Using class member methods as callbacks

[candale]

Hi,

I've been trying to create a class of my own that uses your wrapper but I can't seem to be able to get it working with method callbacks (I'm not experienced with C++). Is there any good way this can be done?

Thank you.

[i-n-g-o]

hi.

you can have a look in the MQTT.h and MQTT.cpp files to see how it is done there.

basically you need to add the signature of the function as the argument of your callback setter function, and store the callback function somewhere.

e.g.:
if you want to pass this function:
void myCallback(int number) { ... }

the setter function looks like this:
void onMyCallback( void(*)(int));

onMyCallback has to store the callback function somewhere, to call it later (see MQTT.cpp)
usually you add a field to your class with the signature of the functionyou want to store:
void (*aCallback) (int);

the implementation of onMyCallback looks like this:

void onMyCallback( void(*func)(int)) { 
    aCallback = func;
}

then you can register myCallback with onMyCallback like this:
onMyCallback(myCallback);

when you want to call the callback, you use the stored function:
aCallback(123);

hope this helps.

[candale]

Thank you for the quick reply!

I've taken a look over your code and I've seen that you have a intermediary function that is passed as a callback to the actual mqtt library and that function calls your method.

But my use case is something like this:

class MyMqttClient
{
    MyMqttClient()
    {
        MQTT mqtt_client(clientId.c_str(), host, port);
        // I would like tor register as callback (with your wrapper) the method this->onConnected
        mqtt_client.onConnected(this->onConnected);
    }

    void onConnected()
    {
        // subscribe to stuff
    }
}

So what I want to do is register as callback a method (MyMwttClient::onConnected). What it tells me is that the signature is different from void (*)(void) and rightfully so, because you have the this parameter sent by default and it's bound to that object.
So this is what I'm fighting with, I can't find a way to set as callback a class' member.
I've tried doing something similar to what you have done in MQTT.cpp but to no avail.

I assume I should be creating another intermediary class that handles the callbacks, specific callbacks for my class. I'm a bit lost in this.

[i-n-g-o]

what is the error you get?

[candale]

So I have this code:

#include <ESP8266WiFi.h>
#include <MQTT.h>

#define SSID "ssid"
#define PASS "pass"

#define ESP_CLIENT_ID ESP.getChipId()
#define MQTT_SERVER "192.168.1.141"
#define MQTT_PORT 1883

class MyMqttClient
{
private:

public:
    MyMqttClient()
    {
        String client_id(ESP_CLIENT_ID);
        MQTT client(client_id.c_str(), MQTT_SERVER, MQTT_PORT);
        client.onConnected(onConnected);
    }
    void onConnected()
    {
        // do smth
    }
};

void setup() {
    Serial.begin(115200);
    delay(1000);

    WiFi.begin(SSID, PASS);

    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
    }

    MyMqttClient client();
}

void loop() {
    delay(100);
}

and I get this error:

main.ino: In constructor 'MyMqttClient::MyMqttClient()':
main.ino:20:31: error: no matching function for call to 'MQTT::onConnected(<unresolved overloaded function type>)'
main.ino:20:31: note: candidate is:
In file included from src/tmp_ino_to.cpp:3:0:
.pioenvs/esp01/esp-mqtt-arduino/MQTT.h:67:7: note: void MQTT::onConnected(void (*)())
void onConnected( void (*)(void) );
^
.pioenvs/esp01/esp-mqtt-arduino/MQTT.h:67:7: note:   no known conversion for argument 1 from '<unresolved overloaded function type>' to 'void (*)()'

I've looked into it and as far as I understood it, it is because a method has a different signature than a function has. So my question is, how would one write it in such a way that a method can be send as callback.

[i-n-g-o]

i see what you mean.
it's not really built to be wrapped like that.
it would need an interface class you could then use to subclass your wrapper from.

can you work around it? -> do you really need to wrap MQTT?

[i-n-g-o]

btw.
i encountered clashing variable names with ESP8266WiFi and pushed changes.
please pull the updated version

[candale]

Hi,

I eventually managed to get it working by making the callbacks in MQTT virtual and unimplemented (e.g. void onConnected() = 0;) and so a subclass is forced to implement them; this way the mechanism you wrote calls the methods that I need. It's not the prettiest but so far it works. When I'll have time I'll write something better.

Thank you for your help!

[i-n-g-o]

great you made it work!
i will have a look into an interface for that usecase.

[everslick]

there is this old, but somehow not recommended trick to declare member functions that should have C ABI as 'static'. but you lose the this pointer this way.