Store Function Pointers to any Member Function

Question

My Event Manager

For a event manager I need to store many pointers to functions in a vector to call them when the event is triggered. (I will provide the source code of the EventFunction helper class at the end of this question.)

// an event is defined by a string name and a number
typedef pair<string, int> EventKey;

// EventFunction holds a pointer to a listener function with or without data parameter
typedef unordered_map<EventKey, vector<EventFunction>> ListEvent;

// stores all events and their listeners
ListEvent List;

Registering an listener could be done by calling the first or the second function, depending on if you want receive additional data or not. (This code is from my event manager class.)

public:
  typedef void (*EventFunctionPointer)();
  typedef void (*EventFunctionPointerData)(void* Data);

  // let components register for events by functions with or without data parameter,
  // internally simple create a EventFunction object and call the private function
  void ManagerEvent::Listen(EventFunctionPointer Function, string Name, int State);
  void ManagerEvent::Listen(EventFunctionPointerData Function, string Name, int State);

private:
  void ManagerEvent::Listen(EventFunction Function, string Name, int State)
  {
    EventKey Key(Name, State);
    List[Key].push_back(Function);
  }  

Member Function Pointers

That code doesn't work because I store function pointers but not member function pointers in my List. All these pointers should be member function pointers because a component like ComponentSound will listen to the event "PlayerLevelup" with on of its member functions ComponentSound::PlayerLevelup to play a nice sound if the event is triggered.

A member function pointer in C++ looks like this.

// ReturnType (Class::*MemberFunction)(Parameters);
void (ComponentSound::*PlayerLevelup)();

The problem is, any component class should be able to listen for events, but storing the member function pointers in the event manager requires me to specify the listening class. As you can see in the example, I need to specify ComponentSound but the event manager should simply have a vector of member function pointers to any class.

Question

An Answer to one of these question would help me a lot.

• How can I store function pointers to any member function in a vector in my event manager? (Maybe it helps that all the listening functions are inherited from one abstract class Component.)
• How can I design my event manager in another way to reach the aimed functionality? (I want to use string and int keys for messages.)

I tried to keep my question general but if you need more informations or code please comment.

Assignments

In my vector of member function pointers I use EventFunction instead of only a pointer to provide two message types. One with, and one without a data parameter.

class EventFunction
{
private: EventFunctionPointer Pointer; EventFunctionPointerData PointerData; bool Data;
public:
    EventFunction(EventFunctionPointer Pointer) : Pointer(Pointer), PointerData(NULL), Data(false) { }
    EventFunction(EventFunctionPointerData PointerData) : PointerData(PointerData), Pointer(NULL), Data(true) { }
    EventFunctionPointer GetFunction() { return Pointer; }
    EventFunctionPointerData GetFunctionData() { return PointerData; } bool IsData() { return Data; }
    void Call(void* Data = NULL){ if(this->Data) PointerData(Data); else Pointer(); }
};

Answer 1

You will have to use std::function. This is the only way to achieve a generic callback. As soon as you involve function pointers instead of function objects, it is not generic, will never be generic, and can never be made to be generic.

unordered_map<string, vector<std::function<void()>>>

Function pointers are bad and should never be explicitly used in C++, only passed to templates like std::bind and std::function's constructor, and member function pointers are even worse.

Answer 2

You can use functors to achieve this. If you wrap a functor around your member functions you can make a vector out of functors. A functor looks like this:

template <class T> class MyFunctor
{
private:
    T* ObjectPtr;
    void (T::*MemberFunction) ();
public:
    void operator () ()
    {
        return (*this->ObjectPtr.*this->MemberFunction)();
    }
};

So basically a functor overrides the () operator and returns the member function stored in the functor class. Functors can be quite complex if you want them to work with different signatures but in this article you can get further information.

http://www.codeproject.com/Articles/7112/Pointers-to-Member-Functions-and-Functors