[Bug libstdc++/119880] fwide must be called before and after std::wcout to change wide orientation to allow std::cout works

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119880

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think using fwide that way leads to undefined behaviour though. The C
standard says:

"Byte input/output functions shall not be applied to a wide-oriented stream and
wide character input/output functions shall not be applied to a byte-oriented
stream."

So once the stdout stream is byte-oriented, using std::wcout is undefined. And
once it's wide-oriented, using std::cout is undefined.

POSIX is clear:

"If the orientation of the stream has already been determined, fwide() shall
not change it."