[cfe-dev] Warning when calling virtual functions from constructor/desctructor?

Nico Weber via cfe-dev cfe-dev at lists.llvm.org
Thu Dec 13 07:16:56 PST 2018

IIRC we tried this and it had a very high false positive rate. I might be
misremembering though.

Note that -Wextra generally tries to be empty, and for -Wall we still don't
want warnings with lots of false positives.

On Thu, Dec 13, 2018 at 10:08 AM Arnaud Bienner via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi all :)
> I noticed that clang doesn't emit a warning in this case.
> It does when calling a *pure* virtual function though (a warning for which
> I added a diagnostic group recently [1]).
> I think one difference is that calling pure virtual function from
> constructor is undefined, while calling virtual function is defined (it
> calls the base class function since the vtable isn't filled yet with
> inherited virtual functions reimplementations).
> So implementing a warning like this will likely trigger false positives.
> However, I think in most cases it denotes a bug, since many IMHO C++
> developers won't get that the function will not behave as they might expect.
> Also, we can have a fixit to this, by suggesting the user to to specify
> the class of the function e.g. write "B::f()" instead of "f()".
> I'm fine (trying to) write a patch for this, but first I wanted to get
> opinions about whether it's worth it.
> I guess it might not be that much difficult to do for simple cases (when
> calling virtual function directly). I expect it might be more difficult to
> catch cases where we call a function that is not virtual, but which calls a
> virtual function behind the scene.
> Also, if you're worried about having too many false positives, I think we
> could put this new warning behind Wextra.
> What do you think?
> Best regards,
> Arnaud
> [1]:
> https://github.com/llvm-mirror/clang/commit/528a1d17a3ff8361fdd4a1a379adf3f64ec68e00
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