February 2014 - Boost-users

boost::python wrapping a container of boost::variant?
by Mario Lang 05 Feb '14

05 Feb '14

Hi. Does anyone have ideas how to convince boost::python to wrap a container of boost::variant? I need this to generate useful python bindings for a project of mine which uses boost::variant internally. Since python supports heterogenous containers natively, it seems to me this should be doable. However, I have attempted to find a solution on my own on several occasions, and always failed. One obvious hint is the indexing_suite. However, now that I looked at it again, I think I know where the problem is: The get_item method of indexing_suite always returns a data_type object, which is value_type from the container in question. However, from what I can see, I'd need something that converts to boost::python::object, so that I can return whatever class is appropriate for the actual variant instance. In other words, get_item would need to return a boost::python::object instead of data_type. To illustrate in code what I want, here is a minimal example: #include <boost/python.hpp> #include <boost/variant.hpp> struct a { int x; }; struct b { double y; }; typedef boost::variant<a, b> variant; typedef std::vector<variant> vector; BOOST_PYTHON_MODULE(bpv) { using namespace boost::python; class_<a>("a", init<a>()) .def(init<>()) .def_readonly("x", &a::x); class_<b>("b", init<b>()) .def(init<>()) .def_readonly("y", &b::y); } The question is, how do I expose std::vector<variant> to Python? I guess I need to define a static_visitor that returns python objects, something like: struct to_python_visitor : boost::static_visitor<boost::python::object> { result_type operator()(a const &a_) const { // return a boost::python::object that refers to a_ } ... }; And hook that up to the container support somehow such that boost::apply_visitor(to_python_visitor(), variant_instance) is called whenever a value is retrieved from the container. Can this even be done with current Boost.Python? Any hints, maybe even concrete code examples, would be highly appreciated. -- CYa, ⡍⠁⠗⠊⠕

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boost::none for boost::shared_ptr
by Chris Stankevitz 03 Feb '14

03 Feb '14

Hello, Does boost::shared_ptr offer a way to create an empty shared pointer without a lot of typing? Perhaps something analogous to boost::none. I am not using c++11. Thank you, Chris == using boost::none; using boost::optional; // declare f(optional<CSomeReallyLongClassName> Item); g(shared_ptr<CSomeReallyLongClassName> pItem); // invoke f with empty object f(none); // invoke g with empty object g(shared_ptr<CSomeReallyLongClassName>());

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multi_array move semantics
by MM 03 Feb '14

03 Feb '14

Hello, 1. Browsing the multi_array doc, there doesn't seem to be a move ctor? 2. Has there been any attempt to implement one? 3. if not, and the authors are reading this, are there any hints to help with this? Regards MM

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serialization: intrusive_ptr
by MM 03 Feb '14

03 Feb '14

Hello, I've found an implementation of a serializer for intrusive_ptr here: http://stellar.cct.lsu.edu/files/hpx_0.9.5/html/code/hpx/util/serialize_int… which i've injected into my boost lib. Hartmut Kaiser, I guess, would there be a proposal to add it serialization for future standard boost? Regards MM

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