Adding Victor directly

On Thu, 17 Oct 2019 at 21:21, Mateusz Pusz <mateusz.pusz@gmail.com> wrote:

Hi everyone,

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: https://github.com/mpusz/units.

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:

10 us vs 10 μs
2 kg*m/s^2 vs 2 kg⋅m/s²

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:
1. For `char`-based types (string_view, ostream) without Unicode signs
2. For Unicode char based types

Yes, with the caveat that you can only output utf-8 to sink that expects it and conversion from Unicode to anything not Unicode will loose information

However, there are a few issues here:
1. As of now, we do not have std::u8cout or even std::u8ostream. So there is really no easy way to create and use a stream for Unicode characters. So even if I implement

template<class CharT, class Traits>
friend std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>& os, const quantity& q)

correctly, we do not have an easy way to use it.

2. In order to implement the above, I could imagine such an interface for a symbol prefix:

template<typename CharT, typename Traits, typename Prefix, typename Ratio>
inline constexpr std::basic_string_view<CharT, Traits> prefix_symbol;

and its partial specializations for different prefixes/ratios:

template<typename CharT, typename Traits>
inline constexpr std::basic_string_view<char, Traits> prefix_symbol<char, Traits, si_prefix, std::micro> = "u";
template<typename CharT, typename Traits>
inline constexpr std::basic_string_view<CharT, Traits> prefix_symbol<CharT, Traits, si_prefix, std::micro> = u8"\u00b5"; // µ
template<typename CharT, typename Traits>
inline constexpr std::basic_string_view<CharT, Traits> prefix_symbol<CharT, Traits, si_prefix, std::milli> = "m";

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.

To solve the second problem fmt and chrono defined something called STATICALLY-WIDEN (http://wg21.link/time.general) 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.

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++?

I would forgo iostream and provide formatters for format.

All of that is locale specific (so the approach you describe above does not work in the general case, for example cm2 will be τ.εκ. in greek [1])

Which means icu

The documentation is sparse [2], but you can play around with some test code

https://github.com/unicode-org/icu/blob/e25796f6e545082af74f0017d55ec2d915c40a3d/icu4c/source/test/intltest/measfmttest.cpp

OSX provide something similar https://developer.apple.com/documentation/foundation/nsmeasurementformatter?language=objc

It seems easy enough for simple units

For more complicated things that are compound units for example grams per cm2, the formatting might be a bit hairy

Ideally at a high level,

std::format(u8"{}", some_unit, std::locale("el_CY"));

would do the right thing.

I am not aware of SG-16 discussing measurements yet.

It's a bigger design space than just providing u8 overloads.

The question is not to provide a "nice" representation but the representation user expect in their preferred locale.

I don't think the committee should be in the business of specifying notation.

[1] https://www.unicode.org/cldr/charts/36/summary/root.html You can explore the CLDR data to list units

[2] https://unicode-org.github.io/icu-docs/apidoc/released/icu4c/classicu_1_1MeasureFormat.html

Sorry to drop a massive curve ball on you

Regards,

Corentin

Please keep in mind that the library is hoped to target C++23.

Best

Mat
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