cpp-httplib/docs-src/pages/en/cookbook/e04-sse-client.md

title, order, status

title	order	status
E04. Receive SSE on the Client	50	draft

cpp-httplib ships a dedicated sse::SSEClient class. It handles auto-reconnect, per-event-name dispatch, and Last-Event-ID tracking for you — so receiving SSE is painless.

Basic usage

#include <httplib.h>

httplib::Client cli("http://localhost:8080");
httplib::sse::SSEClient sse(cli, "/events");

sse.on_message([](const httplib::sse::SSEMessage &msg) {
  std::cout << "data: " << msg.data << std::endl;
});

sse.start(); // blocking

Build an SSEClient with a Client and a path, register a callback with on_message(), and call start(). The event loop kicks in and automatically reconnects if the connection drops.

Dispatch by event name

When the server sends events with an event: field, register a handler per name via on_event().

sse.on_event("message", [](const auto &msg) {
  std::cout << "chat: " << msg.data << std::endl;
});

sse.on_event("join", [](const auto &msg) {
  std::cout << msg.data << " joined" << std::endl;
});

sse.on_event("leave", [](const auto &msg) {
  std::cout << msg.data << " left" << std::endl;
});

on_message() serves as a generic fallback for unnamed events (the default message type).

Connection lifecycle and errors

sse.on_open([] {
  std::cout << "connected" << std::endl;
});

sse.on_error([](httplib::Error err) {
  std::cerr << "error: " << httplib::to_string(err) << std::endl;
});

Hook into connection open and error events. Even when the error handler fires, SSEClient keeps trying to reconnect in the background.

Run asynchronously

If you don't want to block the main thread, use start_async().

sse.start_async();

// main thread continues to do other things
do_other_work();

// when you're done, stop it
sse.stop();

start_async() spawns a background thread to run the event loop. Use stop() to shut it down cleanly.

Configure reconnection

You can tune the reconnect interval and maximum retries.

sse.set_reconnect_interval(5000);    // 5 seconds
sse.set_max_reconnect_attempts(10);  // up to 10 (0 = unlimited)

If the server sends a retry: field, that takes precedence.

Automatic Last-Event-ID

SSEClient tracks the id of each received event internally and sends it back as Last-Event-ID on reconnect. As long as the server sends events with id:, this all works automatically.

std::cout << "last id: " << sse.last_event_id() << std::endl;

Use last_event_id() to read the current value.

Note: SSEClient::start() blocks, which is fine for a one-off command-line tool. For GUI apps or embedded in a server, the start_async() + stop() pair is the usual pattern.

For the server side, see E01. Implement an SSE Server.