我想实现一种变通方法,以使用非静态类作为回调函数。我正在使用Eclipse Paho MQTT代码。以下类型已实现并用作回调:
typedef void MQTTAsync_onSuccess(void* context, MQTTAsync_successData* response);
MQTTAsync_onSuccess* onSuccess;
onSuccess = myStaticCallback;
void myStaticCallback (void* context, MQTTAsync_successData* response)
{
//...callback actions...
}
我想包装此C API(无需修改现有的MQTT C API)并实现属于对象/类的非静态/非集中式回调函数。
typedef void MQTTAsync_onSuccess(void* context, MQTTAsync_successData* response);
class myMQTTClass
{
private:
void myCallback (void* context, MQTTAsync_successData* response);
MQTTAsync_onSuccess* onSuccess;
public:
void foo (void)
{
this->onSuccess = this->myCallback;
}
}
您可能会猜到,上面的代码会导致错误:无法将myCallback从type转换'void (myMQTTClass::) (void*, MQTTAsync_successData*)'
为type 'void (*)(void*, MQTTAsync_successData*)'
。
非常感谢您提供有关如何解决此问题的任何指导或任何解决方法。我愿意提供任何可能的缺失信息。提前致谢。
编辑:实际代码有一些遗漏
namespace rover
{
typedef struct
{
char * clientID;
char * topic;
char * payload;
int qos; // 1
long int timeout; // Such as 10000L usec
} RoverMQTT_Configure_t;
class RoverPahoMQTT
{
public:
RoverPahoMQTT (char * host_name, int port, RoverMQTT_Configure_t MQTT_Configure);
private:
/**
* @brief Host name used for connecting to the Eclipse Paho MQTT server
*/
char * HOST_NAME;
/**
* @brief Port used for connecting to the Eclipse Paho MQTT server
*/
int PORT;
RoverMQTT_Configure_t rover_MQTT_configure;
/* Internal attributes */
MQTTAsync client;
/**
* @brief Connect options
*/
MQTTAsync_connectOptions conn_opts;
/**
* @brief Disconnect options
*/
MQTTAsync_disconnectOptions disc_opts;
//...
static void onPublisherConnect (void* context, MQTTAsync_successData* response);
void onPublisherConnect_ (MQTTAsync_successData* response);
//...
}
}
int rover::RoverPahoMQTT::publish (void)
{
this->flushFlags ();
this->conn_opts = MQTTAsync_connectOptions_initializer;
this->client = new MQTTAsync;
int rc;
char my_addr[20];
this->constructAddress (my_addr);
printf ("address: %s", my_addr);
MQTTAsync_create ( &(this->client),
my_addr,
this->rover_MQTT_configure.clientID,
MQTTCLIENT_PERSISTENCE_NONE,
NULL);
MQTTAsync_setCallbacks(this->client, NULL, onConnectionLost, NULL, NULL);
conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;
conn_opts.onSuccess = rover::RoverPahoMQTT::onPublisherConnect;
conn_opts.onFailure = onConnectFailure;
conn_opts.context = this->client;
if ((rc = MQTTAsync_connect(this->client, &(this->conn_opts))) != MQTTASYNC_SUCCESS)
{
printf("Failed to start connect, return code %d\n", rc);
return rc;
}
/*printf("Waiting for publication of %s\n"
"on topic %s for client with ClientID: %s\n",
PAYLOAD, TOPIC, CLIENTID);*/
while (!mqtt_finished)
usleep(this->rover_MQTT_configure.timeout);
MQTTAsync_destroy(&client);
return rc;
}
void rover::RoverPahoMQTT::onPublisherConnect_(MQTTAsync_successData* response)
{
MQTTAsync_responseOptions opts = MQTTAsync_responseOptions_initializer;
MQTTAsync_message pubmsg = MQTTAsync_message_initializer;
int rc;
printf("Successful connection\n");
opts.onSuccess = onPublisherSend;
opts.context = client;
pubmsg.payload = &default_MQTT_configure.payload;
pubmsg.payloadlen = strlen(default_MQTT_configure.payload);
pubmsg.qos = default_MQTT_configure.qos;
pubmsg.retained = 0;
deliveredtoken = 0;
if ((rc = MQTTAsync_sendMessage(client, default_MQTT_configure.topic, &pubmsg, &opts)) != MQTTASYNC_SUCCESS)
{
printf("Failed to start sendMessage, return code %d\n", rc);
exit(EXIT_FAILURE);
}
}
void rover::RoverPahoMQTT::onPublisherConnect (void* context, MQTTAsync_successData* response)
{
rover::RoverPahoMQTT* m = (rover::RoverPahoMQTT*) context;
m->onPublisherConnect_(response);
//((rover::RoverPahoMQTT*)context)->onPublisherConnect_(response);
// ^^^HERE IS THE SEGMENTATION FAULT
}
由于明确指出这里,回调必须是
通过将其作为参数传递给MQTTAsync_responseOptions向客户端库注册
而context
论点是
指向最初传递给MQTTAsync_responseOptions的上下文值的指针,该值包含任何特定于应用程序的上下文。
我建议为您的类提供一个通用接口,该接口提供与回调原型匹配的静态方法:
class myMQTTClass
{
public:
static void callback(void* context, MQTTAsync_successData* response)
{
myMQTTClass * m = (myMQTTClass*)context;
m->myCallback(response);
}
protected:
virtual void myCallback(MQTTAsync_successData* response) = 0;
};
现在,您可以在子类中实现不同的行为:
class myMQTTClassImpl : public myMQTTClass
{
protected:
void myCallback(MQTTAsync_successData *response)
{
std::cout << "success!!!" << std::endl;
}
};
让我们看看如何使用它:
int main()
{
myMQTTClass * m = new myMQTTClassImpl();
MQTTAsync_responseOptions options;
options.onSuccess = myMQTTClass::callback;
options.context = m;
}
编辑(指实际发布的代码):
在您的publish
方法中,这是正确的:
conn_opts.onSuccess = rover::RoverPahoMQTT::onPublisherConnect;
这是错误的:
conn_opts.context = this->client;
它应该是:
conn_opts.context = this;
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