pybind with array as class attribute - pybind11

Question

I want to wrap the following C++ code into python using pybind
class Galaxy {
public:
double x[3];
double v[3];
};
class GalaxyCatalogue {
public:
long n_tracers;
Galaxy *object;
GalaxyCatalogue(long n_tracers);
~GalaxyCatalogue();
};
GalaxyCatalogue::GalaxyCatalogue(long n_tracers) : n_tracers(n_tracers) {
std::cout << "from galaxies " << n_tracers << std::endl;
object = new Galaxy[n_tracers];
std::cout << "has been allocated " << std::endl;
}
GalaxyCatalogue::~GalaxyCatalogue() {
delete[] object;
}
The first problem I have is that Galaxy doesn't have a constructor, so I'm not sure what to do with that. Even if I declare an empty constructor I don't know how to treat the array in a way that I don't get an error when compiling. This is what I've tried:
#include <pybind11/pybind11.h>
#include <iostream>
namespace py = pybind11;
class Galaxy {
public:
Galaxy();
double x[3];
};
PYBIND11_MODULE(example, m){
py::class_<Galaxy>(m, "Galaxy")
.def(py::init<>())
.def_readwrite("x", &Galaxy::x);
}
This is how I compile it:
c++ -O3 -Wall -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` gal.cpp -o example`python3-config --extension-suffix`
and this is the error I get:
In file included from gal.cpp:1:
/home/florpi/.conda/envs/virtualito/include/python3.5m/pybind11/pybind11.h: In instantiation of ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_readwrite(const char*, D C::*, const Extra& ...) [with C = Galaxy; D = double [3]; Extra = {}; type_ = Galaxy; options = {}]’:
gal.cpp:19:33: required from here
/home/florpi/.conda/envs/virtualito/include/python3.5m/pybind11/pybind11.h:1163:65: error: invalid array assignment
fset([pm](type &c, const D &value) { c.*pm = value; }, is_method(*this));
~~~~~~^~~~~~~
In file included from gal.cpp:1:
/home/florpi/.conda/envs/virtualito/include/python3.5m/pybind11/pybind11.h:64:5: error: ‘pybind11::cpp_function::cpp_function(Func&&, const Extra& ...) [with Func = pybind11::class_<type_, options>::def_readwrite(const char*, D C::*, const Extra& ...) [with C = Galaxy; D = double [3]; Extra = {}; type_ = Galaxy; options = {}]::<lambda(pybind11::class_<Galaxy>::type&, const double (&)[3])>; Extra = {pybind11::is_method}; <template-parameter-1-3> = void]’, declared using local type ‘pybind11::class_<type_, options>::def_readwrite(const char*, D C::*, const Extra& ...) [with C = Galaxy; D = double [3]; Extra = {}; type_ = Galaxy; options = {}]::<lambda(pybind11::class_<Galaxy>::type&, const double (&)[3])>’, is used but never defined [-fpermissive]
cpp_function(Func &&f, const Extra&... extra) {
^~~~~~~~~~~~
Thank you in advance.

Answer 1

In C++, you can't assign directly to an array, which is what pybind11 is trying to do inside its wrapping magic. In general, C++ arrays are not great abstractions for numerical arrays. As you've noticed, you can't even say galaxy.x = other_galaxy.x.
Your best bet is to use a higher-level library for matrices and vectors, which will
a) give you a much better experience writing your C++
b) perform better
c) map more cleanly to Python
Eigen is a good choice. pybind11 automatically knows how to map Eigen matrices and vectors to numpy arrays. Your Galaxy would become:
class Galaxy {
public:
Eigen::Vector3d x;
Eigen::Vector3d v;
};
If you absolutely can't do this, you'll have to supply manual getter/setter functions to the property, where you do your own conversion to and from python types:
https://pybind11.readthedocs.io/en/master/classes.html?highlight=def_property#instance-and-static-fields