Convert Callback Function to DWORD_PTR in C++

Actually newbie in c++. I wrote this code but says cannot convert from "waveOutProc" to "DWORD_PTR".Would you tell me how to fix it?

thanks

void CALLBACK Audio::waveOutProc(HWAVEOUT hWaveOut, unsigned int uMsg,
unsigned long dwInstance, unsigned long dwParam1,
unsigned long dwParam2)
{
/*
* pointer to free block counter
*/
int* freeBlockCounter = (int*)dwInstance;
/*
* ignore calls that occur due to openining and closing the
* device.
*/
if(uMsg != WOM_DONE) { return ; }
EnterCriticalSection(&waveCriticalSection) ;
(*freeBlockCounter)++ ;
LeaveCriticalSection(&waveCriticalSection) ;
}

//////////////////////////////////////////////////////////////////////////////////

void Audio::playSound(const char* filename)
{
HWAVEOUT hWaveOut ;
HANDLE hFile;
WAVEFORMATEX wfx ;
char buffer[1024];
int i;

...

if(waveOutOpen(
&hWaveOut,
WAVE_MAPPER,
&wfx,
(DWORD_PTR)waveOutProc,            ///////////Error Point
(DWORD_PTR)&waveFreeBlockCount,
CALLBACK_FUNCTION
) != MMSYSERR_NOERROR) {
fprintf(stderr, "unable to open wave mapper device\n");
ExitProcess(1);
}

...
}

Solution

Converting function pointers to data pointers is an undefined behaviour so you shouldn't do this in the first place. (I understand the win api function is expecting this).

Also You cannot pass member functions as callbacks in C/C++ unless you handle the implicit this argument.

Your target callback has the following signature

void CALLBACK waveOutProc(
  HWAVEOUT hwo,
  UINT uMsg,
  DWORD_PTR dwInstance,
  DWORD_PTR dwParam1,
  DWORD_PTR dwParam2
);

While Audio::waveOutProc is probably a member function which has an implicit this arguement.

void CALLBACK waveOutProc(Audio*,
      HWAVEOUT hwo,
      UINT uMsg,
      DWORD_PTR dwInstance,
      DWORD_PTR dwParam1,
      DWORD_PTR dwParam2
    );

Just define waveOutProc as static or free function instead.