C++ Programming converting const char* to const wchart_t* - c++

Question

I have problems to make this little piece of code work:
#include <iostream>
#include <Windows.h>
#include <string>
using namespace std;
string Filepath;
string Temp;
int WINAPI WinMain(HINSTANCE instanceHandle, HINSTANCE, char*, int)
{
const char* env_p = std::getenv("TEMP");
std::getenv("TEMP");
Temp = env_p;
Filepath = Temp + "\\File.txt";
Editfile id(Filepath.c_str());
std::cin.get();
return 0;
}
Error C2664 Converting from "const char *" to "const wchar_t *" is not possible
I see the problem, but its not easy for me to fix it.

Answer 1

char based functions like std::getenv can return unusable data in Windows, because the standard narrow encoding in Windows is very limited.
Instead use wide character functions, and for environment stuff, preferably the Windows API itself.
Then there's no need to translate between encodings, because it's all UTF-16, represented as wchar_t based strings in C++:
#include <string>
#include <stdexcept> // runtime error
using namespace std;
#undef UNICODE
#define UNICODE
#undef NOMINMAX
#define NOMINMAX
#undef STRICT
#define STRICT
#include <windows.h>
auto hopefully( bool const e ) -> bool { return e; }
[[noreturn]] auto fail( string const& s ) -> bool { throw runtime_error( s ); }
auto path_to_tempdir()
-> wstring
{
wstring result( MAX_PATH, L'#' );
DWORD const n = GetTempPath( result.size(), &result[0] );
hopefully( 0 < n && n < result.size() )
|| fail( "GetTempPath failed" );
result.resize( n );
return result;
}
auto main()
-> int
{
// Just crash if there's no temp directory.
DWORD const as_infobox = MB_ICONINFORMATION | MB_SETFOREGROUND;
MessageBox( 0, path_to_tempdir().c_str(), L"Temp directory:", as_infobox );
}