C2664 and passing member function pointer

[ifitzgerald]

I am attempting to use a wrapper class to the Win32 timer API in a Visual C++ 6.0 MFC application. The callback function needs to have access to the front panel, and as such needs to be a member of the front panel class. I kept getting compile error C2276 when I tried passing the address of a member function. After implementing a few changes suggested by the author of the class, I now get the following error when compiling:

error C2664: 'setCallbackFun ction' : cannot convert parameter 1 from 'void (__stdcall CTestDlg::*)(in t)' to 'void (__cdecl *)(int)'
There is no context in which this conversion is possible

The line that causes the error is:

timer1.setCallbackFunction(&CTestDlg::timer1Event);

The prototype of the callback function looks like this:

void CALLBACK CTestDlg::timer1Event(int i)

The prototype of the setCallbackFunc tion function looks like this:

VOID LRTimer::setCallbackFunction( VOID (*_pCallback)(int)) {
        m_pCallback = _pCallback;
}

Do you have any ideas as to how I might fix this? I would greatly appreciate it if I could get a fix within the next 48 hours - this is a rather important part of a project I'm working on. I tried contacting the author of the class, but it looks like he hasn't been active for a while.

Thank you!
Ian

[Banfa]

VOID LRTimer::setCallbackFunction( VOID (*_pCallback)(int)) {
        m_pCallback = _pCallback;
}

This function takes a pointer to a function, not a pointer to a function member of an object. That is internally setCallbackFunc tion (and LRTIMER) has no knowledge of the any object containing the callback function and does not perform any of the pointer manipulation (creating and passing this) that would be required to call a object member function. And in fact dopes not have access to the data required to create this.

void CALLBACK CTestDlg::timer1Event(int i)

This function is a member of CTestDlg and before calling requires that this is created and passed to timer1Event.

Solutions

You could make CTestDlg::timer 1Event a static function. This means that timer1Event is a class method, it would be unable to access any object data through this as the this pointer is not set up for class methods. Obviously only you can say if you are able to work with this constraint. May be the data passed would be enough for you t identify the object required and then call a normal method on the object from the class member function.

You could change the prototype of setCallbackFunc tion if you have access to all it's source code (i.e. you are not using it as a library. In this case you change setCallbackFunc tion (or add an overload of it) and all the relevent places in the LRTimer class to accept a pointer to and object of type CTestDlg. And then internally call the relevent member function.

VOID LRTimer::setCallbackFunction( CTestDlg *pCallbackObject)

[ifitzgerald]

VOID LRTimer::setCallbackFunction( VOID (*_pCallback)(int)) {
        m_pCallback = _pCallback;
}

This function takes a pointer to a function, not a pointer to a function member of an object. That is internally setCallbackFunc tion (and LRTIMER) has no knowledge of the any object containing the callback function and does not perform any of the pointer manipulation (creating and passing this) that would be required to call a object member function. And in fact dopes not have access to the data required to create this.

void CALLBACK CTestDlg::timer1Event(int i)

This function is a member of CTestDlg and before calling requires that this is created and passed to timer1Event.

Solutions

You could make CTestDlg::timer 1Event a static function. This means that timer1Event is a class method, it would be unable to access any object data through this as the this pointer is not set up for class methods. Obviously only you can say if you are able to work with this constraint. May be the data passed would be enough for you t identify the object required and then call a normal method on the object from the class member function.

You could change the prototype of setCallbackFunc tion if you have access to all it's source code (i.e. you are not using it as a library. In this case you change setCallbackFunc tion (or add an overload of it) and all the relevent places in the LRTimer class to accept a pointer to and object of type CTestDlg. And then internally call the relevent member function.

VOID LRTimer::setCallbackFunction( CTestDlg *pCallbackObject)

Banfa, thank you very much for your quick response. Unfortunately my automatic notification settings must be incorrect, and I didn't notice your response until this morning.

What I ended up doing was creating a non-member function to use as the callback, and calling member functions in the callback using a global pointer to "this".

VOID timer1Event()
{
     dialogObject->memberFunction(); //dialogObject is a pointer to the form object
}
 

I know global variables aren't exactly the best programming practice, but eventually I'll need several different timers each with a different callback function, and this was the easiest way that I could think of to do it.

Thanks again,
Ian

[Banfa]

Banfa, thank you very much for your quick response. Unfortunately my automatic notification settings must be incorrect, and I didn't notice your response until this morning.

The automatic notification on this site is somewhat broken, I suggest you check back to the site itself every now and then and check your user control panel for updates.

What I ended up doing was creating a non-member function to use as the callback, and calling member functions in the callback using a global pointer to "this".

VOID timer1Event()
{
     dialogObject->memberFunction(); //dialogObject is a pointer to the form object
}
 

I know global variables aren't exactly the best programming practice, but eventually I'll need several different timers each with a different callback function, and this was the easiest way that I could think of to do it.

OK, that is definately the quick and dirty hack :D and you have the prototype of timer1Event wrong, it is supposed to accept an int parameter. This hack will not scale.

Here is what I would say is a better solution

You need a way to associate the LRTimer instance with the CTestDlg instance. Do you access to the code of both classes?

If so then pass a pointer to your CTestDlg instance to the LRTimer, the callback can then be a member function of CTestDlg which LRTimer can call directly, but you would have to handle the case of the CTestDlg instance being deleted before the LRTimer instance had timed out (i.e. the risk of LRTimer trying to use an invalid CTestDlg instance).

If you do not have access to both classes, particularly access to the LRTimer class then:

Firstly make the callback a static member of CTestDlg, if possible make it private. This will only be possible if you create the timer from within the CTestDlg class somewhere.

To associate the 2 instances you will need to have identifying data, this could be the pointer to the class or it could be an id, whatever it is it must uniquely identify the instance. You can then set up a map (again a private member of CTestDlg) which you will be able to consult to get the correct CTestDlg instance for any LRTimer callback and so call the correct object.

When instances are deleted you can simple remove the corresponding map member to prevent use of invalid pointers.