Shiboken requires an XML-based typesystem file that defines the
relationship between C++ and Python types.

It declares the two aforementioned classes. One of them as an
“object-type” and the other as a “value-type”. The main difference is that
object-types are passed around in generated code as pointers, whereas
value-types are copied (value semantics).

By specifying the names of these classes in the typesystem file, Shiboken
automatically tries to generate bindings for all methods of those
classes. You need not mention all the methods manually in the XML file, unless
you want to modify them.

Object ownership rules
----------------------

Shiboken doesn't know if Python or C++ are responsible for freeing the C++
objects that were allocated in the Python code, and assuming this might lead to
errors. There can be cases where Python should release the C++ memory when the
reference count of the Python object becomes zero, but it should never delete
the underlying C++ object just from assuming that it will not be deleted by
underlying C++ library, or if it's maybe parented to another object (like
QWidgets).

In our case, the :code:`clone()` method is only called inside the C++ library,
and we assume that the C++ code takes care of releasing the cloned object.

As for :code:`addIcecreamFlavor()`, we know that a :code:`Truck` owns the
:code:`Icecream` object, and will remove it once the :code:`Truck` is
destroyed. That's why the ownership is set to “c++” in the typesystem file,
so that the C++ objects are not deleted when the corresponding Python names
go out of scope.
