How to use a typedef function pointer to register

Question

I'm trying to implement an observer pattern (of sorts) with C++ and I want to use function pointer to do so, but I keep getting an error when trying to cast a function pointer from class B to a typedef function pointer:

#include <map>

typedef int (*OutputEvent)(const char*, const char*, int);

class A
{
private:
    int nextListenerId;
    std::map<int, OutputEvent> listenerMap;
public:
    A(){ nextListenerId = 0;}
    ~A(){}
    inline int RegisterListener(OutputEvent callback)
    {
        nextListenerId++;
        listenerMap[nextListenerId] = callback;
        return nextListenerId;
    }
};

class B
{
private:
    int listenerId;
public:
    B(const A& a)
    {
        OutputEvent e = &B::CallMeBack;
        listenerId = a.RegisterListener(e);
    }
    ~B(){}

    int CallMeBack(const char* x, const char* y, int z)
    {
        return 0;
    }
};

I created this example and I've pasted it into codepad.org, but when I it fails to compile (it doesn't compile in codepad.org nor in Visual Studio 2010):

Output:

t.cpp: In constructor 'B::B(const A&)':
Line 28: error: cannot convert 'int (B::*)(const char*, const char*, int)' to 'int (*)(const char*, const char*, int)' in initialization
compilation terminated due to -Wfatal-errors.

I don't understand why it can't convert the function pointers. Could anybody help me please?

Answer 1

The function you are trying to cast to OutputEvent is a member function. This is represented clearly in the error message by this:

'int (B::*)(const char*, const char*, int)'

which is a different type than

 int (*OutputEvent)(const char*, const char*, int)

because of the B:: part (that means that the function has an invisible this parameter).

If you define your member function as static, then you will be able to cast it to OutputEvent:

class B
{
   ....
   static int CallMeBack(const char* x, const char* y, int z);
   ...
 };

Answer 2

The member function doesn't match the typedef'd prototype because member functions have an invisible 'this' parameter. Make it a static function and it should work.

Answer 3

The class member function has got a hidden parameter, the object, which the global function does not have.

Do something like this:-

B*pB;

int CallMeBack(const char* x, const char* y, int z)
{
    return pB->CallMeBack(x,y,z);
}

More difficult if you have several callbacks on the go at the same time. But if there is only one you can have a pointer to the object and call via this.