How to create pointer to callback function in MFC

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I'm working on demo bio metric project in MFC
On msdn we have some callback functions but for use that we need to create pointer to member function how to do it.

Getting this error:

Error 1 error C2440: '=' : cannot convert from 'void (__stdcall CBioMetricApplicationDlg::* )(PVOID,HRESULT,WINBIO_UNIT_ID,PWINBIO_BIR,SIZE_T,WINBIO_REJECT_DETAIL)' to 'void (__thiscall CBioMetricApplicationDlg::* )(PVOID,HRESULT,WINBIO_UNIT_ID,PWINBIO_BIR,SIZE_T,WINBIO_REJECT_DETAIL)'

What I have tried:

// in header file CBioMetricApplicationDlg.h ...

VOID CALLBACK CaptureCallback(
__in_opt PVOID CaptureCallbackContext,
__in HRESULT OperationStatus,
__in WINBIO_UNIT_ID UnitId,
__in_bcount(SampleSize) PWINBIO_BIR Sample,
__in SIZE_T SampleSize,
__in WINBIO_REJECT_DETAIL RejectDetail
);


VOID CALLBACK(CBioMetricApplicationDlg:: *pCaptureCallback) (
__in_opt PVOID CaptureCallbackContext,
__in HRESULT OperationStatus,
__in WINBIO_UNIT_ID UnitId,
__in_bcount(SampleSize) PWINBIO_BIR Sample,
__in SIZE_T SampleSize,
__in WINBIO_REJECT_DETAIL RejectDetail
);

// in cpp file CBioMetricApplicationDlg.cpp
//in the OnInitDialog() 

// TODO: Add extra initialization here
pCaptureCallback = &CBioMetricApplicationDlg::CaptureCallback;

Posted 19-Apr-17 1:02am

Premnath Mali

Updated 19-Apr-17 22:35pm

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2 solutions

Solution 1

The address of the object (this)is the first Implicit parm to any class method. So try declaring pcapturecallback as:
Void callback (cbiometricapplicationdlg::*pcapturecallback)(cbiometricapplicationdlg *thisobj, __in__opt pvoid capturecallbackcontext, <rest of params>);
does it work?

Posted 19-Apr-17 20:02pm

RAMASWAMY EKAMBARAM

Comments

Jochen Arndt 20-Apr-17 4:36am

A callback function is static and has therefore no this pointer.

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Jochen Arndt · Accepted Answer · 2017-04-19T22:35:00

Solution 2

It is common to use a typedef for such functions because it makes it easier to declare pointers.

For your case see PWINBIO_CAPTURE_CALLBACK function pointer (Windows)[^]:

typedef VOID ( CALLBACK *PWINBIO_CAPTURE_CALLBACK)(
  _In_opt_ PVOID                CaptureCallbackContext,
  _In_     HRESULT              OperationStatus,
  _In_     WINBIO_UNIT_ID       UnitId,
  _In_     PWINBIO_BIR          Sample,
  _In_     SIZE_T               SampleSize,
  _In_     WINBIO_REJECT_DETAIL RejectDetail
);

Then declare the variable as

PWINBIO_CAPTURE_CALLBACK pCaptureCallback;

[EDIT]
I was not sure how CALLBACK is resolved. But it seems that it does not contain static. Because callback functions must be static, the declaration as class member function must be

/* CBioMetricApplicationDlg.h */
static VOID CALLBACK CaptureCallback(
    __in_opt PVOID CaptureCallbackContext,
    __in HRESULT OperationStatus,
    __in WINBIO_UNIT_ID UnitId,
    __in_bcount(SampleSize) PWINBIO_BIR Sample,
    __in SIZE_T SampleSize,
    __in WINBIO_REJECT_DETAIL RejectDetail
    );

To get access to the class within the callback function pass this as context parameter:

/* CBioMetricApplicationDlg.cpp */
hr = WinBioCaptureSampleWithCallback(
    sessionHandle,
    WINBIO_NO_PURPOSE_AVAILABLE,
    WINBIO_DATA_FLAG_RAW,
    CaptureSampleCallback,
    this);

and cast the parameter in the callback function:

/* CBioMetricApplicationDlg.cpp */
VOID CBioMetricApplicationDlg::CaptureCallback(
    __in_opt PVOID CaptureCallbackContext,
    __in HRESULT OperationStatus,
    __in WINBIO_UNIT_ID UnitId,
    __in_bcount(SampleSize) PWINBIO_BIR Sample,
    __in SIZE_T SampleSize,
    __in WINBIO_REJECT_DETAIL RejectDetail
    )
{
    CBioMetricApplicationDlg *pThis = reinterpret_cast<CBioMetricApplicationDlg*>(CaptureCallbackContext);
    // Use pThis to access class members
    // ...
}

[/EDIT]

Posted 19-Apr-17 22:35pm

Jochen Arndt

Updated 20-Apr-17 2:35am

v2

Comments

Premnath Mali 20-Apr-17 7:21am

getting this error while initialization
pCaptureCallback = &CBioMetricApplicationDlg::CaptureCallback;

Jochen Arndt 20-Apr-17 7:28am

Still the same error?
I can't test it here at the moment.
You can try to simplify the typedef:

typedef VOID (*PWINBIO_CAPTURE_CALLBACK)(
  PVOID                CaptureCallbackContext,
  HRESULT              OperationStatus,
  WINBIO_UNIT_ID       UnitId,
  PWINBIO_BIR          Sample,
  SIZE_T               SampleSize,
  WINBIO_REJECT_DETAIL RejectDetail
);

This assumes that CALLBACK resolves to something containing static.
If not you have to declare the function as static in your header file:

static VOID CALLBACK CaptureCallback(
// ...

Premnath Mali 20-Apr-17 8:10am

//in header file
VOID CALLBACK CaptureCallback(
__in_opt PVOID CaptureCallbackContext,
__in HRESULT OperationStatus,
__in WINBIO_UNIT_ID UnitId,
__in_bcount(SampleSize) PWINBIO_BIR Sample,
__in SIZE_T SampleSize,
__in WINBIO_REJECT_DETAIL RejectDetail
);

typedef VOID ( CALLBACK *PWINBIO_CAPTURE_CALLBACK)(
  _In_opt_ PVOID                CaptureCallbackContext,
  _In_     HRESULT              OperationStatus,
  _In_     WINBIO_UNIT_ID       UnitId,
  _In_     PWINBIO_BIR          Sample,
  _In_     SIZE_T               SampleSize,
  _In_     WINBIO_REJECT_DETAIL RejectDetail
);

PWINBIO_CAPTURE_CALLBACK pCaptureCallback;

//in onInitDialog()
pCaptureCallback = &CBioMetricApplicationDlg::CaptureCallback;

//in Cpp file Defination of callback method
VOID CALLBACK CBioMetricApplicationDlg::CaptureCallback(PVOID CaptureCallbackContext, 
														HRESULT OperationStatus, 
														WINBIO_UNIT_ID UnitId, 
														PWINBIO_BIR Sample, 
														SIZE_T SampleSize, 
														WINBIO_REJECT_DETAIL RejectDetail)
{
	//defination
}

and the error is...

error C2440: '=' : cannot convert from 'void (__stdcall CBioMetricApplicationDlg::* )(PVOID,HRESULT,WINBIO_UNIT_ID,PWINBIO_BIR,SIZE_T,WINBIO_REJECT_DETAIL)' to 'CBioMetricApplicationDlg::PWINBIO_CAPTURE_CALLBACK'

Jochen Arndt 20-Apr-17 8:16am

So CALLBACK is not containing static (use follow symbol in the context menu)?

Then you must declare the callback function as static in the header file (2nd option from my previous comment).

Premnath Mali 20-Apr-17 8:20am

Also this error I'm getting
error C2724: 'CBioMetricApplicationDlg::CaptureCallback' : 'static' should not be used on member functions defined at file scope

Premnath Mali 20-Apr-17 8:16am

if I use static callback method then I will not able to access the other member of the class...

Jochen Arndt 20-Apr-17 8:26am

Callback functions MUST be static.
They usually provide a parameter that can be used to pass a pointer to the class.

In your case it is the CaptureCallbackContext<code> parameter. Call <code>WinBioCaptureSampleWithCallback(this, ...) and cast the parameter in the callback function:
CBioMetricApplicationDlg *pThis = (CBioMetricApplicationDlg*)CaptureCallbackContext;

Premnath Mali 20-Apr-17 8:34am

okay I declare that method as static but how I can give definition
if I give like this

static VOID CALLBACK CBioMetricApplicationDlg::CaptureCallback(PVOID CaptureCallbackContext, 
														HRESULT OperationStatus, 
														WINBIO_UNIT_ID UnitId, 
														PWINBIO_BIR Sample, 
														SIZE_T SampleSize, 
														WINBIO_REJECT_DETAIL RejectDetail)
{
	//defination
}

then I'm getting this error
error C2724: 'CBioMetricApplicationDlg::CaptureCallback' : 'static' should not be used on member functions defined at file scope

and if I'm giving definition without static keyword like then I'm getting following error

Error 1 error LNK2001: unresolved external symbol "public: static void __stdcall CBioMetricApplicationDlg::IdentifyCallback(void *,long,unsigned long,struct _WINBIO_IDENTITY *,unsigned char,unsigned long)" (?IdentifyCallback@CBioMetricApplicationDlg@@SGXPAXJKPAU_WINBIO_IDENTITY@@EK@Z) BioMetricApplicationDlg.obj

Jochen Arndt 20-Apr-17 8:37am

See my updated answer.

The static keyword must be used for the declaration in the header file but not for the function definition in the source file.

Premnath Mali 20-Apr-17 8:48am

Thanks!
Now it's working

Jochen Arndt 20-Apr-17 8:52am

Fine to hear that and thank you for the feedback and accepting my solution.

But I should have noticed earlier that your functon was not static (the __thiscall from the initial error message is the hint).

Premnath Mali 21-Apr-17 0:36am

Sir,
Sorry to interrupt you again, but one little problem is there while using

pThis->ShowMessage(_T("Message"));

where ShowMessage is member function

//def of ShowMessage()
void CBioMetricApplicationDlg::ShowMessage(CString msg)
{
	CString preMsg = _T("\n::::Keep Looking at Instructions::::\n\n");
	m_lblMessage.SetText(preMsg + msg);
}

and the error is occurred run time...

Unhandled exception at 0x003a1d1e in BioMetricApplication.exe: 0xC0000005: Access violation reading location 0x000000a0.

Jochen Arndt 21-Apr-17 2:44am

Generally avoid calling MFC GUI functions from callback functions. MFC functions are not thread safe and such IO callback functions are usually called from different threads (device drivers).

Depending on what you want to do you can use shared buffers for data, shared variables for status, or post user defined messages to your MFC class.

But this is something different and is not really related to the initial question.

However, the error may be sourced also elsewhere (check if pThis / the passed CaptureCallbackContext is a valid pointer to your class).

Premnath Mali 21-Apr-17 3:09am

yes it is valid pointer. An application is stopped at

m_lblMessage.SetText(preMsg+msg);

but as your suggestion I will post messages to mfc class

thank you for the more information about call functions in mfc.

Thanks Once again!