Mosquitto: Use non-static callback function - c++

Question

I've running Mosquitto on my RPI4. But right know I only can set static callback functions. Is there a way to use class members?
I've tried to use std::bind to pass a class member function as callback:
main.cpp
#include <stdio.h>
#include <mosquitto.h>
#include "mqtt.h"
#include <string>
int main(int argc, char **argv)
{
MqttConnector * mqtt = new MqttConnector("piClient", "send", "rc", 1883, "localhost", 60);
mqtt->startClient();
return 0;
}
mqtt.h (only important parts
#include <mosquitto.h>
#include <string>
#include <stdio.h>
class MqttConnector
{
public:
MqttConnector(std::string id,
std::string sendTopic,
std::string receiveTopic,
int port,
std::string host,
int keepalive);
~MqttConnector();
void startClient();
private:
void messageCallback(struct mosquitto *mosq,
void *userdata,
const struct mosquitto_message *message);
struct mosquitto *mosqClient = NULL;
int keepalive;
int port;
std::string id;
std::string host;
std::string sendTopic;
std::string receiveTopic;
};
mqtt.cpp
#include "mqtt.h"
#include <stdio.h>
#include <string>
#include <string.h>
#include <mosquitto.h>
#include <functional>
using namespace std::placeholders;
MqttConnector::MqttConnector(std::string id, std::string sendTopic, std::string receiveTopic, int port, std::string host, int keepalive)
{
mosquitto_lib_init();
mosqClient = mosquitto_new(NULL, true, NULL);
if(!mosqClient){
fprintf(stderr, "Error: Out of memory.\n");
}
this->keepalive = keepalive;
this->id = id;
this->host = host;
this->port = port;
this->sendTopic = sendTopic;
this->receiveTopic = receiveTopic;
}
MqttConnector::~MqttConnector()
{
mosquitto_destroy(mosqClient);
mosquitto_lib_cleanup();
}
void MqttConnector::messageCallback(struct mosquitto *mosq, void *userdata, const struct mosquitto_message *message)
{
// I want to access class members like sendTopic / receiveTopic here
}
void MqttConnector::startClient()
{
// try to bind class members function
mosquitto_message_callback_set(mosqClient, std::bind(&MqttConnector::messageCallback, this, _1, _2, _3));
//other stuff
}
This gives me the following error while compiling:
cannot convert 'std::_Bind_helper<false, void (MqttConnector::*)(mosquitto*, void*, const mosquitto_message*), MqttConnector*, const std::_Placeholder<1>&, const std::_Placeholder<2>&, const std::_Placeholder<3>&>::type' {aka 'std::_Bind<void (MqttConnector::*(MqttConnector*, std::_Placeholder<1>, std::_Placeholder<2>, std::_Placeholder<3>))(mosquitto*, void*, const mosquitto_message*)>'} to 'void (*)(mosquitto*, void*, const mosquitto_message*)'
83 | mosquitto_message_callback_set(mosqClient, std::bind(&MqttConnector::messageCallback, this, _1, _2, _3));
Why doesn't it work?
Thanks!

Answer 1

This is a problem of using C-api from C++. What is the difference between a member function and a free function? When you provide a pointer to a member function a pointer to the class object is implicitly passed as the first parameter. Since C-api doesn't do that, but the problem is well known, the solution was introduced and it is called passing a context. Usually it is done through a void pointer. Functions that register callbacks usually take the pointer to the free function and a pointer to context. Then this pointer will be passed as one of the callback parameters.
In mosquitto case this context pointer is passed beforehand at the creation of a mosquitto object with mosquitto_new.
In order to make the callback function behave like a C function, we declare it static.
Inside the callback function we use static_cast to cast the void pointer to the object that we have provided.
mqtt.h
#include <mosquitto.h>
#include <string>
#include <stdio.h>
class MqttConnector
{
public:
MqttConnector(std::string id,
std::string sendTopic,
std::string receiveTopic,
int port,
std::string host,
int keepalive);
~MqttConnector();
void startClient();
private:
// make this function static
---->
static void messageCallback(struct mosquitto *mosq,
void *userdata,
const struct mosquitto_message *message);
struct mosquitto *mosqClient = NULL;
int keepalive;
int port;
std::string id;
std::string host;
std::string sendTopic;
std::string receiveTopic;
};
mqtt.cpp
#include "mqtt.h"
#include <stdio.h>
#include <string>
#include <string.h>
#include <mosquitto.h>
#include <functional>
using namespace std::placeholders;
MqttConnector::MqttConnector(std::string id, std::string sendTopic, std::string receiveTopic, int port, std::string host, int keepalive)
{
mosquitto_lib_init();
// provide apointer to this as user data
mosqClient = mosquitto_new(NULL, true, this);
---->
if(!mosqClient){
fprintf(stderr, "Error: Out of memory.\n");
}
this->keepalive = keepalive;
this->id = id;
this->host = host;
this->port = port;
this->sendTopic = sendTopic;
this->receiveTopic = receiveTopic;
}
MqttConnector::~MqttConnector()
{
mosquitto_destroy(mosqClient);
mosquitto_lib_cleanup();
}
void MqttConnector::messageCallback(struct mosquitto *mosq, void *userdata, const struct mosquitto_message *message)
{
// Use static cast to get pointer to your class object from userdata
MqttConnector *connector = static_cast<MqttConnector>(userdata);
connector->sendTopic;
}
void MqttConnector::startClient()
{
// static callback
mosquitto_message_callback_set(mosqClient, &MqttConnector::messageCallback);
// lambda callback
// beware, you can't use capture here
mosquitto_message_callback_set(&m, [/*no capture possible*/] (struct mosquitto *, void *userdata, const struct mosquitto_message *)
{
MqttConnector *connector = static_cast<MqttConnector>(userdata);
connector->sendTopic;
});
}