Why callback functions needs to be static when declared in class

Question

I was trying to declare a callback function in class and then somewhere i read the function needs to be static but It didn't explain why?

#include <iostream>
using std::cout;
using std::endl;

class Test
{
public:
    Test() {}

    void my_func(void (*f)())
    {
        cout << "In My Function" << endl;
        f(); //Invoke callback function
    }

    static void callback_func()
    {cout << "In Callback function" << endl;}
};

int main()
{
    Test Obj;
    Obj.my_func(Obj.callback_func);
}

Answer 1

A member function is a function that need a class instance to be called on. Members function cannot be called without providing the instance to call on to. That makes it harder to use sometimes.

A static function is almost like a global function : it don't need a class instance to be called on. So you only need to get the pointer to the function to be able to call it.

Take a look to std::function (or std::tr1::function or boost::function if your compiler doesn't provide it yet), it's useful in your case as it allow you to use anything that is callable (providing () syntax or operator ) as callback, including callable objects and member functions (see std::bind or boost::bind for this case).