CP_ACP : CP_OEMCP

    // filesystem windows_file_codecvt.cpp -----------------------------------------//

     // Copyright Beman Dawes 2009

     // Distributed under the Boost Software License, Version 1.0.
     // See http://www.boost.org/LICENSE_1_0.txt

     // Library home page: http://www.boost.org/libs/filesystem

     //--------------------------------------------------------------------------------------//

     // define BOOST_FILESYSTEM_SOURCE so that <boost/system/config.hpp> knows
     // the library is being built (possibly exporting rather than importing code)
     #define BOOST_FILESYSTEM_SOURCE

     #ifndef BOOST_SYSTEM_NO_DEPRECATED
     # define BOOST_SYSTEM_NO_DEPRECATED
     #endif

     #include <boost/filesystem/config.hpp>
     #include <cwchar> // for mbstate_t

     #ifdef BOOST_WINDOWS_API

     #include "windows_file_codecvt.hpp"

     // Versions of MinGW prior to GCC 4.6 requires this
     #ifndef WINVER
     # define WINVER 0x0500
     #endif

     #include <windows.h>

     std::codecvt_base::result windows_file_codecvt::do_in(
     std::mbstate_t &,
     const char* from, const char* from_end, const char*& from_next,
     wchar_t* to, wchar_t* to_end, wchar_t*& to_next) const
     {
     UINT codepage = AreFileApisANSI() ? CP_ACP : CP_OEMCP;

     int count;
     if ((count = ::MultiByteToWideChar(codepage, MB_PRECOMPOSED, from,
     from_end - from, to, to_end - to)) == )
     {
     return error; // conversion failed
     }

     from_next = from_end;
     to_next = to + count;
     *to_next = L'\0';
     return ok;
     }

     std::codecvt_base::result windows_file_codecvt::do_out(
     std::mbstate_t &,
     const wchar_t* from, const wchar_t* from_end, const wchar_t* & from_next,
     char* to, char* to_end, char* & to_next) const
     {
     UINT codepage = AreFileApisANSI() ? CP_ACP : CP_OEMCP;

     int count;
     if ((count = ::WideCharToMultiByte(codepage, WC_NO_BEST_FIT_CHARS, from,
     from_end - from, to, to_end - to, , )) == )
     {
     return error; // conversion failed
     }

     from_next = from_end;
     to_next = to + count;
     *to_next = '\0';
     return ok;
     }

     # endif // BOOST_WINDOWS_API
 

https://github.com/chigraph/chigraph/blob/master/third_party/boost/filesystem/windows_file_codecvt.cpp