| Inheritance

In the previous examples, we dealt with classes that are not polymorphic. This is not often the case. Much of the time, we will be wrapping polymorphic classes and class hierarchies related by inheritance. We will often have to write Boost.Python wrappers for classes that are derived from abstract base classes.

Consider this trivial inheritance structure:

`

        struct Base { virtual ~Base(); };
        struct Derived : Base {};

`

And a set of C++ functions operating on `Base` and `Derived` object instances:

`

        void b(Base*);
        void d(Derived*);
        Base* factory() { return new Derived; }

`

We've seen how we can wrap the base class `Base`:

`

        class_<Base>("Base")
            /*...*/
            ;

`

Now we can inform Boost.Python of the inheritance relationship between `Derived` and its base class `Base`. Thus:

`

        class_<Derived, bases<Base> >("Derived")
            /*...*/
            ;

`

Doing so, we get some things for free:

1. Derived automatically inherits all of Base's Python methods (wrapped C++ member functions)
2. If Base is polymorphic, `Derived` objects which have been passed to Python via a pointer or reference to `Base` can be passed where a pointer or reference to `Derived` is expected.

Now, we shall expose the C++ free functions `b` and `d` and `factory`:

`

        def("b", b);
        def("d", d);
        def("factory", factory);

`

Note that free function `factory` is being used to generate new instances of class `Derived`. In such cases, we use `return_value_policy<manage_new_object>` to instruct Python to adopt the pointer to `Base` and hold the instance in a new Python `Base` object until the the Python object is destroyed. We shall see more of Boost.Python call policies later.

`

        // Tell Python to take ownership of factory's result
        def("factory", factory,
            return_value_policy<manage_new_object>());

`

Copyright © 2002 David Abrahams
Copyright © 2002 Joel de Guzman
Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.