Question

I’m actually coding a program in C++ using the library PortAudio.
This library is using a callback function tu manage audio input and output.
In C++, I implemented this callback function in my class “Audio” and I can’t send it to Pa_OpenDefaultStream(). The compiler says “this argument is incompatible with parameter of type PaStreamCallback*” with this line :

Pa_OpenDefaultStream(&this->_stream, 1, 2, paFloat32, this->_sampleRate, this->_framesPerBuffer, callbackFunction, NULL);

When I use C, sending my callbackFunction like this works well.
How can I send my callback function to this OpenDefaultStream function ?

Answer 1

You need to implement a plain global function with the same signature as PaStreamCallback.

It looks like PaStreamCallback takes an argument, void* userData. This is what’s usually called a context argument. Without knowledge of PortAudio I guess that you can use this to represent your class instance. When you call Pa_OpenDefaultStream, pass “this” for the userData pointer, where you wrote NULL in your example.

Here is a sample implememtation of the function wrapper you need:

int MyPaStreamCallback (const void *input, void *output, unsigned long frameCount, const PaStreamCallbackTimeInfo *timeInfo, PaStreamCallbackFlags statusFlags, void *userData)
{
     MyClass *myClass = reinterpret_cast<MyClass*>(userData);
     return myClass->callbackFunction(input, output, frameCount, timeInfo, statusFlags);
}

Then replace your original code with:

Pa_OpenDefaultStream(&this->_stream, 1, 2, paFloat32, this->_sampleRate, this->_framesPerBuffer, MyPaStreamCallback , this); 

I assumed you would make your callbackFunction to take all the other args.