reactos/sdk/lib/3rdparty/stlport/doc/README.utf8

  Here is a description of how you can use STLport to read/write utf8 files.
utf8 is a way of encoding wide characters. As so, management of encoding in
the C++ Standard library is handle by the codecvt locale facet which is part
of the ctype category. However utf8 only describe how encoding must be
performed, it cannot be used to classify characters so it is not enough info
to know how to generate the whole ctype category facets of a locale
instance.

In C++ it means that the following code will throw an exception to
signal that creation failed:

#include <locale>
// Will throw a std::runtime_error exception.
std::locale loc(".utf8");

For the same reason building a locale with the ctype facets based on
UTF8 is also wrong:

// Will throw a std::runtime_error exception:
std::locale loc(locale::classic(), ".utf8", std::locale::ctype);

The only solution to get a locale instance that will handle utf8 encoding
is to specifically signal that the codecvt facet should be based on utf8
encoding:

// Will succeed if there is necessary platform support.
locale loc(locale::classic(), new codecvt_byname<wchar_t, char, mbstate_t>(".utf8"));

  Once you have obtain a locale instance you can inject it in a file stream to
read/write utf8 files:

std::fstream fstr("file.utf8");
fstr.imbue(loc);

You can also access the facet directly to perform utf8 encoding/decoding operations:

typedef std::codecvt<wchar_t, char, mbstate_t> codecvt_t;
const codecvt_t& encoding = use_facet<codecvt_t>(loc);

Notes:

1. The dot ('.') is mandatory in front of utf8. This is a POSIX convention, locale
names have the following format:
language[_country[.encoding]]

Ex: 'fr_FR'
    'french'
    'ru_RU.koi8r'

2. utf8 encoding is only supported for the moment under Windows. The less common
utf7 encoding is also supported.