No, PlusCallback back won’t help you. libusb is a C library, so it’s using C callbacks.

The pointer typedef you have, is a pointer to a function returning void and taking one argument. The single argument is a pointer to a libusb_transfer struct. So just define a function following those guidelines. It must be a static or free function. Member functions aren’t supported by C.

To pass the function’s address around, generally you just use its name.

BTW, StackOverflow is an excellent site for these types of questions. You would probably get this question answered there in about 90 seconds flat.

I now have a device which requires that I use the asynchronous interface. Therefore I am trying (rather slowly) to understand templates, function pointers, etc. in c++.

To the point, tho:

The libusb library (written in c) requires me to create a structure and then set a member of this structure to a pointer to a function with the following definition:

typedef void(* libusb_transfer_cb_fn)(struct libusb_transfer *transfer)

I have tried several different syntaxes, methods, etc. and I seem unable, with my limited knowledge, to make this work. I have tried member functions as well as static functions, and I cannot seem to get the required function to execute when the appropriate time comes.

Would PlusCallback assist me in this, and if so, would you be willing to provide an example as to how?

I added the fix and a test to detect the problem.

if (mCallback)
  mCallback->deinc();
mCallback = rhs.mCallback;
mCallback->inc();

Since you deincrement the refcount before incrementing, the code does not work in case of self-assignment:

cb::Callback1 callback(&func);
callback(4);
callback = callback;
callback(4); // SEGFAULT!

Once mCallback->deinc() is called, the ref counted object is destroyed leaving callback pointing to a dead object.

The correct sequence would be:

rhs.mCallback->inc();
if (mCallback) mCallback->deinc();
mCallback = rhs.mCallback;