helper.hpp

#if !defined HELPER_HPP
#define      HELPER_HPP

#include <codecvt>
#include <locale>
#include <string>

namespace helper {

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

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

}

#endif

test.cc

#include <iostream>
#include "helper.hpp"

auto main() -> int {
  const std::string str("std::string!!");
  const std::wstring wstr(L"std::wstring!!");

  const std::string from_wstring = helper::to_string(wstr);
  const std::wstring from_string = helper::to_wide(str);

  std::cout << from_wstring << '\n';
  std::wcout << from_string << '\n';
}