cpp-httplib v0.42.0
DRAFT

S16. Detect When the Client Has Disconnected

During a long-running response, the client might close the connection. There's no point continuing to do work no one's waiting for. In cpp-httplib, check req.is_connection_closed().

Basic usage

svr.Get("/long-task", [](const httplib::Request &req, httplib::Response &res) {
  for (int i = 0; i < 1000; ++i) {
    if (req.is_connection_closed()) {
      std::cout << "client disconnected" << std::endl;
      return;
    }

    do_heavy_work(i);
  }

  res.set_content("done", "text/plain");
});

svr.Get("/long-task", [](const httplib::Request &req, httplib::Response &res) {
  for (int i = 0; i < 1000; ++i) {
    if (req.is_connection_closed()) {
      std::cout << "client disconnected" << std::endl;
      return;
    }

    do_heavy_work(i);
  }

  res.set_content("done", "text/plain");
});

is_connection_closed is a std::function<bool()>, so call it with (). It returns true when the client is gone.

With a streaming response

The same check works inside set_chunked_content_provider(). Capture the request by reference.

svr.Get("/events", [](const httplib::Request &req, httplib::Response &res) {
  res.set_chunked_content_provider(
    "text/event-stream",
    [&req](size_t offset, httplib::DataSink &sink) {
      if (req.is_connection_closed()) {
        sink.done();
        return true;
      }

      auto event = generate_next_event();
      sink.write(event.data(), event.size());
      return true;
    });
});

svr.Get("/events", [](const httplib::Request &req, httplib::Response &res) {
  res.set_chunked_content_provider(
    "text/event-stream",
    [&req](size_t offset, httplib::DataSink &sink) {
      if (req.is_connection_closed()) {
        sink.done();
        return true;
      }

      auto event = generate_next_event();
      sink.write(event.data(), event.size());
      return true;
    });
});

When you detect a disconnect, call sink.done() to stop the provider from being called again.

How often should you check?

The call itself is cheap, but calling it in a tight inner loop doesn't add much value. Check at boundaries where interrupting is safe — after producing a chunk, after a database query, etc.

Warning: is_connection_closed() is not guaranteed to reflect reality instantly. Because of how TCP works, sometimes you only notice the disconnect when you try to send. Don't expect pixel-perfect real-time detection — think of it as "we'll notice eventually."

ESC