How to set the callback from a c++ class?

First of all, sorry that I'm asking here... it seems to me like a generic C/C++ question but it's probably too generic to be searched on Google. I was grown somewhere between Z80 assembler and high-level languages like Objective-C, Swift and JavaScript, so the C-side is temporarily unfamiliar to me.

In your header, you declare:

//! Set the callback function
PubSubClient& set_callback(callback_t cb) { _callback = cb; return *this; }

In my .h:

class MyClass {
 public: 
    PubSubClient *mqtt_client;
    // I had to inline the callback here because otherwise linker has issues with undefined attribute types
     inline void mqtt_callback(const MQTT::Publish& pub) {
      Serial.println("*TH: MQTT callback...");
      if (pub.has_stream()) {
        Serial.print(pub.topic());
        Serial.print(" => ");
        if (pub.has_stream()) {
          uint8_t buf[MQTT_BUFFER_SIZE];
          int read;
          while (read = pub.payload_stream()->read(buf, MQTT_BUFFER_SIZE)) {
            // Do something with data in buffer
            Serial.write(buf, read);
          }
          pub.payload_stream()->stop();
          Serial.println("stop.");
        } else {
          Serial.println(pub.payload_string());
        }
      }
    }
}

In my .cpp:

mqtt_client = new PubSubClient(*wifi_client, mqtt_url.c_str());
if (mqtt_client->connect(MQTT::Connect(id)
                .set_clean_session()
                .set_will(willTopic, disconnected_response.c_str())
                .set_auth(user, pass)
                .set_keepalive(30)
              )) {
    // The callback function does not get called. Why?
    mqtt_client->set_callback([this](const MQTT::Publish &pub) {
      Serial.print("*MQTT Callback called...");
      this->mqtt_callback(pub);
    });
}

The callback function does not get called. Why?

Imroy / pubsubclient

How to set the callback from a c++ class? #81