in main:

   ...

   SineWave sine;
   AudioCallback dac;

   dac.startAudio(&sine);

   ...


in AudioCallback.cpp

  ... 

int tick (char *buffer, int bufferSize, void *dataPointer) {
  
   ...

   SineWave *sine = (SineWave*) dataPointer;
   sine->useSomeFunction();

   ...
}


void AudioCallback::startAudio(void *data) {

   ...
   
   a_data.dac->setStreamCallback(&tick, &data); //setStreamCallback sets a function which will be called by an RealTime AudioSystem,
                                                // the 2nd argument is used for passing user data to this function.

   ...
}