guru of the Week (GotW) article on this topic > (http://www.gotw.ca/publications/mill18.htm), it says: > > Guideline #4: A base class destructor should be either public and virtual, or > protected and nonvirtual. The compiler cannot know Derived is not a base class (unless you mark it "final" to prevent derivation, or "__final" as a GCC extension in C++03). You have virtual functions, so it's reasonable to assume you plan to derive from it. > 3) If I modify 'virtual void foo()' to 'void foo()' in my previous example the > warning vanishes, this makes no sense. Then the class isn't polymorphic, so the compiler assumes you don't plan to derive from it. > 4) In your MoreDerived example you can make the Derived dtor virtual, this way > the protected non-virtual Base destructor is absolutely valid. Obviously. That fixes the undefined behaviour, so there's no warning, that's the change the compiler is suggesting you make! (In reply to comment #4) > I found another post on the web > (http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20110530/042333.html) > that explains the warning, it says: > > "If Derived (see above) gets subclassed in the future, deletion in "main()" > will be undefined behavior. Ideally Derived should be marked "final" and then > there will also be no warning" That's exactly what I demonstrated in my MoreDerived example. > Based on this explanation I think the warning is valid. Clang's warning > message > is somewhat more clear: > > "warning: delete called on 'Derived' that has virtual functions but > non-virtual > destructor [-Wdelete-non-virtual-dtor]" A polymorphic class is one with virtual functions. That's standard terminology. I prefer the GCC message, it tells you using delete might (or will) be undefined behaviour, instead of just warning about using delete. > This also explains why there is no warning in test2.cpp of my previous post. Exactly.
