cpp-httplib/docs-src/pages/en/cookbook/c11-progress-callback.md

title, order, status

title	order	status
C11. Use the Progress Callback	11	draft

To display download or upload progress, pass a DownloadProgress or UploadProgress callback. Both take two arguments: (current, total).

Download progress

httplib::Client cli("http://localhost:8080");

auto res = cli.Get("/large-file",
  [](size_t current, size_t total) {
    auto percent = (total > 0) ? (current * 100 / total) : 0;
    std::cout << "\rDownloading: " << percent << "% ("
              << current << "/" << total << ")" << std::flush;
    return true; // return false to abort
  });
std::cout << std::endl;

The callback fires each time data arrives. total comes from the Content-Length header — if the server doesn't send one, it may be 0. In that case, you can't compute a percentage, so just display bytes received.

Upload progress

Uploads work the same way. Pass an UploadProgress as the last argument to Post() or Put().

httplib::Client cli("http://localhost:8080");

std::string body = load_large_body();

auto res = cli.Post("/upload", body, "application/octet-stream",
  [](size_t current, size_t total) {
    auto percent = current * 100 / total;
    std::cout << "\rUploading: " << percent << "%" << std::flush;
    return true;
  });
std::cout << std::endl;

Cancel mid-transfer

Return false from the callback to abort the transfer. This is how you wire up a "Cancel" button in a UI — flip a flag, and the next progress tick stops the transfer.

std::atomic<bool> cancelled{false};

auto res = cli.Get("/large-file",
  [&](size_t current, size_t total) {
    return !cancelled.load();
  });

Note: ContentReceiver and the progress callback can be used together. When you want to stream to a file and show progress at the same time, pass both.

For a concrete example of saving to a file, see C01. Get the Response Body / Save to a File.