This example shows how you can pass a member function to a global function as a pointer.

Passing-Member-Function-to-Global-Functions.cpp

//-------------------------------------------------------------//

#include <iostream>
using namespace std;

//-------------------------------------------------------------//
//the type of the callback

typedef void (*func)(void); //the only pointer type the global accetps

//-------------------------------------------------------------//
//forward declarations

class wish;
wish* wishPtr[3] = {0}; //pointer to the object
int objCount;

//-------------------------------------------------------------//
//the library function that accepts a callback

void saySomething(func say) {
    say();  //invokes the member function explicitly
}

//-------------------------------------------------------------//
//one function for each object

void func_1();
void func_2();
void func_3();

//save the function ptrs to an array so that they can be automatically accessed
func funcPtr[3] = {func_1, func_2, func_3};

//-------------------------------------------------------------//
//main class

class wish {
    public:
    int number; //just a variable to identify the object
    func callback;
    
    //constructor
    wish(int n) {
        number = n;
        callback = funcPtr[objCount];   //get a copy of global callback function
        wishPtr[objCount++] = this; //save the object pointer so the callback can access
    }
    void wishSomething() {
        saySomething(callback); //send the callback
    }
};

//-------------------------------------------------------------//
//each function knows the object they want to point to

void func_1() {
    if(wishPtr[0] != 0) {
        cout << "Foo " << wishPtr[0]->number << "\n";
    }
}

void func_2() {
    if(wishPtr[1] != 0) {
        cout << "Bar " << wishPtr[1]->number << "\n";
    }
}

void func_3() {
    if(wishPtr[2] != 0) {
        cout << "Baz " << wishPtr[2]->number << "\n";
    }
}

//-------------------------------------------------------------//
//instantiate three objects

wish A(10);
wish B(11);
wish C(12);

//-------------------------------------------------------------//

int main() {
	A.wishSomething();
	B.wishSomething();
	C.wishSomething();
	return 0;
}

//-------------------------------------------------------------//