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#include <stdio.h>
#include <locale>
#include <codecvt>

const std::string ws2s( const std::wstring& src )
{
 std::locale sys_locale("");

 const wchar_t* data_from = src.c_str();
 const wchar_t* data_from_end = src.c_str() + src.size();
 const wchar_t* data_from_next = 0;

 int wchar_size = 4;
 char* data_to = new char[(src.size() + 1) * wchar_size];
 char* data_to_end = data_to + (src.size() + 1) * wchar_size;
 char* data_to_next = 0;

 memset( data_to, 0, (src.size() + 1) * wchar_size );

 typedef std::codecvt<wchar_t, char, mbstate_t> convert_facet;
 mbstate_t out_state = 0;
 auto result = std::use_facet<convert_facet>(sys_locale).out(
  out_state, data_from, data_from_end, data_from_next,
  data_to, data_to_end, data_to_next );
 if( result == convert_facet::ok )
 {
  std::string dst = data_to;
  delete[] data_to;
  return dst;
 }
 else
 {
  printf( "convert error!\n" );
  delete[] data_to;
  return std::string("");
 }
}

const std::wstring s2ws( const std::string& src )
{
 std::locale sys_locale("");
  
 const char* data_from = src.c_str();
 const char* data_from_end = src.c_str() + src.size();
 const char* data_from_next = 0;

 wchar_t* data_to = new wchar_t[src.size() + 1];
 wchar_t* data_to_end = data_to + src.size() + 1;
 wchar_t* data_to_next = 0;

 wmemset( data_to, 0, src.size() + 1 );

 typedef std::codecvt<wchar_t, char, mbstate_t> convert_facet;
 mbstate_t in_state = 0;
 auto result = std::use_facet<convert_facet>(sys_locale).in(
  in_state, data_from, data_from_end, data_from_next,
  data_to, data_to_end, data_to_next );
 if( result == convert_facet::ok )
 {
  std::wstring dst = data_to;
  delete[] data_to;
  return dst;
 }
 else
 {
  printf( "convert error!\n" );
  delete[] data_to;
  return std::wstring(L"");
 }
}

const std::string ws2utf8( const std::wstring& src )
{
 std::wstring_convert<std::codecvt_utf8<wchar_t>> conv;
 return conv.to_bytes( src );
}

const std::wstring utf8_2_ws( const std::string& src )
{
 std::wstring_convert<std::codecvt_utf8<wchar_t> > conv;
 return conv.from_bytes( src );
}

http://it.cppreference.com/w/cpp/locale/wstring_convert

Vedi anche

Character conversions	narrow multibyte (char)	UTF-8 (char)	UTF-16 (char16_t)
UTF-16	mbrtoc16 / c16rtomb	codecvt<char16_t, char, mbstate_t> codecvt_utf8_utf16<char16_t> codecvt_utf8_utf16<char32_t> codecvt_utf8_utf16<wchar_t>	N/A
UCS2	No	codecvt_utf8<char16_t>	codecvt_utf16<char16_t>
UTF-32/UCS4 (char32_t)	mbrtoc32 / c32rtomb	codecvt<char32_t, char, mbstate_t> codecvt_utf8<char32_t>	codecvt_utf16<char32_t>
UCS2/UCS4 (wchar_t)	No	codecvt_utf8<wchar_t>	codecvt_utf16<wchar_t>
wide (wchar_t)	codecvt<wchar_t, char, mbstate_t> mbsrtowcs / wcsrtombs	No	No