[cfe-dev] Puzzling clang behavior with classes in unions

[David Majnemer]

The latest version of MSVC does not reject:
Compiled with /EHsc /nologo /W4 /c
main.cpp
main.cpp(3): warning C4100: 'x': unreferenced formal parameter
main.cpp(14): warning C4100: 'argv': unreferenced formal parameter
main.cpp(14): warning C4100: 'argc': unreferenced formal parameter
main.cpp(16): warning C4101: 'list': unreferenced local variable

Compilation successful!

Total compilation time: 109ms

You are quoting the C++98 text, C++11 has a feature called "unrestricted
unions": http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2544.pdf

It looks like there is a bug here though.  We allow unrestricted unions in
C++98 mode if the original union declaration is dependent.

On Mon, Dec 15, 2014 at 1:04 AM, Florian Merz <florian.merz at kit.edu> wrote:
>
> Hi everyone,
>
> I stumbled upon a piece of code that Visual Studio rejects (and I believe
> for a good reason), but which clang and gcc both seem to accept (but should
> not, AFAIK).
>
> I tried to extract a minimal example for this:
>
> class X {
> public:
>     X(int x) {}
> };
>
> template<class Payload>
> class List {
>     union PayloadOrNext {
>         Payload payload;
>         PayloadOrNext *next;
>     };
> };
>
> int main(int argc, char **argv)
> {
>     List<X> list;
> }
>
> The standard says (in 9.5.1):
>
> "A union can have member functions (including constructors and
> destructors), but not virtual functions. A union shall not have base
> classes. A union shall not be used as a base class. An object of a class
> with a non-trivial constructor, a non-trivial copy-constructor, a
> non-trivial destructor, or a non-trivial copy assignment operator cannot be
> a member of a union, nor can an array of such objects. If a union contains
> a static data member, or a member of a reference type, the program is
> ill-formed."
>
> X is a class with a non-trivial constructor and is used as a member of the
> union PayloadOrNext (in the template class List), so I'd expect this code
> to be rejected. If I use the union directly (still as a template though) or
> if I replace the template class by a non-template version, the code is
> indeed rejected.
>
> So my question is: Why does template-with-a-union-member make a difference
> and what does the generated code actually look like?
>
> Best regards,
>  Florian Merz
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20141215/40774374/attachment.html>