Programming callbacks in C++ classes

[ninevoltz]

I know this question may be slightly off-topic for this board, but I'll ask anyway.

I'm a noob to C++ programming, and I've been trying for a couple of days to implement a NIDAQmx callback in a C++ class.

Is there a way to create multiple instances of a callback method? As far as I can tell, DAQmxRegisterEveryNSamplesEvent will only allow you to pass a static pointer as DAQmxEveryNSamplesEventCallbackPtr.

when using:
class niscope
{
    public:
       niscope(void);
    private:
        DAQmxEveryNSamplesEventCallbackPtr callback;
        int32 CVICALLBACK EveryNCallback(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples, void *callbackData);
};

niscope::niscope
{
    callback = this->EveryNCallback;
}

I get compile errors like this:
error: argument of type ‘int32 (niscope::)(TaskHandle, int32, uInt32, void*)’ does not match ‘int32 (*)(TaskHandle, int32, uInt32, void*)’

but if I declare EveryNCallback as static it compiles fine, but a static member cannot access other class instances, so it's a catch-22.

Can anyone help?

Thanks,
John

[ninevoltz]

Ignore my last, I now realize that this was a stupid question. There is no need to implement the DAQ routines and callbacks as class members.

[Born2Burn]

I am trying to create a class using the DAQmxRegisterEveryNSamplesEvent function that will call another member of the function.  I am also receiving the same error and although I would like to change things this must be wrapped in a class.  Is there anyway to make this work?

[Brad_K]

Hi Born2Burn,

You need to create a free function that calls your member function. You can use the callbackData argument to hold a pointer to your object. Note that I haven't compiled or tested this, so it may have some bugs, but hopefully it gives you the right idea:

class A
{
   ...
   int32 everyN(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples);
   ...
};

int32 CVICALLBACK A_everyN_function(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples, void* callbackData)
{
   A* a = reinterpret_cast<A*>(callbackData);
   return a->everyN(taskHandle, everyNsamplesEventType, nSamples);
}

int32 registerEveryN(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples, uInt32 options, A* a)
{
   return DAQmxRegisterEveryNSamplesEvent(taskHandle, everyNsamplesEventType, nSamples, options, &A_everyN_function, reinterpret_cast<void*>(a));
}

Brad