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Using function pointer in callback function

P: n/a

Hi,

I am writing an app which encapsulates a multimedia timer. I implemented
a TimerProc as static member function and a static member variable pThis
as pseudo this variable to access in the static TimerProc function.
timeSetEvent uses TimerProc to set the callback function.

m_pCallback is a function that is passed using the
SetTimerCallbackFunction. It's a function that an object owning the
CMMTimer object can pass to the CMMTimer class. What I want is that
every time the TimerProc function is called, the m_pCallback function is
executed, resulting in the object owning the timer processing the timer.

Header file looks like this:

/***************************************/

typedef void (CALLBACK * MMTIMERCALLBACK) (TRIGGER eTrigger, UINT uID,
int iVal);

CMMTimer

{

public:

BOOL Stop();

BOOL Start(UINT uPeriod);

BOOL Initialize(UINT uResolution);

CMMTimer();

virtual ~CMMTimer();

void SetTimerCallbackFunction(MMTIMERCALLBACK pCallback);

private:

MMTIMERCALLBACK m_pCallback;

UINT m_uResolution;

UINT m_uPeriod;

UINT m_uTimerID;

BOOL m_bInitialized;

static CMMTimer *pThis;

static void CALLBACK TimerProc(UINT wTimerID, UINT msg, DWORD dwUser,
DWORD dw1, DWORD dw2);

}

/***************************************/

Parts of cpp file look like this

/***************************************/

CMMTimer* CMMTimer::pThis = NULL;

void CMMTimer::SetTimerCallbackFunction(MMTIMERCALLBACK pCallback)

{

m_pCallback = pCallback;

}

BOOL CMMTimer::Start(UINT uPeriod)

{

if (!m_bInitialized) return FALSE;

m_uPeriod = uPeriod;

pThis = this;

m_uTimerID =

timeSetEvent(

m_uPeriod,

m_uResolution,

TimerProc,

(DWORD) this,

TIME_PERIODIC );

if(! m_uTimerID)

return FALSE;

else

return TRUE;

}

void CALLBACK TimerProc(UINT wTimerID, UINT msg, DWORD dwUser, DWORD
dw1, DWORD dw2)

{

// I tried this

CMMTimer * pseudoThis = (CMMTimer *) dwUser;

pseudoThis->m_pCallback(<TIMERT, 0, 0); #error#

// and this

pThis->m_pCallback(<TIMERT, 0, 0); #error#

}

I also tried helper functions.

The problem is that this code compiles perfectly, but when I execute the
code, the debugger shows problems at the lines #error#. m_pCallback
cannot be called, the pointer points at memory address 0xcccccccccccccc

I really would like this problem solved.

Thank you

pvdm

}
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Jul 19 '05 #1
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1 Reply


P: n/a
On Tue, 09 Sep 2003 05:02:58 -0400, pvdm <me*********@dbforums.com>
wrote:

Hi,

I am writing an app which encapsulates a multimedia timer. I implemented
a TimerProc as static member function and a static member variable pThis
as pseudo this variable to access in the static TimerProc function.
Why do you need pThis? You are passing the this argument to the
callback anyway.
timeSetEvent uses TimerProc to set the callback function. m_pCallback is a function that is passed using the
SetTimerCallbackFunction. It's a function that an object owning the
CMMTimer object can pass to the CMMTimer class. What I want is that
every time the TimerProc function is called, the m_pCallback function is
executed, resulting in the object owning the timer processing the timer.
Right, sounds reasonable.
Header file looks like this:

typedef void (CALLBACK * MMTIMERCALLBACK) (TRIGGER eTrigger, UINT uID,
int iVal);

CMMTimer
class CMMTimer?

{

public:

BOOL Stop();

BOOL Start(UINT uPeriod);

BOOL Initialize(UINT uResolution);

CMMTimer();

virtual ~CMMTimer();

void SetTimerCallbackFunction(MMTIMERCALLBACK pCallback);

private:

MMTIMERCALLBACK m_pCallback;

UINT m_uResolution;

UINT m_uPeriod;

UINT m_uTimerID;

BOOL m_bInitialized;

static CMMTimer *pThis;
I don't see any use of pThis. Remove it.

static void CALLBACK TimerProc(UINT wTimerID, UINT msg, DWORD dwUser,
DWORD dw1, DWORD dw2);

}
CMMTimer* CMMTimer::pThis = NULL;
Again, scrub this.
void CMMTimer::SetTimerCallbackFunction(MMTIMERCALLBACK pCallback)

{

m_pCallback = pCallback;

}

BOOL CMMTimer::Start(UINT uPeriod)
{
if (!m_bInitialized) return FALSE;
m_uPeriod = uPeriod;
pThis = this;
What if you have more than one timer at a time?

In any case, you need to check the callback here:

if (!m_pCallback)
{
return FALSE;
}

and make sure m_pCallback is initialized to NULL by your constructor.
m_uTimerID =
timeSetEvent(
m_uPeriod,
m_uResolution,
TimerProc,
(DWORD) this,
TIME_PERIODIC );
if(! m_uTimerID)
return FALSE;
else
return TRUE;
}
That looks ok.
void CALLBACK TimerProc(UINT wTimerID, UINT msg, DWORD dwUser, DWORD
dw1, DWORD dw2)

{

// I tried this

CMMTimer * pseudoThis = (CMMTimer *) dwUser;

pseudoThis->m_pCallback(<TIMERT, 0, 0); #error#
That looks correct, apart from "<TIMERT" which isn't valid C++.
// and this

pThis->m_pCallback(<TIMERT, 0, 0); #error#
No need for this. Where is pThis initialized, anyway?
}

I also tried helper functions.
How so?
The problem is that this code compiles perfectly, but when I execute the
code, the debugger shows problems at the lines #error#. m_pCallback
cannot be called, the pointer points at memory address 0xcccccccccccccc
That suggests that m_pCallback hasn't been initialized. Perhaps you
are calling SetTimerCallbackFunction on a different instance of your
timer class to the one you call Start on.
I really would like this problem solved.


The code you've posted looks generally ok. It must be the code using
your timer class that is at fault. However, drop the static member
variable, and add the test to Start, and you'll probably find your
problem.

Tom
Jul 19 '05 #2

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