[-Wdelete-non-virtual-dtor] fires for protected destructor in base class

[BenPope]

Bugzilla Link	12246
Version	3.0
OS	Linux
CC	@BenPope,@DougGregor,@efriedma-quic

Extended Description

This is also applicable to trunk as of r152547

struct Base { virtual void func() {} protected: ~Base() {} };

struct Derived : Base {};

int main() { Derived * foo = new Derived; delete foo; }

clang++ -Wall file.cpp

If Base::~Base were public, (but nobody deletes through a base) the warning is desirable (with a suggestion to make it protected). If foo is of static type Base and Base::~Base is protected, an error is correctly emitted. If foo is of static type Base* and Base::~Base is public, the warning is correctly emitted.

This appears to be a new warning; in 2.9 it doesn't exist, I suspect -Wnon-virtual-dtor to be related but it wasn't part of -Wall in 2.9.

Motivation: http://www.gotw.ca/publications/mill18.htm Guideline #​4

Additional motivation: g++ 4.6.2 gets this right (and I'm led to believe since 4.3) ;)

[efriedma-quic]

I'm pretty sure you just end up with different false positives with that approach... but please send an email to cfe-commits if you're looking for more people to comment.

[BenPope]

Thank you, I did do some searching but clearly in the wrong places.

Now I see that without dynamic analysis this is less tractable.

The solution is to take note of Herb Sutter (Declare your destructor public and virtual or protected and non-virtual) in which case Derived would have a protected destructor (which is close enough to abstract for our purposes) and take note of Scott Meyers (item 33 of More Effective C++, "Make non-leaf classes abstract."); to ensure the latter with simple static analysis we need to insist that Derived is a leaf class by using final as you suggested.

I wonder, lets think about it another way, can you see any problems with a warning that looks like this:

struct Base { virtual void func() {} protected: ~Base() {} };

struct Derived : Base {};

struct FurtherDerived : Derived {};

clang++ TestSample.cpp -Wderive-public-non-virtual-dtor

TestSample.cpp:9:1: warning: class 'FurtherDerived' is being derived from a class that has virtual functions but public non-virtual destructor [-Wderive-public-non-virtual-dtor] struct FurtherDerived : Derived {}; ^

If you see no major gotchas in that approach, I can take a look at producing a patch (it sounds like fun).

[efriedma-quic]

Yes, this issue was brought up when we initially added the warning; see http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20110530/042332.html . You can eliminate the warning in your code by making Derived a final class, as in "struct Derived final : Base {};".