Hi Jonathan,

> Is the pseudocode above the intention?

Yes, it looks about right to me. {fmt} does something simpler because it only relies on (limited) standard facilities. For this reason it implements a subset of functionality (UTF conversions) which might also be conforming considering "an implementation-defined set of locales" but obviously less useful.

HTH,

Victor

On Wed, Jan 10, 2024 at 3:25 PM Jonathan Wakely <cxx@kayari.org> wrote:

What's the intended implementation strategy for https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2419r2.html on POSIX?

My best guess is something like this, where loc is the formatting locale ([time.format] p2):

if (narrow literal encoding is UTF-8)

  if (locale_t cloc = ::newlocale(loc.name()))

    if (const char* enc = ::nl_langinfo_l(CODESET, cloc))

      if (/*enc is not UTF-8 */) {

        iconv_t ic = ::iconv_open("UTF-8", enc);

        if (ic != (iconv_t)-1) {

          // use ::iconv to convert from locale's encoding to UTF-8

But that seems pretty involved ... and {fmt} doesn't do any of that.

I tried testing the example from the paper on Linux, and fmt::format fails with an exception. Debugging it shows that it tries to use the formatting locale's std::codecvt<char32_t, char, mbstate_t> facet to convert the string. But that's not right, because that codecvt specialization is defined by the standard to convert between UTF-8 and UTF-32 only. So it can only work if the input is ASCII, or the locale uses UTF-8, in which case there's nothing that needs converting anyway.

AFAIK the standard doesn't provide a way to convert from an arbitrary locale's encoding to the execution charset, or even to get the name of an arbitrary locale's encoding (C++23 provides a way to get the name of the execution environment's encoding, but not an arbitrary std::locale's encoding).

Is the pseudocode above the intention? Or am I misinterpreting something in P2419?

I've read the SG16 minutes when P2419 was discussed, and I don't see an answer.