How to properly pass member function as a parameter - c++

Question

How do I properly pass member function as a parameter?
MyCode:
#include <iostream>
using namespace std;
class Test
{
public:
int add(int a, int b)
{
return a + b;
}
int sub(int a, int b)
{
return a - b;
}
typedef int (*funcPtr)(int a, int b);
int myFunc(funcPtr func, int a, int b)
{
return func(a, b);
}
void setup()
{
cout << myFunc(&Test::add, 5, 3) << endl;
cout << myFunc(&Test::sub, 5, 3) << endl;
}
};
int main()
{
Test test;
test.setup();
}
Result:
Error: Cannot initialize a parameter of type 'Test::funcPtr' (aka 'int
()(int, int)') with an rvalue of type 'int (Test::)(int, int)'
Expected Result:
8
2

Answer 1

Your methods should be "regular" functions. add static to them to allow to use them with function pointers:
class Test
{
public:
static int add(int a, int b)
{
return a + b;
}
static int sub(int a, int b)
{
return a - b;
}
// ...
};
If you really pointer on method, you should replace int (*funcPtr)(int a, int b) by int (Test::*funcPtr)(int a, int b) and use something like that instead:
class Test
{
public:
int add(int a, int b)
{
return a + b;
}
int sub(int a, int b)
{
return a - b;
}
typedef int (Test::*funcPtr)(int a, int b);
int myFunc(funcPtr func, int a, int b)
{
return (this->*func)(a, b);
}
void setup()
{
cout << myFunc(&Test::add, 5, 3) << endl;
cout << myFunc(&Test::sub, 5, 3) << endl;
}
};

Answer 2

You should read about std:: function and std::bind. The first thing will allow you to store a function pointer with multiple form (Functor, lamda, binded), and the second will allow you to bind parameters to your function call (In your case, you want to bind the instance of the class that is needed to call your function).
std:: function<int(int, int)> func = std::bind(&Class::Method, instancePtr, std::placeholders::_1, std:: placeholders::_2);
int result = func(a, b);
However, in your context, your methods should be marked as static (They are not using any non-static member of your class), but the example and the explanation I provided answer to your base question

Answer 3

Firstly, based on your question the best solution here will be without using the pointer instead declare your methods as static and call them directly as given below.
That solution will generate the right results without using the complexity of pointers.
Simple is better if you do not need to use pointers it is better to not use them. code will be more readable as well.
Following code works i tested it:
#include <iostream>
using namespace std;
class Test
{
public:
static int add(int a, int b)
{
return a + b;
}
static int sub(int a, int b)
{
return a - b;
}
void setup()
{
cout << add( 5, 3) << endl;
cout << sub(5, 3) << endl;
}
};
int main()
{
Test test;
test.setup();
}