How do I convert wchar_t* to std::string?

std::wstring ws( args.OptionArg() );
std::string test( ws.begin(), ws.end() );

You can convert a wide char string to an ASCII string using the following function:

#include <locale>
#include <sstream>
#include <string>

std::string ToNarrow( const wchar_t *s, char dfault = '?', 
                      const std::locale& loc = std::locale() )
{
  std::ostringstream stm;

  while( *s != L'\0' ) {
    stm << std::use_facet< std::ctype<wchar_t> >( loc ).narrow( *s++, dfault );
  }
  return stm.str();
}

Be aware that this will just replace any wide character for which an equivalent ASCII character doesn't exist with the dfault parameter; it doesn't convert from UTF-16 to UTF-8. If you want to convert to UTF-8 use a library such as ICU.

You could just use wstring and keep everything in Unicode