boost::shared_ptr<boost::asio::ip::udp::socket> ptr_udp;
template<typename Callback>
void recieve_request(boost::asio::yield_context yield, boost::asio::io_service& service, size_t port_num, Callback callback) {
...
        ptr_udp = boost::make_shared<boost::asio::ip::udp::socket>(service, boost::asio::ip::udp::endpoint(boost::asio::ip::udp::v4(), port_num));
....
boost::asio::spawn(ptr_udp->get_executor(), boost::bind(...) }
void server_run(boost::asio::io_service& service, size_t port_num, Callback callback) {
        boost::asio::spawn(service, boost::bind(&UDP_server::recieve_request, ... , boost::ref(callback)));
        service.run();
    }