pybind11: stuck with unresolved overloaded function type

Question

I am working on a pybind11 wrapper for libnest2d. Unfortuately I am not well versed in modern C++ templating and got stuck with this overload problem. I tried to resolve it using the py::overload_cast (see src/main.cpp at the bottom) but the problem still remains. Any hints on how to analyse/solve this would be more than welcome.

what the compiler has to say:

    /usr/bin/c++  -DBOOST_ALL_NO_LIB -DLIBNEST2D_GEOMETRIES_clipper -DLIBNEST2D_OPTIMIZER_nlopt -DLIBNEST2D_THREADING_std -Dnest2d_EXPORTS -I/home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include -I/usr/include/python3.8 -I/home/mark/devel/14_2D_3D/nest2d/lib/libnest2d/include -isystem /usr/include/polyclipping  -DVERSION_INFO=\"0.1.0\" -O3 -DNDEBUG -fPIC -fvisibility=hidden   -std=c++14 -flto -fno-fat-lto-objects -o CMakeFiles/nest2d.dir/src/main.cpp.o -c /home/mark/devel/14_2D_3D/nest2d/src/main.cpp
    /home/mark/devel/14_2D_3D/nest2d/src/main.cpp: In function ‘void pybind11_init_nest2d(pybind11::module&)’:
    /home/mark/devel/14_2D_3D/nest2d/src/main.cpp:62:85: error: no match for call to ‘(const pybind11::detail::overload_cast_impl<std::vector<libnest2d::_Item<ClipperLib::Polygon>, std::allocator<libnest2d::_Item<ClipperLib::Polygon> > >&, const libnest2d::_Box<ClipperLib::IntPoint>&>) (<unresolved overloaded function type>)’
       62 |     m.def("nest", py::overload_cast<std::vector<Item>&, const Box&>(&libnest2d::nest)
          |                                                                                     ^
    In file included from /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/pytypes.h:12,
                     from /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/cast.h:13,
                     from /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/attr.h:13,
                     from /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/pybind11.h:44,
                     from /home/mark/devel/14_2D_3D/nest2d/src/main.cpp:1:
    /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/detail/common.h:736:20: note: candidate: ‘template<class Return> constexpr decltype (pf) pybind11::detail::overload_cast_impl<Args>::operator()(Return (*)(Args ...)) const [with Return = Return; Args = {std::vector<libnest2d::_Item<ClipperLib::Polygon>, std::allocator<libnest2d::_Item<ClipperLib::Polygon> > >&, const libnest2d::_Box<ClipperLib::IntPoint>&}]’
      736 |     constexpr auto operator()(Return (*pf)(Args...)) const noexcept
          |                    ^~~~~~~~
    /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/detail/common.h:736:20: note:   template argument deduction/substitution failed:
    /home/mark/devel/14_2D_3D/nest2d/src/main.cpp:62:85: note:   couldn’t deduce template parameter ‘Return’
       62 |     m.def("nest", py::overload_cast<std::vector<Item>&, const Box&>(&libnest2d::nest)
          |                                                                                     ^
    In file included from /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/pytypes.h:12,
                     from /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/cast.h:13,
                     from /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/attr.h:13,
                     from /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/pybind11.h:44,
                     from /home/mark/devel/14_2D_3D/nest2d/src/main.cpp:1:
    /home/mark/devel/14_2D_3D/nest2d/lib/pybind11/include/pybind11/detail/common.h:740:20: note: candidate: ‘template<class Return, class Class> constexpr decltype (pmf) pybind11::detail::overload_cast_impl<Args>::operator()(Return (Class::*)(Args ...), std::false_type) const [with Re

src/main.cpp:

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>

#include <libnest2d/libnest2d.hpp>

namespace py = pybind11;

using Point = libnest2d::Point;
using Box = libnest2d::Box;
using Item = libnest2d::Item;

PYBIND11_MODULE(nest2d, m)
{
    m.doc() = "2D irregular bin packaging and nesting for python";

    py::class_<Point>(m, "Point", "2D Point")
        .def(py::init<int, int>(),  py::arg("x"), py::arg("y"))
        //.def_property_readonly("x", &Point::X) // TODO
        //.def_property_readonly("y", &Point::Y)
        .def("__repr__",
             [](const Point &p) {
                 std::string r("Point(");
                 r += boost::lexical_cast<std::string>(p.X);
                 r += ", ";
                 r += boost::lexical_cast<std::string>(p.Y);
                 r += ")";
                 return r;
             }
        )
        .def("__eq__",
            [](const Point &p, const Point & q) {
                return p == q;
            }
        );

    // see lib/libnest2d/include/libnest2d/geometry_traits.hpp
    py::class_<Box>(m, "Box", "2D Box point pair")
        .def(py::init<int, int>())
        ;

    // Item is a shape defined by points
    // see lib/libnest2d/include/libnest2d/nester.hpp
    py::class_<Item>(m, "Item", "An item to be placed on a bin.")
        .def(py::init<std::vector<Point>>())
        .def("__repr__",
             [](const Item &i) {
                 std::string r("Item area: ");
                 r += boost::lexical_cast<std::string>(i.area());
                 return r;
             }
        )
        ;

    // The nest function takes two parameters input and box
    // see lib/libnest2d/include/libnest2d/libnest2d.hpp
    m.def("nest", py::overload_cast<std::vector<Item>&, const Box&>(&libnest2d::nest)
        )
        ;

}

to make this a MRE we need the CMakeLists.txt (cmake build) as well:

cmake_minimum_required(VERSION 2.8.12)

set(CMAKE_VERBOSE_MAKEFILE ON)

project(nest2d)

# Boost and its components
find_package( Boost REQUIRED )

if ( NOT Boost_FOUND )

    message(STATUS "This project requires the Boost library, and will not be compiled.")

    return()

endif()

add_subdirectory(lib/pybind11)
add_subdirectory(lib/libnest2d)

pybind11_add_module(nest2d src/main.cpp)
target_link_libraries(nest2d PUBLIC libnest2d)

Simply means that the overload (i.e. one with parameters std::vector<Item>&, const Box&) can not be matched. If I found the correct header, then it looks like nest requires at least 5 parameters. — Wim Lavrijsen
– Wim Lavrijsen, Commented Dec 26, 2019 at 20:41
the "simple" c++ example from the README.md uses only two parameters (this works): size_t bins = nest(input, Box(150000000, 150000000)); so it appears the parameter count is not the problem. — moin moin
– moin moin, Commented Dec 26, 2019 at 22:35
That is very different: when calling a function, defaults are considered and do not need to be provided explicitly. Here, however, is a template resolution to create a cast, not a function call. — Wim Lavrijsen
– Wim Lavrijsen, Commented Dec 26, 2019 at 23:09
that is great insight and was instrumental for resolving this issue. Thank you so much! — moin moin
– moin moin, Commented Dec 29, 2019 at 9:09

moin moin · Accepted Answer · 2019-12-29 09:23:28Z

5

The issue was related to the number of parameters used in the cast. I was too focused on the implementation of the simple case to see this at first. Thanks to Wim Lavrijsen for pointing this out (see comments). I found that in this case I don't need the overload_cast at all but can explicitly relay the call.

m.def("nest", [](std::vector<Item> input, const Box& box) {
        return libnest2d::nest(input, box);
    }
    );

answered Dec 29, 2019 at 9:23

moin moin

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