c++ – How do I combine multiple boost::asio::experimental::promise?

Yes, using boost::asio::async_compose is the approach here. It allows you to compose multiple asynchronous operations (like TCP connect followed by SSL handshake) into a single operation that returns a boost::asio::experimental::promise, which can be used with co_await or .operator().

You can implement it like this:

boost::asio::experimental::promise>
SocketWrapper::asyncConnectAndHandshake(
    const boost::asio::ip::basic_resolver_results<:asio::ip::tcp>& resolver_results)
{
    namespace asio = boost::asio;

    auto executor = underlyingSocket().get_executor();

    return asio::async_compose<
        decltype(asio::experimental::use_promise),
        void(boost::system::error_code)>(
        [this, resolver_results](auto& self) mutable {
            asio::async_connect(underlyingSocket(), resolver_results,
                [this, &self](boost::system::error_code ec, const boost::asio::ip::tcp::endpoint&) mutable {
                    if (ec || !isSSL()) {
                        self.complete(ec);
                        return;
                    }

                    stream().async_handshake(asio::ssl::stream_base::client,
                        [self = std::move(self)](boost::system::error_code ec) mutable {
                            self.complete(ec);
                        });
                });
        },
        asio::experimental::use_promise,
        executor);
}