C ++ Passing a pointer to a non-stationary member function

Question

Hello everyone :) I have a problem with function pointers
My arguments to the callback function:
1) this function: int (* fx) (int, int)
2) variable int: int a 3) another int: int b
Well, the problem that the function that I want to pass to the "callback" is a non-stationary member of the function :( and there are many problems
If someone smarter than me has time to spend, he can see my code :)

#include <iostream>
using namespace std;

class A{
private:
    int x;
public:
    A(int elem){
        x = elem;
    }

    static int add(int a, int b){
        return a + b;
    }

    int sub(int a, int b){
        return x - (a + b);
    }
};

void callback( int(*fx)(int, int), int a, int b)
{
    cout << "Value of the callback: " << fx(a, b) << endl;
}

int main()
{
A obj(5);

    //PASSING A POINTER TO A STATIC MEMBER FUNCTION -- WORKS!!
    // output = 'Value of the callback: 30'
    callback(A::add, 10, 20);

    //USING A POINTER TO A NON-STATIC MEMBER FUNCTION -- WORKS!!
    int(A::*function1)(int, int) = &A::sub;
    // output = 'Non static member: 3'
    cout << "Non static member: " << (obj.*function1)(1, 1) << endl;

    //PASSING A POINTER TO A NON-STATIC MEMBER FUNCTION -- aargh
    // fallita! tutto quello sotto non funziona --> usa i funtori???
    // puoi creare una classe wrapper ma non riuscirai mai a chiamare da callback
    int(A::*function2)(int, int) = &A::sub;
    int(*function3)(int, int) = obj.*function2; //[error] invalid use of non-static member function
    callback(function3, 1, 1);
}

Is there a way to create my pointer in the way I tried to write, for example int (* fx) (int, int) = something?
I searched a lot, but no one could give me an answer (well, there was an answer: "NO", but I still think I can do something)

, ?

PS:

EDIT1: - :

template <class T>
void callback2( T* obj, int(T::*fx)(int, int), int a, int b)
{
    cout << "Value of the callback: " << (obj->*fx)(a, b) << endl;
}
void callback2( void* nullpointer, int(*fx)(int, int), int a, int b)
{
    cout << "Value of the callback: " << fx(a, b) << endl;
}

:

callback2(NULL, &mul, 5, 3); // generic function, it like: int mul(int a, int b){return a*b;}
callback2(NULL, &A::add, 5, 3); //static member function
callback2(&obj, &A::sub, 1, 1); //non static member function

, "callback2" ()...
, () , : callback2?

void callback2(int(*fx)(int, int), int a, int b)<br>

:

callback2(&obj.sub, 1, 3);

Answer 1

:

int (*function3)(int, int) = obj.*function2;

:

int (*function3)(int, int) = std::mem_fn(&A::sub, obj);
//                           ^^^^^^^^^^^^^^^^^^^^^^^^^

function2 , .

Answer 2

std-, , :

#include <iostream>
#include <functional>
using namespace std;

class A{
private:
    int x;
public:
    A(int elem){
        x = elem;
    }

    static int add(int a, int b){
        return a + b;
    }

    int sub(int a, int b) const{
        return x - (a + b);
    }
};

void callback( std::function<int(const A& ,int,int )> fx, A obj, int a, int b)
{
    cout << "Value of the callback: " << fx( obj, a, b) << endl;
}

int main()
{
A obj(5);


    std::function<int(const A& ,int,int )> Aprinter= &A::sub;

    callback(Aprinter,obj,1,2);
}