std::variant<string, int, bool> v;
  ...
  if (auto pstr = std::get_if<string>(&v)) {
    cout << *pstr << endl;
  }
  else if (auto pint = std::get_if<int>(&v)) {
    cout << *pint << endl;
  }
  else if (auto pbool = std::get_if<bool>(&v)) {
    cout << *pbool << endl;
  }
  else {
    count << "null" << endl;
  }