Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of boost. Click here for the latest Boost documentation.
Constructors

Our previous example didn't have any explicit constructors. Since World is declared as a plain struct, it has an implicit default constructor. Boost.Python exposes the default constructor by default, which is why we were able to write

    >>> planet = hello.World()

We may wish to wrap a class with a non-default constructor. Let us build on our previous example:

    struct World
    {
        World(std::string msg): msg(msg) {} // added constructor
        void set(std::string msg) { this->msg = msg; }
        std::string greet() { return msg; }
        std::string msg;
    };

This time World has no default constructor; our previous wrapping code would fail to compile when the library tried to expose it. We have to tell class_<World> about the constructor we want to expose instead.

    #include <boost/python.hpp>
    using namespace boost::python;

    BOOST_PYTHON_MODULE(hello)
    {
        class_<World>("World", init<std::string>())
            .def("greet", &World::greet)
            .def("set", &World::set)
        ;
    }

init<std::string>() exposes the constructor taking in a std::string (in Python, constructors are spelled ""__init__"").

We can expose additional constructors by passing more init<...>s to the def() member function. Say for example we have another World constructor taking in two doubles:

    class_<World>("World", init<std::string>())
        .def(init<double, double>())
        .def("greet", &World::greet)
        .def("set", &World::set)
    ;

On the other hand, if we do not wish to expose any constructors at all, we may use no_init instead:

    class_<Abstract>("Abstract", no_init)

This actually adds an __init__ method which always raises a Python RuntimeError exception.