Using std::smart_ptr with Pybind11

Question

I am starting to use Pybind11. I am planning to use Pybind11 in a complex project for exposing c++ structures to python, will be helpful to see an example showing how to expose populate from python an array using the following structures:

template <typename T>
struct point;

template <typename T>
struct quaternion;

template <typename T>
struct point{
T _x, _y, _z;
};

template <typename T>
struct quaternion{
T _a, _b_i, _c_j, _d_k;
};

It is a good idea to use std::shared_ptr pointers. How to use std::smart_ptr with pybind11? In more concrete terms, I want to be able of populating the array from python and once the population is complete, I want to be capable of passing a range of pointers(std::smart_ptr) pointing to a region of the array from python to a routine in C++ to do something else with the data. It could be passing a pointer to the beginning and another the end of a chunk of data in the array scan the data using pointer arithmetics.

Disclaimer: I don't know if Pybin11 might be using std::shared_ptr under the hood by default.

What is std::smart_ptr? Do you mean smart pointers in general? — Melebius
– Melebius, Commented Jun 17, 2020 at 13:00
yes, in particular, std::share_ptr en.cppreference.com/w/cpp/memory/shared_ptr — user3116936
– user3116936, Commented Jun 17, 2020 at 13:23
can you provide examples of what you're expecting the python code that is consuming the bindings to look like? — Jesse C
– Jesse C, Commented Jun 17, 2020 at 16:40

Piotr Barejko · Accepted Answer · 2020-07-16 23:16:50Z

I believe that for point, quaternion etc, a floating point type template argument is used to accommodate required precision. If in your situation, aforementioned structures, are used to build aliases such as:

using Point = point<float>

Then the solution is simple, you wrap them in following way:

    py::class_<Point>(m, "Point")
            .def(py::init<>())
            .def_property("x", [](const Point& o){ return o._x; }, [](Point& o, float v){ o._x = v;});

Situation gets more complicated when in the C++ code, point<float>, point<double> etc appear. Those are two different types, and they'd have to be wrapped independently:

template <typename T>
struct point{
    T _x, _y, _z;
};

template<typename T>
void wrap_point(const char* type_name, py::module& m)
{
    py::class_<point<T>>(m, type_name)
            .def(py::init<>())
            .def_property("x", [](const point<T>& o){ return o._x; }, [](point<T>& o, T v){ o._x = v;});
}

PYBIND11_MODULE(example, m) {
    wrap_point<float>("pointf", m);
    wrap_point<double>("pointd", m);
}

Your question about the usage of shared_ptr is unfortunately too vague, you have to be more specific and give more context.

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