This is really the issue with C++ and not Qt. If you want to keep binary compatibility while improving a library there really are not any other options. With

This is really the issue with C++ and not Qt. If you want to keep binary compatibility while improving a library there really are not any other options. With a private class you can have new private variables, virtual functions, function renaming, and well anything goes. But probably something you care more about I have seen on more then one occasion a class completely re-written (for the better). The private class changes completely but the public API stays the same. This would only be possible in c++ with a private class.