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Boost.Python converter for C++11 tuple

Date:
Tags: cpp, cpp11, python
Author: Andrey Antukh

With c++11 certain improvements have been introduced, some improvements are for language and some for the standard library. One of the interesting things is the library of tuples.

With tuples can define fixed-size containers with heterogeneous types. Boost.Python by default no have automatic conversion to C++11 tuple, and here an example of how to implement it.

// Compile with:
// clang++ -std=c++11 -shared -l boost_python3 -I /usr/include/python3.2mu -fPIC -o bptuple.so tuple-test.cpp

#include <tuple>
#include <string>
#include <boost/python.hpp>

namespace py = boost::python;

using std::string;

template<int ...> struct seq{};
template<int N, int ...S> struct gens : gens<N-1, N-1, S...>{};
template<int ...S> struct gens<0, S...> {typedef seq<S...> type;};

template <typename ...Args>
struct cpptuple2pytuple_wrapper {
    std::tuple<Args...> params;
    cpptuple2pytuple_wrapper(const std::tuple<Args...>& _params):params(_params){}

    py::tuple delayed_dispatch() {
        return callFunc(typename gens<sizeof...(Args)>::type());
    }

    template<int ...S>
    py::tuple callFunc(seq<S...>) {
        return py::make_tuple(std::get<S>(params) ...);
    }
};

template <typename ...Args>
struct pytuple2cpptuple_wrapper {
    py::tuple params;
    pytuple2cpptuple_wrapper(const py::tuple& _params):params(_params){}

    std::tuple<Args...> delayed_dispatch() {
        return callFunc(typename gens<sizeof...(Args)>::type());
    }

    template<int ...S>
    std::tuple<Args...> callFunc(seq<S...>) {
        return std::make_tuple((static_cast<Args>(py::extract<Args>(params[S])))...);
    }
};

// Convert (C++) tuple to (Python) tuple as PyObject*.
template<typename ... Args> PyObject* cpptuple2pytuple(const std::tuple<Args...>& t) {
    cpptuple2pytuple_wrapper<Args...> wrapper(t);
    py::tuple bpt = wrapper.delayed_dispatch();
    return py::incref(py::object(bpt).ptr());
}

// Convert (Python) tuple to (C++) tuple.
template<typename ... Args> std::tuple<Args...> pytuple2cpptuple(PyObject* obj) {
    py::tuple tup(py::borrowed(obj));
    pytuple2cpptuple_wrapper<Args...> wrapper(tup);
    std::tuple<Args...> bpt = wrapper.delayed_dispatch();
    return bpt;
}

template<typename ... Args>
struct cpptuple_to_python_tuple {
    static PyObject* convert(const std::tuple<Args...>& t) {
        return cpptuple2pytuple<Args...>(t);
    }
};

template<typename ... Args>
struct cpptuple_from_python_tuple {
    cpptuple_from_python_tuple() {
        py::converter::registry::push_back(&convertible, &construct, py::type_id<std::tuple<Args...> >());
    }

    static void* convertible(PyObject* obj_ptr) {
        if (!PyTuple_CheckExact(obj_ptr)) return 0;
        return obj_ptr;
    }

    static void construct(PyObject* obj_ptr, py::converter::rvalue_from_python_stage1_data* data) {
        void* storage = ((py::converter::rvalue_from_python_storage<std::tuple<Args...> >*)data)->storage.bytes;
        new (storage) std::tuple<Args...>(pytuple2cpptuple<Args...>(obj_ptr));
        data->convertible = storage;
    }
};

template<typename ...Args> void create_tuple_converter() {
    py::to_python_converter<std::tuple<Args...>, cpptuple_to_python_tuple<Args...> >();
    cpptuple_from_python_tuple<Args...>();
}

void export_cpptuple_conv() {
    create_tuple_converter<int, float>();
    create_tuple_converter<int, double, string>();
}

std::tuple<int, float> tupid1(std::tuple<int, float> t){return t;}
std::tuple<int, double, string> tupid2(std::tuple<int, double, string> t){return t;}

BOOST_PYTHON_MODULE(bptuple) {
    export_cpptuple_conv();
    py::def("tupid1", tupid1);
    py::def("tupid2", tupid2);
}