Storing member function of an object as a callback function in another object

Question

I'm new with call-back function. I wrote a Timer class. It stores callback functions in it and calls them back after their respective periods.

Timer.h

#ifndef UTILITY_CLASSES__TIMER_H
#define UTILITY_CLASSES__TIMER_H

#include <stdint.h>
#include <list>
#include <Windows.h>

class Timer
{
    public:
        static uint64_t LAST_TIMER_ID;
        uint64_t TimerId;
        uint64_t KillId;
        typedef void (*TimerCallBackFunction) (DWORD, uint64_t);

        Timer(UINT Period, TimerCallBackFunction CbFunction);
        ~Timer();

    protected:
        class TimerInfo
        {
            public:
                TimerInfo(TimerCallBackFunction Cbf, uint64_t Tid, uint64_t Kid)
                    : Callback(Cbf), TimerId(Tid), KillId(Kid) {}
                TimerCallBackFunction Callback;
                uint64_t TimerId;
                uint64_t KillId;
        };

        static VOID CALLBACK TimerProc( _In_  HWND hwnd,
                                        _In_  UINT uMsg,
                                        _In_  UINT_PTR idEvent,
                                        _In_  DWORD dwTime);

        static std::list<TimerInfo> Timers;
};

#endif  // UTILITY_CLASSES__TIMER_H

Timer.cpp

#include "Timer.h"

#include <Windows.h>

uint64_t Timer::LAST_TIMER_ID = 0;

Timer::Timer(UINT Period, TimerCallBackFunction CbFunction)
{
    TimerId = ++LAST_TIMER_ID;
    KillId = SetTimer(NULL, TimerId, Period, Timer::TimerProc);
    if (KillId == NULL)
    {
        LAST_TIMER_ID--;    // roll back
        TimerId = 0;
    } 
    else
    {
        Timers.push_back(TimerInfo(CbFunction, TimerId, KillId));
    }
}

Timer::~Timer()
{
    for (std::list<TimerInfo>::const_iterator it=Timers.begin(); it!=Timers.end(); ++it)
    {
        if (it->TimerId == TimerId)
        {
            Timers.erase(it);
            break;
        }
    }
}

VOID CALLBACK Timer::TimerProc( _In_ HWND hwnd,
                                _In_ UINT uMsg,
                                _In_ UINT_PTR idEvent,
                                _In_ DWORD dwTime)
{
    for (std::list<TimerInfo>::const_iterator it=Timers.begin(); it!=Timers.end(); ++it)
    {
        if (it->TimerId == idEvent)
        {
            it->Callback(dwTime, it->TimerId);
            break;
        }
    }
}

main.cpp

#include <Windows.h>
#include "Timer.h"

class MyClass
{
    public:
        void CallMe(DWORD Time, uint64_t TimerId)
        {
            MessageBoxW(NULL, L"Callback function is called!",
                        L"Notification", MB_ICONINFORMATION);
        }
};

int APIENTRY wWinMain(  _In_        HINSTANCE   hInstance,
                        _In_opt_    HINSTANCE   hPrevInstance,
                        _In_        LPTSTR      lpCmdLine,
                        _In_        int         nCmdShow)
{
    // ...

    MyClass MyObject;
    Timer TimerObject(10000, MyObject.CallMe);  // Line 114

    //...
    return 0;
}

I get this error message when I compile this code:

Line 114
error C3867:
'MyClass::CallMe': function call missing argument list; use '&MyClass::CallMe' to create a pointer to member

I can't get my code run when I change MyObject.CallMe to &MyClass::CallMe or &MyObject.CallMe at line 114.

What am I doing wrong in my code?

---

IDE: Microsoft Visual Studio 2012
Documentations: SetTimer, TimerProc

Answer 1

typedef void (*TimerCallBackFunction) (DWORD, uint64_t);

declares a typedef for a pointer to free function and these can't be used to point to member functions. You need a pointer to member function:

typedef void (MyClass::*TimerCallBackFunction) (DWORD, uint64_t);

You'll need to forward declare MyClass before you make that typedef and also do what the error message says:

Timer TimerObject(10000, &MyObject::CallMe);
                      // ^^^^^^^^^^^^^^^^^

Recommended reading: Pointers to member functions section on C++ FAQ Lite.

Answer 2

I would do with using std::function rather than a function pointer, and then use std::bind with placeholders:

Change the typedef to

    typedef std::function<void (DWORD, uint64_t)> TimerCallBackFunction;

and in WinMain(), bind a std::function to CallMe() of the instance MyObject, which will then be called with the two parameters replacing the placeholders.

MyClass MyObject;
Timer TimerObject(10000, std::bind(&MyClass::CallMe,
                                   MyObject,
                                   std::placeholders::_1,
                                   std::placeholders::_2));