bun.sh/packages/bun-uws/examples/UpgradeAsync.cpp

/* We simply call the root header file "App.h", giving you uWS::App and uWS::SSLApp */
#include "App.h"

/* This is a simple WebSocket "async" upgrade example.
 * You may compile it with "WITH_OPENSSL=1 make" or with "make" */

int main() {
    /* ws->getUserData returns one of these */
    struct PerSocketData {
        /* Define your user data */
        int something;
    };

    /* Keep in mind that uWS::SSLApp({options}) is the same as uWS::App() when compiled without SSL support.
     * You may swap to using uWS:App() if you don't need SSL */
    uWS::SSLApp({
        /* There are example certificates in uWebSockets.js repo */
	    .key_file_name = "misc/key.pem",
	    .cert_file_name = "misc/cert.pem",
	    .passphrase = "1234"
	}).ws<PerSocketData>("/*", {
        /* Settings */
        .compression = uWS::SHARED_COMPRESSOR,
        .maxPayloadLength = 16 * 1024,
        .idleTimeout = 10,
        .maxBackpressure = 1 * 1024 * 1024,
        /* Handlers */
        .upgrade = [](auto *res, auto *req, auto *context) {

            /* HttpRequest (req) is only valid in this very callback, so we must COPY the headers
             * we need later on while upgrading to WebSocket. You must not access req after first return.
             * Here we create a heap allocated struct holding everything we will need later on. */

            struct UpgradeData {
                std::string secWebSocketKey;
                std::string secWebSocketProtocol;
                std::string secWebSocketExtensions;
                struct us_socket_context_t *context;
                decltype(res) httpRes;
                bool aborted = false;
            } *upgradeData = new UpgradeData {
                std::string(req->getHeader("sec-websocket-key")),
                std::string(req->getHeader("sec-websocket-protocol")),
                std::string(req->getHeader("sec-websocket-extensions")),
                context,
                res
            };

            /* We have to attach an abort handler for us to be aware
             * of disconnections while we perform async tasks */
            res->onAborted([=]() {
                /* We don't implement any kind of cancellation here,
                 * so simply flag us as aborted */
                upgradeData->aborted = true;
                std::cout << "HTTP socket was closed before we upgraded it!" << std::endl;
            });

            /* Simulate checking auth for 5 seconds. This looks like crap, never write
             * code that utilize us_timer_t like this; they are high-cost and should
             * not be created and destroyed more than rarely!
             *
             * Also note that the code would be a lot simpler with capturing lambdas, maybe your
             * database client has such a nice interface? Either way, here we go!*/
            struct us_loop_t *loop = (struct us_loop_t *) uWS::Loop::get();
            struct us_timer_t *delayTimer = us_create_timer(loop, 0, sizeof(UpgradeData *));
            memcpy(us_timer_ext(delayTimer), &upgradeData, sizeof(UpgradeData *));
            us_timer_set(delayTimer, [](struct us_timer_t *t) {
                /* We wrote the upgradeData pointer to the timer's extension */
                UpgradeData *upgradeData;
                memcpy(&upgradeData, us_timer_ext(t), sizeof(UpgradeData *));

                /* Were'nt we aborted before our async task finished? Okay, upgrade then! */
                if (!upgradeData->aborted) {
                    std::cout << "Async task done, upgrading to WebSocket now!" << std::endl;

                    /* If you don't want to upgrade you can instead respond with custom HTTP here,
                    * such as res->writeStatus(...)->writeHeader(...)->end(...); or similar.*/

                    /* This call will immediately emit .open event */
                    upgradeData->httpRes->template upgrade<PerSocketData>({
                        /* We initialize PerSocketData struct here */
                        .something = 13
                    }, upgradeData->secWebSocketKey,
                        upgradeData->secWebSocketProtocol,
                        upgradeData->secWebSocketExtensions,
                        upgradeData->context);
                } else {
                    std::cout << "Async task done, but the HTTP socket was closed. Skipping upgrade to WebSocket!" << std::endl;
                }

                delete upgradeData;

                us_timer_close(t, 0);
            }, 5000, 0);

        },
        .open = [](auto *ws) {
            /* Open event here, you may access ws->getUserData() which points to a PerSocketData struct.
             * Here we simply validate that indeed, something == 13 as set in upgrade handler. */
            std::cout << "Something is: " << static_cast<PerSocketData *>(ws->getUserData())->something << std::endl;
        },
        .message = [](auto *ws, std::string_view message, uWS::OpCode opCode) {
            /* We simply echo whatever data we get */
            ws->send(message, opCode);
        },
        .drain = [](auto */*ws*/) {
            /* Check ws->getBufferedAmount() here */
        },
        .ping = [](auto */*ws*/, std::string_view) {
            /* You don't need to handle this one, we automatically respond to pings as per standard */
        },
        .pong = [](auto */*ws*/, std::string_view) {
            /* You don't need to handle this one either */
        },
        .close = [](auto */*ws*/, int /*code*/, std::string_view /*message*/) {
            /* You may access ws->getUserData() here, but sending or
             * doing any kind of I/O with the socket is not valid. */
        }
    }).listen(9001, [](auto *listen_socket) {
        if (listen_socket) {
            std::cout << "Listening on port " << 9001 << std::endl;
        }
    }).run();
}