Converting Member Function Pointer to TIMERPROC

Question

How do I convert a member function pointer to the TIMERPROC type for use with the WINAPI SetTimer? The code snippet below shows how I'm doing it now, but when I compile I get this error:

error C2664: 'SetTimer' : cannot convert parameter 4 from 'void (__stdcall CBuildAndSend::* )(HWND,UINT,UINT_PTR,DWORD)' to 'TIMERPROC'

The callback needs to be tied to its originating class instance. If there is a better way to do that, I'm all ears. Thanks.

class CMyClass
{
public:
    void (CALLBACK CBuildAndSend::*TimerCbfn)( HWND hwnd, UINT uMsg, UINT_PTR idEvent, DWORD dwTime );

private:
    void CALLBACK TimeoutTimerProc( HWND hwnd, UINT uMsg, UINT_PTR idEvent, DWORD dwTime );
};

CMyClass::CMyClass()
{
    ...

    this->TimerCbfn = &CBuildAndSend::TimeoutTimerProc;

    ...

    ::CreateThread(
        NULL,                           // no security attributes
        0,                              // use default initial stack size
        reinterpret_cast<LPTHREAD_START_ROUTINE>(BasThreadFn), // function to execute in new thread
        this,                           // thread parameters
        0,                              // use default creation settings
        NULL                            // thread ID is not needed
        )
}

void CALLBACK CMyClass::TimeoutTimerProc( HWND hwnd, UINT uMsg, UINT_PTR idEvent, DWORD dwTime )
{
    ...
}

static DWORD MyThreadFn( LPVOID pParam )
{
    CMyClass * pMyClass = (CMyClass *)pParam;

    ...

    ::SetTimer( NULL, 0, BAS_DEFAULT_TIMEOUT, pMyClass->TimerCbfn ); // <-- Error Here

    ...
}

BTW, any time you have a cast with a function (or function pointer) in a win32 API, it usually indicates there's some sort of error. If it doesn't compile as-is, then usually the function needs to be changed; adding a cast is just papering over the problem. Lucky for you, turns out that C++ (pointers to) member functions are a completely different beast than (pointers to) plain functions, so not even reinterpret_cast is possible in this case. — BrendanMcK

Nawaz Nawaz · Accepted Answer · 2011-06-24T19:45:16

Member-function and TIMEPROC are not compatible types.

You need to make the member function static. Then it will work, assuming parameter-list is same in both, static member function and TIMEPROC.

class CMyClass
{
public:
    //modified
    void (CALLBACK *TimerCbfn)(HWND hwnd, UINT uMsg, UINT_PTR idEvent, DWORD dwTime);

private:
    //modified
    static void CALLBACK TimeoutTimerProc(HWND hwnd, UINT uMsg, UINT_PTR idEvent, DWORD dwTime );
};

Function pointer as well as member function both are modified. Now it should work.

Now since the callback function became static, it cannot access the non-static members of the class, because you don't have this pointer in the function.

To access the non-static members, you can do this:

class CMyClass
{
public:

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

    //add this static member
    static std::map<UINT_PTR, CMyClass*> m_CMyClassMap; //declaration
};

//this should go in the  CMyClass.cpp file
std::map<UINT_PTR, CMyClass*> CMyClass::m_CMyClassMap;  //definition

static DWORD MyThreadFn( LPVOID pParam )
{
    CMyClass * pMyClass = (CMyClass *)pParam;

    UINT_PTR id = ::SetTimer( NULL, 0, BAS_DEFAULT_TIMEOUT, CMyClass::TimerProc);

    //store the class instance with the id as key!        
    m_CMyClassMap[id]= pMyClass; 
}

void CALLBACK CMyClass::TimerProc( HWND hwnd, UINT uMsg, UINT_PTR idEvent, DWORD dwTime )
{
    //retrieve the class instance
    CMyClass *pMyClass= m_CMyClassMap[idEvent];

    /*
      now using pMyClass, you can access the non-static 
      members of the class. e.g
      pMyClass->NonStaticMemberFunction();
    */
}

I removed TimerCbfn from my implementation, as it doesn't really needed. You can pass TimerProc directly to SetTimer as last argument.

Converting Member Function Pointer to TIMERPROC

1 Answers