mqtt api: future style

opencv-motion-detect/mqtt_helper.hpp

@@ -321,7 +321,7 @@ void on_subscribed(void* context, MQTTAsync_successData* response)
 void on_subscribed_fail(void* context, MQTTAsync_failureData* response)
 {
     MqttHelper *self = (MqttHelper *) context;
-    string msg = fmt::format("Subscribe failed: {}", MQTTAsync_strerror(response ? response->code : 0));
+    //string msg = fmt::format("subscribe failed: {}", MQTTAsync_strerror(response ? response->code : 0));
 	// spdlog::error(msg);
     MqttHelper::AsyncResult as;
     as.state = MqttHelper::State::SubScribeFail;