I'm working with a C++ class-library that provides a Thread base-class where the user has to
I'm not sure about C++, but that's how you would do it in Java. You'd have a class that extends Thread (or implements Runnable) and a constructor with the parameters you'd like to pass. Then, when you create the new thread, you have to pass in the arguments, and then start the thread, something like this:
Thread t = new MyThread(args...); t.start();
Must be the same in your case.