https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122
--- Comment #21 from Paul A. Bristow <pbristow at hetp dot u-net.com> --- I also note that https://en.cppreference.com/w/cpp/io/manip/fixed Notes Hexadecimal floating-point formatting ignores the stream precision specification, as required by the specification of std::num_put::do_put. https://en.cppreference.com/w/cpp/locale/num_put/put Additionally, if floatfield != (ios_base::fixed | ios_base::scientific), then (since C++11) precision modifier is added, set to str.precision() "When formatting a floating point value as hexfloat (i.e., when floatfield == (std::ios_base::fixed | std::ios_base::scientific)), the stream's precision is not used; instead, the number is always printed with enough precision to exactly represent the value." (since C++11) but this is not what MSVC currently 16.7.0.Preview 3 is implementing as noted https://developercommunity.visualstudio.com/content/problem/520472/hexfloat-stream-output-does-not-ignore-precision-a.html *It would be extremely unfortunate if various compilers implemented this differently.* I see no case for precision to change the output from std::hexfloat because it specifies a *decimal* precision, not a hexadecimal one. std::hexfloat output should always be enough precision to *exactly represent the value*, however large. It would be nice if std::showpoint could also respect output of trailing zeros, so that the default std::showpoint output displays neat columns, and that std::noshowpoint made output with minimum size (for example, minimizing serialized data). (In passing, I note that std::scientific always includes (often many) trailing zeros, ignoring showpoint. Changing this would be logical but a breaking change.)
