CALLBACK aka __stdcall

[Igor Mihailov]

Hello all. Help me please: I need to make a callback function as a member of my class (Win32 API, no MFC). Is it possible? Thanks.

[palbano]

sure but it has to be static which means there is no object instance scope when the function is called. -pete

[Lost User]

Sorry, I can't fully understand the idea. Can you give a code example?

[Michael Dunn]

See the FAQ 6.1 Why can't I use a member function as a callback?[^] --Mike-- Personal stuff:: Ericahist | Homepage Shareware stuff:: 1ClickPicGrabber | RightClick-Encrypt CP stuff:: CP SearchBar v2.0.2 | C++ Forum FAQ ---- There is a saying in statistics that a million monkeys pounding on typewriters would eventually create a work of Shakespeare. Thanks to the Internet, we now know that this is not true.

[palbano]

/** Win32 CreateThread class wrapper */
class base_w32thread{
protected:
HANDLE _handle;
DWORD _dwTID;
base_w32thread():_handle(0), _dwTID(0L){}
virtual ~base_w32thread()=0;

public:
bool start(LPSECURITY_ATTRIBUTES psecattrs = NULL, DWORD dwCreateFlags = 0L);
operator HANDLE(){ return _handle; }
protected:
virtual void run()=0;
virtual void onEndThread(){}
static long WINAPI threadfnc( LPARAM lp);
};

threadfnc is the callback that is sent as the argument to CreateThread() below

/** start the thread */
bool base_w32thread::start(LPSECURITY_ATTRIBUTES psecattrs, DWORD dwCreateFlags){

assert( !\_handle);
if( \_handle)
	return false;

\_handle = ::CreateThread( psecattrs, 0, 
	(LPTHREAD\_START\_ROUTINE)threadfnc, this, dwCreateFlags, &\_dwTID);
return (\_handle)?true:false;

}

Then in the callback function there is no "this" pointer so we have sent the object pointer as the User Parameter to CreateThread and we cast it to the correct type

/** Thread function */
long WINAPI base_w32thread::threadfnc( LPARAM lp){

assert( lp);
base\_w32thread\* pThis = (base\_w32thread\*)lp;
if( pThis)
	pThis->run();

pThis->\_handle = 0L;		// thread exited
pThis->onEndThread();
return 0L;

}

Hope that helps

"No matter where you go, there your are." - Buckaroo Banzai

-pete

[Igor Mihailov]

Thanks a lot. I'll try my best.

[Indrawati]

Just curious... is CALLBACK exaactly the same ad __stdcall? Can I use them interchangeably?

[Archer282]

I had a problem with CALLBACK functions in MFC when i made my process/windows viewer someone on CodePrject showed me a way to get around this, ill declare one function as static BOOL CALLBACK myfunc( LPARAM lParam ); and one as (you can use the same name if you like VC++ will pick the right one ) BOOL myfunc2( LPARAM lParam ); when call your callback call the static one with the lparam as "this" ie. EnumWindows( myfunc, (LPARAM) this); then have your static function call the second BOOL CALLBACK myfunc( LPARAM lParam ) { CMyDialog * me = (CMyDialog*) lParam; return me->myfunc2(); }