Sharing class member callback with other class

I have two classes, WebUiCoroutine and NADC338 (Worker). The NADC338 class handles some networking, after which I want it to "post" to the WebUiCoroutine class (which is written in header file). Essentially I want to pass/assign postHandler from WebUiCoroutine.h to postUpdate in NADC338 classdsdsdsdsdsdsd

I tried to pass the callback as seen below, and many other ways :/ This one fails with message '=': function as left operand

It’s loosely based upon this question and many other Stack Overflow posts. I’m sure i have misunderstood/overlooked something essential.

WebUiCoroutine.h

class WebUiCoroutine {
    NADC338 nadAPI = NADC338("someIPAdress", port);
public: 
    WebUiCoroutine() {
        nadAPI.register_callback(std::bind(&WebUiCoroutine::postHandler, this, std::placeholders::_1));
    }

    void postHandler(crow::json::wvalue) {
      //call another class member method of WebUiCoroutine
    }
}

NADC338.cpp

void NADC338::register_callback(std::function<void(crow::json::wvalue)> callback) {
    postUpdate = callback;
}

NADC338.h

class NADC338 {
    NADC338();
    NADC338(string ip, int port);

public:
    void (*postUpdate) (crow::json::wvalue);
    void register_callback(std::function<void(crow::json::wvalue)> callback);

}

Hope my problem is understandable and the code not too redacted.

Any input is appreciated

>Solution :

Make PostUpdate a std::function rather than a direct function pointer.

It is not possible to cast an std::function when that std::function carries state (as yours must due to the binding).

Instead, make it:

std::function<void(crow::json::wvalue)> PostUpdate;