...

class CoapClient {
    struct mg_mgr mgr;
    struct mg_connection *nc;
    std::string address{"udp://127.0.0.1:5683"};

public:
    static int s_time_to_exit;
    CoapClient() {
        std::cout << "[Client] Using %s as CoAP server" << address << std::endl;
        mg_mgr_init(&mgr, 0);


       nc = mg_connect(&mgr, address.c_str(), coap_handler);

        
        
    }

    static  void coap_handler(struct mg_connection *nc, int ev, void *p) {
        switch (ev) {
            case MG_EV_CONNECT: {
                struct mg_coap_message cm;
                uint32_t res;

                memset(&cm, 0, sizeof(cm));
                cm.msg_id = 1;
                cm.msg_type = MG_COAP_MSG_CON;
                std::cout << "[Client] Sending CON..." << std::endl;
                res = mg_coap_send_message(nc, &cm);
                if (res == 0) {
                    std::cout << "[Client] Sent CON with msg_id = " << cm.msg_id << std::endl;
                } else {
                    std::cout << "[Client] Error: " << res << std::endl;
                    s_time_to_exit = 1;
                }
                break;
            }
            case MG_EV_COAP_ACK:
            case MG_EV_COAP_RST: {
                struct mg_coap_message *cm = (struct mg_coap_message *) p;
                std::cout << "[Client] ACK/RST for message with msg_id =" << cm->msg_id << " received"<<std::endl;
                s_time_to_exit = 1;
                break;
            }
        }
    }
   

   };