I don’t have the reference on hand, but I’m pretty sure writing to one union member and reading the value back through another is undefined behaviour. I believe this is inherited from C99 or earlier.
> On 19 Mar 2018, at 12:00, via cfe-users <cfe-users@lists.llvm.org> wrote: > > Send cfe-users mailing list submissions to > cfe-users@lists.llvm.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users > or, via email, send a message with subject or body 'help' to > cfe-users-requ...@lists.llvm.org > > You can reach the person managing the list at > cfe-users-ow...@lists.llvm.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of cfe-users digest..." > > > Today's Topics: > > 1. Undefined behaviour in a small code snippet > (Łukasz Szczur via cfe-users) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 13 Mar 2018 01:14:51 +0100 > From: Łukasz Szczur via cfe-users <cfe-users@lists.llvm.org> > To: cfe-users@lists.llvm.org > Subject: [cfe-users] Undefined behaviour in a small code snippet > Message-ID: > <cadw1asr2he1c2yd+v8be5yysgfmfi7jzcoszw2h_vym2ebn...@mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > Hello, > > The program at the bottom prints > 1 > 2 > 1 > 2 > > Does the code contain undefined behaviour in C++17 (note that the structs > have common initial sequence )? > > Łukasz > > > #include<iostream> > > using namespace std; > > struct A { > int first; > int second; > }; > > struct B { > int first; > int second; > }; > > struct Test { > union { > A a; > B b; > }; > }; > > int main() { > Test t; > t.a.first = 1; > t.b.second = 2; > cout << t.a.first << '\n'; > cout << t.a.second << '\n'; > cout << t.b.first << '\n'; > cout << t.b.second << '\n'; > return 0; > } > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://lists.llvm.org/pipermail/cfe-users/attachments/20180313/89f47154/attachment-0001.html> > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > cfe-users mailing list > cfe-users@lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users > > > ------------------------------ > > End of cfe-users Digest, Vol 62, Issue 9 > **************************************** _______________________________________________ cfe-users mailing list cfe-users@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
