How to properly pass a pointer to a member function for a 3rd party code?

Question

I have this 3rd party type definition:

typedef void (*NexTouchEventCb)(void *ptr);

Which is used in a function in the 3rd party library:

void NexTouch::attachPop(NexTouchEventCb pop, void *ptr)

If I have a single .cpp file then I can successfully call the attachPop without any compiler errors:

NexButton b0 = NexButton(0, 1, "b0");

void b0PopCallback(void *ptr)
{
    // do something here
}

b0.attachPop(b0PopCallback, &b0);

But I do OOP, so I have my own header and class files:

MyNextion.h

class MyNextion {
    public:
        void init();
        void b0PopCallback(void *ptr);
    private:
        NexButton *b0;
}

MyNextion.cpp

#include "MyNextion.h"

void MyNextion::init() {
    b0 = new NexButton(0, 1, "b0");
    b0->attachPop(/* what should I put here? */);
}

void MyNextion::b0PopCallback(void *ptr) {
    // do something here
}

I went through several similar questions like this, this and this one as well. But I just don't get it.

Note that this code is compiled for Arduino, which is a AVR microcontroller. Special C++ syntaxes may not be available.

Can you please tell me how should I call b0->attachPop and why should I call that way?

Please note, that I'm not able to modify the 3rd party library, which can be found here: https://github.com/itead/ITEADLIB_Arduino_Nextion

I'll take a look on that question too. I also added a note that I'm compiling for Arduino which is an AVR microcontroller. — papaiatis, Nov 30 '16 at 19:12
The possbile duplicate does not mention the static member functions way to do. — papaiatis, Nov 30 '16 at 19:56
There's many, many other possible duplicates but you can only flag a question with one. — MrEricSir, Nov 30 '16 at 20:19

score 2 · Accepted Answer · answered Nov 30 '16 at 19:09

You can only use a static member function (or non-member function) in this way. Fortunately, your API passes an additional void* pointer where you can put the object you want to call on:

class MyNextion {
public:
    void init();
    void b0PopCallback();
private:
    static void b0PopStaticCallback(void *ptr);
    NexButton *b0;
};

void MyNextion::b0PopStaticCallback(void *ptr)
{
    static_cast<MyNextion*>(ptr)->b0PopCallback();
}

void MyNextion::init()
{
    b0 = new NexButton(0, 1, "b0");
    b0->attachPop(b0PopStaticCallback, this);
}

Thank you so much! I would have never figured this out. – papaiatis Nov 30 '16 at 19:55 — papaiatis, Nov 30 '16 at 19:55

How to properly pass a pointer to a member function for a 3rd party code?

1 Answers1