Use overridden locale in ostream

[dlaugt]

I'm wondering if the overridden locale can be passed to ostream. Here is a code with the current behavior:

struct S
{
  string a;
  int b;
};

ostream& operator<<(ostream& os, const S& s)
{ return os << "{" << s.a << ": " << s.b << "}"; }

locale::global (locale ("fr-FR"));
format (locale("en-US"), "{} {:n} {}", 1234, 1234, S {"test", 1234});
// gives 1234 1,234 {test: 1 234}

For known types (including chrono), format is using C locale or the overridden locale. In this example, I don't see the interest to display the result of ostream with the global locale...

I can override the current behavior by implementing the following for each of my own classes:

namespace fmt
{
  template <>
  struct formatter<S> : formatter<basic_string_view<char>, char>
  {
    using base_type = formatter<basic_string_view<char>, char>;

    auto format (const S& value, format_context& ctx)
    {
      basic_memory_buffer<char> buffer;
      internal::formatbuf<char> format_buffer (buffer);
      std::basic_ostream<char> out (&format_buffer);
      out.imbue (ctx.locale ().get<std::locale> ());
      out.exceptions (std::ios_base::failbit | std::ios_base::badbit);
      out << value;

      buffer.resize (buffer.size ());
      basic_string_view<char> str (buffer.data (), buffer.size ());
      return base_type::format (str, ctx);
    }
  };
}

But I'm wondering if this can be done as standard for ostream.

[vitaut]

Thanks for the PR. Mostly looks good but let's get rid of the duplicate format_value overload.

[vitaut]

Please remove this and make loc default to no locale in the format_value defined in ostream.h.

[dlaugt]

I've committed the change for having only one format_value. In my benchmark under msvc 2019, constructing a copy of the global locale has a 9% impact on performance results. I've added an operator bool in locale_ref to avoid this.

[vitaut]

Good call.