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)) == 0)
     {
     return error; // conversion failed
     }
      
     from_next = from_end;
     to_next = to + count;
     *to_next = L'';
     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, 0, 0)) == 0)
     {
     return error; // conversion failed
     }
      
     from_next = from_end;
     to_next = to + count;
     *to_next = '';
     return ok;
     }
      
     # endif // BOOST_WINDOWS_API
 

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