Function usage

gcs-fast-transfers/download.cc:157–178

char const* kPositional[] = {"bucket", "object", "destination"};

[[noreturn]] void usage(std::string const& argv0,
                        po::options_description const& desc,
                        std::string const& message = {}) {
  auto exit_status = EXIT_SUCCESS;
  if (not message.empty()) {
    exit_status = EXIT_FAILURE;
    std::cout << "Error: " << message << "\n";
  }

  // format positional args
  auto const positional_names =
      std::accumulate(std::begin(kPositional), std::end(kPositional),
                      std::string{" [options]"}, [](auto a, auto const& b) {
                        a += ' ';
                        a += b;
                        return a;
                      });

  // print usage + options help, and exit normally
  std::cout << "usage: " << argv0 << positional_names << "\n\n" << desc << "\n";
  std::exit(exit_status);
}

po::variables_map parse_command_line(int argc, char* argv[]) {
  auto const default_minimum_slice_size = 64 * 1024 * 1024L;