VC, CALLBACK and non-static member function problem!

Sridha
#1 / 13

CALLBACK and non-static member function problem!

Hi all,
Here is my problem. For now, I have a dummy class (Let's call it class A)
which just makes an object of a class (class B)which is inherits CDialog.
Now, B has some private member variables which are accessed by public
accessor methods. I am trying to put a Timer on B which is going to do
something every so often. It cannot compile because I am trying to call a
non-static Get1() and Get2() in TimerProc2(). But, if I convert TimerProc
and ThreadProc to Retrieval::TimerProc and Retrieval::ThreadProc, then I get
the following error.

"Retrieval.cpp(282) : error C2440: 'type cast' : cannot convert from '' to
'void (__stdcall *)(struct HWND__ *,unsigned int,unsigned int,unsigned
long)'
None of the functions with this name in scope match the target type
Retrieval.cpp(323) : error C2664: 'CreateThread' : cannot convert parameter
3 from 'unsigned long (void *)' to 'unsigned long (__stdcall *)(void *)'
None of the functions with this name in scope match the target type
Error executing cl.exe."

Any help would be appreciated.

Thanks,
Sridhar.

CodeSnippet from B.h:

class Retrieval : public CDialog
{
public:
CString Get1();
CString Get2();

private:
CString var1;
CString var2;

Quote:

}

Code Snipppet From B.cpp:

CString Get1()
{
return var1; //Where var1 is a private variable.

Quote:

}

CString Get2()
{
return var2; //Where var2 is a private variable.

Quote:

}

VOID CALLBACK TimerProc2(
HWND hwnd, // handle to window
UINT uMsg, // WM_TIMER message
UINT_PTR idEvent, // timer identifier
DWORD dwTime // current system time
)
{
Retrieval::DoSomething(Retrieval::Get1(), Retrieval::Get2());

Quote:

}

DWORD WINAPI ThreadProc(
LPVOID lpParameter // thread data
)
{
UINT uiTimer = ::SetTimer(NULL, 12, 60000, (TIMERPROC)TimerProc2);
MSG msg;
while (GetMessage(&msg, NULL, 0, 0))
{
DispatchMessage(&msg);
}
::KillTimer(NULL, uiTimer);
return 0;

Quote:

}

void Retrieval::OnGetMail()
{
// TODO: Add your control notification handler code here

HANDLE hThread = CreateThread(NULL, NULL, ThreadProc, NULL, NULL, NULL);
CloseHandle(hThread);

//Sleep(30000);
this->ShowWindow(SW_SHOWMINIMIZED);

Quote:

}

Mon, 23 Aug 2004 02:08:01 GMT

James Brow
#2 / 13

CALLBACK and non-static member function problem!

Uhh...
Make the callback a function static function of your class.

James.
--
www.catch22.uk.net
Free win32 software, sourcecode, and tutorials
------
Please remove "NOSPAM" when replying.

Quote:

> Hi all,
> Here is my problem. For now, I have a dummy class (Let's call it class A)
> which just makes an object of a class (class B)which is inherits CDialog.
> Now, B has some private member variables which are accessed by public
> accessor methods. I am trying to put a Timer on B which is going to do
> something every so often. It cannot compile because I am trying to call a
> non-static Get1() and Get2() in TimerProc2(). But, if I convert TimerProc
> and ThreadProc to Retrieval::TimerProc and Retrieval::ThreadProc, then I
get
> the following error.

> "Retrieval.cpp(282) : error C2440: 'type cast' : cannot convert from ''
to
> 'void (__stdcall *)(struct HWND__ *,unsigned int,unsigned int,unsigned
> long)'
> None of the functions with this name in scope match the target
type
> Retrieval.cpp(323) : error C2664: 'CreateThread' : cannot convert
parameter
> 3 from 'unsigned long (void *)' to 'unsigned long (__stdcall *)(void *)'
> None of the functions with this name in scope match the target
type
> Error executing cl.exe."

> Any help would be appreciated.

> Thanks,
> Sridhar.

> CodeSnippet from B.h:

> class Retrieval : public CDialog
> {
> public:
> CString Get1();
> CString Get2();

> private:
> CString var1;
> CString var2;
> }

> Code Snipppet From B.cpp:

> CString Get1()
> {
> return var1; //Where var1 is a private variable.
> }

> CString Get2()
> {
> return var2; //Where var2 is a private variable.
> }

> VOID CALLBACK TimerProc2(
> HWND hwnd, // handle to window
> UINT uMsg, // WM_TIMER message
> UINT_PTR idEvent, // timer identifier
> DWORD dwTime // current system time
> )
> {
> Retrieval::DoSomething(Retrieval::Get1(), Retrieval::Get2());
> }

> DWORD WINAPI ThreadProc(
> LPVOID lpParameter // thread data
> )
> {
> UINT uiTimer = ::SetTimer(NULL, 12, 60000, (TIMERPROC)TimerProc2);
> MSG msg;
> while (GetMessage(&msg, NULL, 0, 0))
> {
> DispatchMessage(&msg);
> }
> ::KillTimer(NULL, uiTimer);
> return 0;

> }

> void Retrieval::OnGetMail()
> {
> // TODO: Add your control notification handler code here

> HANDLE hThread = CreateThread(NULL, NULL, ThreadProc, NULL, NULL, NULL);
> CloseHandle(hThread);

> //Sleep(30000);
> this->ShowWindow(SW_SHOWMINIMIZED);
> }

Mon, 23 Aug 2004 02:33:38 GMT

Sridha
#3 / 13

CALLBACK and non-static member function problem!

James,
In that case, I get a "'TimerProc2' : 'static' should not be used on member
functions defined at file scope" error.

Sridhar.

Quote:

> Uhh...
> Make the callback a function static function of your class.

> James.
> --
> www.catch22.uk.net
> Free win32 software, sourcecode, and tutorials
> ------
> Please remove "NOSPAM" when replying.

> > Hi all,
> > Here is my problem. For now, I have a dummy class (Let's call it class
A)
> > which just makes an object of a class (class B)which is inherits
CDialog.
> > Now, B has some private member variables which are accessed by public
> > accessor methods. I am trying to put a Timer on B which is going to do
> > something every so often. It cannot compile because I am trying to call
a
> > non-static Get1() and Get2() in TimerProc2(). But, if I convert
TimerProc
> > and ThreadProc to Retrieval::TimerProc and Retrieval::ThreadProc, then I
> get
> > the following error.

> > "Retrieval.cpp(282) : error C2440: 'type cast' : cannot convert from ''
> to
> > 'void (__stdcall *)(struct HWND__ *,unsigned int,unsigned int,unsigned
> > long)'
> > None of the functions with this name in scope match the target
> type
> > Retrieval.cpp(323) : error C2664: 'CreateThread' : cannot convert
> parameter
> > 3 from 'unsigned long (void *)' to 'unsigned long (__stdcall *)(void *)'
> > None of the functions with this name in scope match the target
> type
> > Error executing cl.exe."

> > Any help would be appreciated.

> > Thanks,
> > Sridhar.

> > CodeSnippet from B.h:

> > class Retrieval : public CDialog
> > {
> > public:
> > CString Get1();
> > CString Get2();

> > private:
> > CString var1;
> > CString var2;
> > }

> > Code Snipppet From B.cpp:

> > CString Get1()
> > {
> > return var1; //Where var1 is a private variable.
> > }

> > CString Get2()
> > {
> > return var2; //Where var2 is a private variable.
> > }

> > VOID CALLBACK TimerProc2(
> > HWND hwnd, // handle to window
> > UINT uMsg, // WM_TIMER message
> > UINT_PTR idEvent, // timer identifier
> > DWORD dwTime // current system time
> > )
> > {
> > Retrieval::DoSomething(Retrieval::Get1(), Retrieval::Get2());
> > }

> > DWORD WINAPI ThreadProc(
> > LPVOID lpParameter // thread data
> > )
> > {
> > UINT uiTimer = ::SetTimer(NULL, 12, 60000, (TIMERPROC)TimerProc2);
> > MSG msg;
> > while (GetMessage(&msg, NULL, 0, 0))
> > {
> > DispatchMessage(&msg);
> > }
> > ::KillTimer(NULL, uiTimer);
> > return 0;

> > }

> > void Retrieval::OnGetMail()
> > {
> > // TODO: Add your control notification handler code here

> > HANDLE hThread = CreateThread(NULL, NULL, ThreadProc, NULL, NULL,
NULL);
> > CloseHandle(hThread);

> > //Sleep(30000);
> > this->ShowWindow(SW_SHOWMINIMIZED);
> > }

Mon, 23 Aug 2004 05:04:53 GMT

Scot T Brenneck
#4 / 13

CALLBACK and non-static member function problem!

Sridhar,
CALLBACK functions are not prototyped to have the hidden 'this' pointer of a
member function, so cannot access members of a class. Making the CALLBACK
function a static member of the class doesn't help, because it still has no
'this' to get to object members (there is no object in the context of a
callback).
For ThreadProcs, you can get around this by passing a pointer to a class object
as the LPVOID parameter. The thread can then cast that back to a class pointer,
and call a public member function to do the actual thread work.
For TimerProcs, however, there is no good workaround, since you can't pass a
pointer to them. You'd have to save the object pointer in some file-scope
variable so the TimerProc could get to it. That would only work if there is
only one object of the class, however. The better solution is to not use a
TimerProc, but handle the WM_TIMER message instead. Since you are using MFC,
this is easy. Just add a handler for WM_TIMER to your class (if it's derived
from CWnd). Then when the message is sent, your handler code executes within
the context of a class object.

Mon, 23 Aug 2004 13:36:42 GMT

Jochen Kalmbac
#5 / 13

CALLBACK and non-static member function problem!

Quote:

> Here is my problem. For now, I have a dummy class (Let's call it class A)
> which just makes an object of a class (class B)which is inherits CDialog.
> Now, B has some private member variables which are accessed by public
> accessor methods. I am trying to put a Timer on B which is going to do
> something every so often. It cannot compile because I am trying to call a
> non-static Get1() and Get2() in TimerProc2(). But, if I convert TimerProc
> and ThreadProc to Retrieval::TimerProc and Retrieval::ThreadProc, then I
get
> the following error.

I dont know if this helps, but you can make callbakcs into a class without
static members. For this you have to use templates... Here is an small
example:

template<class T>
class TCallbacker
{
typedef void (T::*pFunction)(void);
pFunction pFunc;
T *This;

public:
TCallbacker(T *T,pFunction p)
: pFunc(p),This(T)
{
}
void call()
{
(This->*pFunc)();
}

Quote:

};

class CallbackClass
{
public:
virtual void Callback(void)
{
_tprintf(_T("Callback called!\n"));
}

Quote:

};

int _tmain(int argc, _TCHAR* argv[])
{
CallbackClass a;
TCallbacker<CallbackClass> b(&a,CallbackClass::Callback);
b.call();
return 0;

Quote:

}

Greetings
Jochen

Mon, 23 Aug 2004 14:52:29 GMT

Fred Jackso
#6 / 13

CALLBACK and non-static member function problem!

See below.

Quote:

> Sridhar,
> CALLBACK functions are not prototyped to have the hidden 'this' pointer of
a
> member function, so cannot access members of a class.

Absolutely correct for non-static methods.

Quote:

> Making the CALLBACK
> function a static member of the class doesn't help, because it still has
no
> 'this' to get to object members (there is no object in the context of a
> callback).

If I'm you're claiming what I think you are, this is incorrect.
I use static methods all the time for callbacks and thread procs.
It works fine, and aids encapsulation. Think about it. You can
call static methods without having a class instance (using the
class::method() syntax) - so clearly there is no hidden "this"
parameter.

Quote:

> For ThreadProcs, you can get around this by passing a pointer to a class
object
> as the LPVOID parameter. The thread can then cast that back to a class
pointer,
> and call a public member function to do the actual thread work.

Exactly. I make the static method call a non-static method
using the "this" pointer thus passed - then the non-static
method can start using class member data and calling other
methods.

Quote:

> For TimerProcs, however, there is no good workaround, since you can't pass
a
> pointer to them. You'd have to save the object pointer in some file-scope
> variable so the TimerProc could get to it. That would only work if there
is
> only one object of the class, however.

I know - it's a bad callback design - no user parameter provided
in the function that sets the callback. BTW, the multimedia
timers (see timeSetEvent) are designed better - a user parameter
is provided.

Quote:

> The better solution is to not use a
> TimerProc, but handle the WM_TIMER message instead. Since you are using
MFC,
> this is easy. Just add a handler for WM_TIMER to your class (if it's
derived
> from CWnd). Then when the message is sent, your handler code executes
within
> the context of a class object.

Mon, 23 Aug 2004 21:40:55 GMT

Fred Jackso
#7 / 13

CALLBACK and non-static member function problem!

You have to put the "static" qualifier in the method declaration
inside the class declaration - not in the method definition if
the latter is outside the class declaration.

It doesn't make a difference to your problem, though. TimerProc2
should work equally well as a standalone function or a static
method.

By the way, if you use any C/C++ runtime library functions in
your thread proc, it won't work safely unless you use
_beginthread or _beginthreadex to start it (instead of
CreateThread). Otherwise, CreateTHread is OK.

Quote:

> James,
> In that case, I get a "'TimerProc2' : 'static' should not be used on
member
> functions defined at file scope" error.

> Sridhar.

> > Uhh...
> > Make the callback a function static function of your class.

> > James.
> > --
> > www.catch22.uk.net
> > Free win32 software, sourcecode, and tutorials
> > ------
> > Please remove "NOSPAM" when replying.

> > > Hi all,
> > > Here is my problem. For now, I have a dummy class (Let's call it class
> A)
> > > which just makes an object of a class (class B)which is inherits
> CDialog.
> > > Now, B has some private member variables which are accessed by public
> > > accessor methods. I am trying to put a Timer on B which is going to do
> > > something every so often. It cannot compile because I am trying to
call
> a
> > > non-static Get1() and Get2() in TimerProc2(). But, if I convert
> TimerProc
> > > and ThreadProc to Retrieval::TimerProc and Retrieval::ThreadProc, then
I
> > get
> > > the following error.

> > > "Retrieval.cpp(282) : error C2440: 'type cast' : cannot convert from
''
> > to
> > > 'void (__stdcall *)(struct HWND__ *,unsigned int,unsigned int,unsigned
> > > long)'
> > > None of the functions with this name in scope match the target
> > type
> > > Retrieval.cpp(323) : error C2664: 'CreateThread' : cannot convert
> > parameter
> > > 3 from 'unsigned long (void *)' to 'unsigned long (__stdcall *)(void
*)'
> > > None of the functions with this name in scope match the target
> > type
> > > Error executing cl.exe."

> > > Any help would be appreciated.

> > > Thanks,
> > > Sridhar.

> > > CodeSnippet from B.h:

> > > class Retrieval : public CDialog
> > > {
> > > public:
> > > CString Get1();
> > > CString Get2();

> > > private:
> > > CString var1;
> > > CString var2;
> > > }

> > > Code Snipppet From B.cpp:

> > > CString Get1()
> > > {
> > > return var1; //Where var1 is a private variable.
> > > }

> > > CString Get2()
> > > {
> > > return var2; //Where var2 is a private variable.
> > > }

> > > VOID CALLBACK TimerProc2(
> > > HWND hwnd, // handle to window
> > > UINT uMsg, // WM_TIMER message
> > > UINT_PTR idEvent, // timer identifier
> > > DWORD dwTime // current system time
> > > )
> > > {
> > > Retrieval::DoSomething(Retrieval::Get1(), Retrieval::Get2());
> > > }

> > > DWORD WINAPI ThreadProc(
> > > LPVOID lpParameter // thread data
> > > )
> > > {
> > > UINT uiTimer = ::SetTimer(NULL, 12, 60000, (TIMERPROC)TimerProc2);
> > > MSG msg;
> > > while (GetMessage(&msg, NULL, 0, 0))
> > > {
> > > DispatchMessage(&msg);
> > > }
> > > ::KillTimer(NULL, uiTimer);
> > > return 0;

> > > }

> > > void Retrieval::OnGetMail()
> > > {
> > > // TODO: Add your control notification handler code here

> > > HANDLE hThread = CreateThread(NULL, NULL, ThreadProc, NULL, NULL,
> NULL);
> > > CloseHandle(hThread);

> > > //Sleep(30000);
> > > this->ShowWindow(SW_SHOWMINIMIZED);
> > > }

Mon, 23 Aug 2004 21:51:46 GMT

Fred Jackso
#8 / 13

CALLBACK and non-static member function problem!

Here is how I do this kind of thing in VC6; should work
as well in VC7 (excerpted/adapted from working code but
excerpting edits not rigorously checked):

class CMyClass
{
public:
/* ctor */ CMyClass();
...

private:
void Thread();
static UINT __stdcall ThreadHelper(void* arg);
void Timer();
static void CALLBACK TimerHelper(UINT uID, UINT uMsg,
DWORD dwUser,
DWORD dw1, DWORD dw2);
HANDLE m_threadHandle;
UINT m_threadId;
MMRESULT m_timerId;
...

Quote:

};

/* ctor */
CMyClass::CMyClass()
{
unsigned long temp = ::_beginthreadex(NULL, 0, ThreadHelper,
this, 0, &m_ThreadId);
m_threadHandle = reinterpret_cast<HANDLE>(temp);
m_timerId = ::timeSetEvent(1, 0, TimerHelper,
reinterpret_cast<DWORD>(this),
TIME_PERIODIC);
...

Quote:

}

// The "real" non-static thread proc method.
void
CMyClass::Thread()
{
...

Quote:

}

// The static thread proc helper method.
// Just retrieves the "this" pointer and calls the
// "real" non-static thread proc method.
UINT __stdcall
CMyClass::ThreadHelper(void* arg)
{
CMyClass* thisPtr = static_cast<CMyClass*>(arg);
thisPtr->Thread();
return 0;

Quote:

}

// The "real" non-static timer proc method.
void
CMyClass::Timer()
{
...

Quote:

}

// The static timer proc helper method.
// Just retrieves the "this" pointer and calls the
// "real" non-static timer proc method.
void CALLBACK
CMyClass::TimerHelper(UINT uID, UINT uMsg, DWORD dwUser,
DWORD dw1, DWORD dw2)
{
CMyClass* thisPtr = reinterpret_cast<CMyClass*>(dwUser);
thisPtr->Timer();

Quote:

}

Tue, 24 Aug 2004 00:06:59 GMT

Scot T Brenneck
#9 / 13

CALLBACK and non-static member function problem!

I'm not implying that you can't use a static member for a TimerProc, only that
it doesn't help solve the problem of accessing object member-specific data from
the callback function. This is because, as you acknowledged later in your
reply, the SetTimer doesn't provide for a 32-bit user value to be passed back
(except for the timer ID). Your solution of using a multimedia timer
(timeSetEvent) instead is clever.

Quote:

>> Making the CALLBACK
>> function a static member of the class doesn't help, because it still has no
>> 'this' to get to object members (there is no object in the context of a
>> callback).

> If I'm you're claiming what I think you are, this is incorrect.
> I use static methods all the time for callbacks and thread procs.
> It works fine, and aids encapsulation. Think about it. You can
> call static methods without having a class instance (using the
> class::method() syntax) - so clearly there is no hidden "this"
> parameter.

Tue, 24 Aug 2004 02:38:48 GMT

Scot T Brenneck
#10 / 13

CALLBACK and non-static member function problem!

Jochen,
Can you show how this method would assist in the case of the SetTimer callback
problem?

Is it just a coincidence that your name, Kalmbach, sounds like Callback? ;)

Tue, 24 Aug 2004 02:42:54 GMT

Craig Power
#11 / 13

CALLBACK and non-static member function problem!

Quote:

> I'm not implying that you can't use a static member for a TimerProc, only that
> it doesn't help solve the problem of accessing object member-specific data from
> the callback function. This is because, as you acknowledged later in your
> reply, the SetTimer doesn't provide for a 32-bit user value to be passed back
> (except for the timer ID).

A workaround for this is to generate a map of timer IDs to class
instances. This is how I dealt with the issue in a VB implementation.

Tue, 24 Aug 2004 03:48:48 GMT

Fred Jackso
#12 / 13

CALLBACK and non-static member function problem!

OK, I was misinterpreting what you said. We are definitely
in sync.

Quote:

> I'm not implying that you can't use a static member for a TimerProc, only
that
> it doesn't help solve the problem of accessing object member-specific data
from
> the callback function. This is because, as you acknowledged later in your
> reply, the SetTimer doesn't provide for a 32-bit user value to be passed
back
> (except for the timer ID). Your solution of using a multimedia timer
> (timeSetEvent) instead is clever.

> >> Making the CALLBACK
> >> function a static member of the class doesn't help, because it still
has no
> >> 'this' to get to object members (there is no object in the context of a
> >> callback).

> > If I'm you're claiming what I think you are, this is incorrect.
> > I use static methods all the time for callbacks and thread procs.
> > It works fine, and aids encapsulation. Think about it. You can
> > call static methods without having a class instance (using the
> > class::method() syntax) - so clearly there is no hidden "this"
> > parameter.

Tue, 24 Aug 2004 04:56:56 GMT

Fred Jackso
#13 / 13

CALLBACK and non-static member function problem!

Wow, that's right! std::map would work.

Quote:

> > I'm not implying that you can't use a static member for a TimerProc,
only that
> > it doesn't help solve the problem of accessing object member-specific
data from
> > the callback function. This is because, as you acknowledged later in
your
> > reply, the SetTimer doesn't provide for a 32-bit user value to be passed
back
> > (except for the timer ID).

> A workaround for this is to generate a map of timer IDs to class
> instances. This is how I dealt with the issue in a VB implementation.

Tue, 24 Aug 2004 04:59:32 GMT