Function Pointer (Within Class)

Question

I'm trying to use a function pointer (fnPtr) to reference a class method (fnEmpty) from within another class method, (in this case the constructor).

The setup program main.cpp is

#include <iostream>
#include "test.hpp"

int main(int argc, char* argv[]){
    Test test;
    return 0;
}

and a test.hpp that reads

#ifndef _test_h
#define _test_h
#include <iostream>

class Test {
    private:

    public:
        void (*ptrEmpty)(void);
        void fnEmpty(){ std::cout << "Got Here" << std::endl;};
        Test(){ ptrEmpty = fnEmpty; };
        ~Test(){};
};

#endif

Most of the material on the net details being able to reference a class method from outside the class (see microsoft page and newty.de), but not from inside the class (as I have done).

Using the above files gives the error

In file included from main.cpp:6: ./test.hpp:11:21: error: reference to non-static member function must be called;

So 1) why does this error result?

If I change fnPtr declaration on line 10 to be a static so it reads

static void fnEmpty(){ std::cout << "Got Here" << std::endl;};

This seems to solve the problem. But that begs the question 2) Why does it accept a 'static' if the error says only a 'non-static' is allowed?

If I remove the static and instead change line 11 (by adding the &) so it reads

        Test(){ ptrEmpty = &fnEmpty; };

I get a whole new error that reads

./test.hpp:11:21: error: must explicitly qualify name of member function when taking its address

Unfortunately replacing with

        Test(){ Test::ptrEmpty = Test::&fnEmpty; };

Or variations not including the Test:: prefix as was done in the links given above do not solve the problem.

I was under the impression that as far as function pointers go, when referencing a function (say fn),

ptr = fn;

is equivalent to

ptr = &fn;

3) So why am I getting two different sets of errors when I include and exclude the '&'

So the ultimate question I have is::

4) What is the correct way to point to a class method (from within a different method of the same class)? Do I need to include the 'Classname::' qualifier when Its within the same class?

Thanks, Jeff

Answer 1

fnEmpty is a non-static member function, which means it takes an implicit first argument, a pointer to the Test instance it's being invoked on (the this pointer). So the type of a pointer to that member function is

void (Test::*ptrEmpty)();

Next, to form a pointer to a member function, you need to use the syntax &ClassName::MemFnName So, within your constructor

ptrEmpty = &Test::fnEmpty;

Now your example should compile. You can invoke the function that ptrEmpty points to by using

Test t;
(t.*(t.ptrEmpty))();

When you change fnEmpty to be static it no longer receives the implicit this pointer argument, so it's just like any other pointer to function, and your code compiles.