Add support for std::byte to fmt::join

[hellow554]

There is the std::byte type since C++17.

It would be nice to have a built-in way to format them properly.

My current ad-hoc solution is

template<> struct fmt::formatter<std::byte> : fmt::formatter<int> {
    template<class FormatContext> auto format(const std::byte &b, FormatContext &ctx) {
        return fmt::formatter<int>::format(std::to_integer<int>(b), ctx);
    }
};

[vitaut]

std::byte is already supported:

#include <fmt/core.h>

int main() {
  fmt::print("{}", std::byte(42));
}

prints 42 (https://godbolt.org/z/MKccMT).

[hellow554]

My bad:

https://godbolt.org/z/f3oTno

#include <cstddef>
#include <vector>

#include <fmt/core.h>
#include <fmt/format.h>

int main() {
    std::vector<std::byte> vv;

    fmt::print("{}", fmt::join(vv, ","));
}

doesn't work, so this is an issue

[MikeGitb]

Could this be related to #2014?

Btw.: I'd find it much more reasonable, if std::byte got formatted as hex by default (ideally with leading zeros if necessary) - it is supposed to represent raw memory after all, not an integral number.

[hellow554]

@MikeGitb it was an example to get it working for me. You can of course use {:x}, but having that as default? Hmm, not sure about it.

[MikeGitb]

@hellow554 : That was more meant for @vitaut, referring to the result of fmt::print("{}", std::byte(42)); not about your workaournd for container<std::byte>, which of course should be consistent with formatting of a single std::byte. but I guess thats off topic for the problem at hand.

[vitaut]

Added a formatter specialization for std::byte in 4a6eadb, so it should work with join and other APIs. Thanks for reporting.

[phprus]

Without a #include <cstddef>, there will be a compilation error in GCC-7:

include/fmt/format.h:3556:20: error: ‘byte’ is not a member of ‘std’
 FMT_FORMAT_AS(std::byte, unsigned);