Convert TCHAR * -> std::wstring in both unicode and non-unicode environments

Question

I have some code in a library which has to internally work with wstring, that's all nice and fine. But it's called with a TCHAR string parameter, from both unicode and non-unicode projects, and I'm having trouble finding a neat conversion for both cases.

I see some ATL conversions and so on but can't see the right way, without defining multiple code paths using #define

Answer 1

Assuming TCHAR expands to wchar_t in Unicode builds:

inline std::wstring convert2widestr(const wchar_t* const psz)
{
  return psz;
}
inline std::wstring convert2widestr(const char* const psz)
{
  std::size_t len = std::strlen(psz);
  if( psz.empty() ) return std::wstring();
  std::vector<wchar_t> result;
  const int len = WideCharToMultiByte( CP_ACP
                                     , 0
                                     , reinterpret_cast<LPCWSTR>(psz)
                                     , static_cast<int>(len)
                                     , NULL
                                     , 0
                                     , NULL
                                     , NULL
                                     );

  result.resize( len );
  if(result.empty()) return std::wstring();
  const int cbytes = WideCharToMultiByte( CP_ACP
                                        , 0
                                        , reinterpret_cast<LPCWSTR>(psz)
                                        , static_cast<int>(len)
                                        , reinterpret_cast<LPSTR>(&result[0])
                                        , static_cast<int>(result.size())
                                        , NULL
                                        , NULL
                                        );
  assert(cbytes);
  return std::wstring( result.begin(), result.begin() + cbytes );
}

Use like this:

void f(const TCHAR* psz)
{
   std::wstring str = convert(psz);
   // ...
}