Using an instance method as callback.

[MichMich]

First of all, thanks for the awesome library. It really helps me in the development of my MQTT projects.

I'm currently trying to use an instance method as the callback, but unfortunately, the set mqtt.set_callback(mqttCallback) method expects a general function. mqtt.set_callback(this.mqttCallback) or something like mqtt.set_callback(MyObj::mqttCallback, this) does not work.

Do you know a way around this, without making my instance static?

[av1024]

Look at std:bind:

#include<functional>
mqtt.set_callback(std::bind(&MyClass::myMethod, MyClassInstance))

//Or lambda:

set_callback([]() { return myInstance.get(); })

[MichMich]

Thanks for the info. I tried the std::bind route:

mqtt.set_callback(std::bind(&DataManager::mqttCallback, this));

Where DataManager is my class. And this refers to the instance.

But I get the following compiler error:

src/DataManager.cpp:49:67: error: no matching function for call to 'PubSubClient::set_callback(std::_Bind_helper<false, void (DataManager::)(const MQTT::Publish&), DataManager const>::type)'

Any idea?

[av1024]

I'm not guru in c++ :( do just an idea.

May be wrong definition of mqttCallback.

And at least wrong usage: You shouldn't pass 'this' to std::bind, but instance of global variable. There is a way to store this pointer in lambdas (http://en.cppreference.com/w/cpp/language/lambda) but I tried with no success (and looks like it is very similar with passing global).

As an a way:

• pass global variable as 2'nd arg
• use lambda syntax and pass some const (I tried to pass index in sensors array for example) as arg to lambda

For now with my meteo sensor I use following syntax:

sens[0].setReadFn([] () { return _bmp.readTemperature(); } ); //less mem usage, but equivalent to: std::bind(&BME280::readTemperature, _bmp));

UPD: btw, look at pubsubclient.h - there is ifdef switching between functional and plain syntax - _GLIBC_FUNCTIONAL

[MichMich]

Tried using a lambda (I'm not a real C++, programmer, but is seems to look a lot like closures which other languages use.) This solved the issue. The following works:

// define the callback method for the object
void DataManager::mqttCallback(const MQTT::Publish& pub) {
    Serial.print("Received data: ");
    Serial.println(pub.payload_string());
}

// register the callback using the lambda
mqtt.set_callback([this](const MQTT::Publish& pub) {
    this->mqttCallback(pub);
});

Of course, the above two could also be combined to.

mqtt.set_callback([](const MQTT::Publish& pub) {
    Serial.print("Received data: ");
    Serial.println(pub.payload_string());
});

(Note the tiny difference that it doesn't capture this between the [] since this isn't used in the lambda anyway.)

So with this, the issue is solved. Thanks for your help!