cpp-httplib/docs-src/pages/en/cookbook/c07-multipart-upload.md

title, order, status

title	order	status
C07. Upload a File as Multipart Form Data	7	draft

When you want to send a file the same way an HTML <input type="file"> does, use multipart form data (multipart/form-data). cpp-httplib offers two APIs — UploadFormDataItems and FormDataProviderItems — and you pick between them based on file size.

Send a small file

Read the file into memory first, then send it. For small files, this is the simplest path.

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

std::ifstream ifs("avatar.png", std::ios::binary);
std::string content((std::istreambuf_iterator<char>(ifs)),
                     std::istreambuf_iterator<char>());

httplib::UploadFormDataItems items = {
  {"name", "Alice", "", ""},
  {"avatar", content, "avatar.png", "image/png"},
};

auto res = cli.Post("/upload", items);

Each UploadFormData entry is {name, content, filename, content_type}. For plain text fields, leave filename and content_type empty.

Stream a large file

To avoid loading the whole file into memory, use make_file_provider(). It reads the file in chunks as it sends — so even huge files won't blow up your memory footprint.

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

httplib::UploadFormDataItems items = {
  {"name", "Alice", "", ""},
};

httplib::FormDataProviderItems provider_items = {
  httplib::make_file_provider("video", "large-video.mp4", "", "video/mp4"),
};

auto res = cli.Post("/upload", httplib::Headers{}, items, provider_items);

The arguments to make_file_provider() are (form name, file path, file name, content type). Leave the file name empty to use the file path as-is.

Note: You can mix UploadFormDataItems and FormDataProviderItems in the same request. A clean split is: text fields in UploadFormDataItems, files in FormDataProviderItems.

To show upload progress, see C11. Use the progress callback.