locale::facet::_S_create_c_locale 名称对空本地名称无效

Question

I was writing a function for printing u8string to ostream . This is what I came up with:

auto operator<<(std::ostream& out, const std::u8string_view str) -> std::ostream& {
    std::locale::global(std::locale{".utf8"});
    auto& ret = out << std::string_view{std::bit_cast<const char*>(str.data()), str.size()};
    std::locale::global(std::locale{""});
    return ret;
}

This compiles fine. But while executing, this fails saying:

terminate called after throwing an instance of 'std::runtime_error'
  what():  locale::facet::_S_create_c_locale name not valid

First I thought ".utf8" locale might not be supported on my system (that's true). But even after I comment out that line, I'm still getting the same error. So the error must have been caused by using "" .

I always thought an empty locale name sets the locale to the user's preference. If this is not happening, how should I restore the user-preferred locale after writing to the ostream ?

I am using g++ 12.2.0 Rev6, built by MSYS2 project on Windows 11 x64 .

Answer 1

std::locale::global returns the previous locale, so you can use that to restore it:

auto operator<<(std::ostream& out, const std::u8string_view str) -> std::ostream& {
    auto locale = std::locale::global(std::locale{".utf8"});
    auto& ret = out << std::string_view{std::bit_cast<const char*>(str.data()), str.size()};
    std::locale::global(locale);
    return ret;
}

But why are you messing with the locale in the first place? Writing chars to an ostream is not affected by the locale. And when you actually do something with the stream that is affected by locale, you should change the stream's locale, not the global locale:

my_stream.imbue(std::locale{"en_US.utf8"})

PS: that is not a sensible use of bit_cast , you should use reinterpret_cast .