Tips_CPlusPlus

h1. Tips - C++

You do not need to use any special bindings to use PortAudio in C++ code. The only difficulty most people have is creating a callback function. The best way to do that is to use a static function, which appears to the compiler a a normal C function, and have the static function call a non-static function. (You can assign a pointer to your class as the userData argument when creating a stream).

For example, your .h file might look like this:

class MyPortaudioClass
{
[snip]

int myMemberCallback(const void *input, void *output,
  unsigned long frameCount,
  const PaStreamCallbackTimeInfo* timeInfo,
  PaStreamCallbackFlags statusFlags);

static int myPaCallback(
  const void *input, void *output,
  unsigned long frameCount,
  const PaStreamCallbackTimeInfo* timeInfo,
  PaStreamCallbackFlags statusFlags,
  void *userData )
{
  return ((MyPortaudioClass*)userData)
     ->myMemberCallback(input, output, frameCount, timeInfo, statusFlags);
}

[snip]
};

and your .cpp file might look like this:

int MyPortaudioClass::myMemberCallback(const void *input, void *output,
  unsigned long frameCount,
  const PaStreamCallbackTimeInfo* timeInfo,
  PaStreamCallbackFlags statusFlags)
{
  // Do what you need (having access to every part of MyPortaudioClass)

  return paContinue;
}

Once that's setup, you can open your stream using "this" as your userData:

PaError pa_error = Pa_OpenStream(
                    [snip]...
                    &MyPortaudioClass::myPaCallback,      // streamCallback
                    this);   // userData

You could also just use a reference to your class in your static function like (make sure the static function is a member of the class so that you have access to "private" declared variables):

MyPortaudioClass* pThis = (MyPortaudioClass*)userData;

but this adds some complexity to the code.

This topic is covered on many C++ web sites, so try googling for the following terms: function pointer C++ static

Thanks to Robert Bielik for the sample code.