<div dir="ltr">Hi everyone,<div><br></div><div>Right now I am in the process of designing and implementing a Physical Units library that hopefully will be a start for having such a feature in the C++ Standard Library. You can find more info on the library here: <a href="https://github.com/mpusz/units">https://github.com/mpusz/units</a>.</div><div><br></div><div>Recently, I started to work on the text output of quantities. Quantities consist of value and a unit symbol. The latter is a perfect use case for Unicode. Consider:</div><div><br></div><div><font face="monospace" color="#660000">10 us vs 10 μs</font></div><div><font face="monospace" color="#660000">2 kg*m/s^2 vs 2 kg⋅m/s²<br></font></div><div><br></div><div>Before C++20 we could get away with a hack by providing Unicode characters to `char`-based types and streams, but with the introduction of `char8_t` in C++20 it seems it will be a bigger issue from now on. The library implementors will have to provide 2 separate implementations:</div><div>1. For `char`-based types (string_view, ostream) without Unicode signs</div><div>2. For Unicode char based types</div><div><br></div><div>However, there are a few issues here:</div><div>1. As of now, we do not have <font face="monospace">std::u8cout</font> or even <font face="monospace">std::u8ostream</font>. So there is really no easy way to create and use a stream for Unicode characters. So even if I implement</div><div><br></div><div><font face="monospace" color="#660000">template<class CharT, class Traits><br>friend std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>& os, const quantity& q)<br></font></div><div><br></div><div>correctly, we do not have an easy way to use it.</div><div><br></div><div>2. In order to implement the above, I could imagine such an interface for a symbol prefix:</div><div><br></div><div><font face="monospace" color="#660000">template<typename CharT, typename Traits, typename Prefix, typename Ratio><br>inline constexpr std::basic_string_view<CharT, Traits> prefix_symbol;<br></font></div><div><br></div><div>and its partial specializations for different prefixes/ratios:</div><div><br></div><div><font color="#660000" face="monospace">template<typename CharT, typename Traits></font></div><div><font color="#660000" face="monospace">inline constexpr std::basic_string_view<char, Traits> </font><span style="color:rgb(102,0,0);font-family:monospace">prefix_symbol<</span><span style="color:rgb(102,0,0);font-family:monospace">char, Traits, </span><span style="color:rgb(102,0,0);font-family:monospace">si_prefix, std::micro> = "u";</span></div><div><span style="color:rgb(102,0,0);font-family:monospace">template<typename CharT, typename Traits></span><br></div><div><font color="#660000" face="monospace">inline constexpr std::basic_string_view<CharT, Traits> prefix_symbol<CharT, Traits, si_prefix, std::micro> = u8"\u00b5"; // µ</font></div><div><font color="#660000" face="monospace">template<typename CharT, typename Traits></font></div><div><font color="#660000" face="monospace">inline constexpr std::basic_string_view<CharT, Traits> prefix_symbol<CharT, Traits, si_prefix, std::milli> = "m";</font></div><div><br></div><div>The problem is that the above code will not compile. Specialization for all `CharT` will not be possible to be initialized with a literal like "m". Also, there is no generic mechanism to initialize all Unicode-based versions of the type with the same literal as each of them requires a different prefix (u8, u, U). Providing a specialization for every character type here is going to be a nightmare for library authors.</div><div><br></div><div>To solve the second problem fmt and chrono defined something called STATICALLY-WIDEN (<a href="http://wg21.link/time.general">http://wg21.link/time.general</a>) but it seems that it is more a specification hack rather than the implementation technique. I call it a hack as it currently addresses only `char` and `wchar_t` and does not mention
Unicode characters
at all as of now.</div><div><br></div><div>Dear SG16 members, do you have any BKMs or suggestions on how to write a library that is Unicode aware and safe in an easy and approachable way? Should we strive to provide a nice-looking representation of units for outputs that support Unicode (console, files, etc) or should we, as ever before, just support only `char` and `wchar_t` and ignore the existence of Unicode in C++?</div><div><br></div><div>Please keep in mind that the library is hoped to target C++23.</div><div><br></div><div>Best</div><div><br></div><div>Mat</div></div>