return the base of a given file path.

basepath.cpp

std::string base_path(std::string in) {
  const size_t last1 = in.rfind('/');
  const size_t last2 = in.rfind('\\');
  const size_t last = std::max(last1 == std::string::npos ? 0 : last1,
                               last2 == std::string::npos ? 0 : last2);
  in.resize(last);
  return in;
}