Re: function pointer help!

"Ron H." <rnh@xxxxxxx> wrote in message news:473b2e5f$0$7573$8d2e0cab@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I can't get this function pointer to work! Here's the scoop: I am using a
third party I/O board. The manufacturer supplied a lib and header file with
the following typedef and cbEnableEvent function prototype:

typedef void (__stdcall *EVENTCALLBACK)(int, unsigned, unsigned, void*);

int EXTCCONV cbEnableEvent(int BoardNum, unsigned EventType, unsigned Count,
EVENTCALLBACK CallbackFunc, void *UserData);

I created a function to deal with the callback:

//the return void and input prameters are defined in the manual...
void MyProjectView::CallHandler(int,unsigned int, unsigned int, void*)
{
}

So my challenge is to call cbEnableEvent with a pointer to CallHandler().

How about this?

EVENTCALLBACK pCallHandler;
pCallHandler = CallHandler;

The compiler complains with:

error C2440: '=' : cannot convert from 'void (__stdcall
CMyProjectView::* )(int,unsigned int,unsigned int,void *)' to
'EVENTCALLBACK'

There is no context in which this conversion is possible



Any help will be greatly appreciated!!!


You are attempting to use a C++ member function as the callback, but the callback is defined in terms or C, not C++. You can declare your C++ function as static, or simply make it a nonmember function. The underlying problem is that C++ functions receive a hidden parameter, the 'this' pointer, so their signature is incompatible with C definitions.

Your static/global function will not be able to call other C++ member functions, but the usual solution to that is to pass your 'this' pointer in the void* UserData parameter. Then in your callback function you can cast it back to an object pointer and call object functions with it.

--
Scott McPhillips [VC++ MVP]

.

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