int main()
{
  string line;
  ifstream ifs;
  string fname;

  cout << "Address file name: ";
  cin >> fname;
  ifs.open(fname.c_str());
  if (!ifs.is_open()) {
    cout << "Unable to open input file '" << fname << "'" << endl;
    exit(0);
  }
  // Now process the file
  ...
}