以数组作为类属性的pybind

Question

我想使用 pybind 将以下 C++ 代码包装到 python 中

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;

}

我遇到的第一个问题是 Galaxy 没有构造函数，所以我不知道该怎么做。即使我声明了一个空的构造函数，我也不知道如何以编译时不会出错的方式处理数组。这是我尝试过的：

#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);
 }

我是这样编译的：

c++ -O3 -Wall -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` gal.cpp -o example`python3-config --extension-suffix`

这是我得到的错误：

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) {
     ^~~~~~~~~~~~

提前谢谢你。

Answer 1

在 C++ 中，您不能直接分配给数组，这正是 pybind11 在其包装魔法中试图做的事情。一般来说，C++ 数组对于数值数组来说并不是很好的抽象。正如您所注意到的，您甚至不能说galaxy.x = other_galaxy.x。

最好的办法是使用更高级别的矩阵和向量库，这将 a) 为您提供更好的 C++ 编写体验 b) 表现更好 c) 更干净地映射到 Python

Eigen 是个不错的选择。 pybind11 自动知道如何将特征矩阵和向量映射到 numpy 数组。你的 Galaxy 会变成：

class Galaxy {
  public:
    Eigen::Vector3d x;
    Eigen::Vector3d v;
};

如果您绝对不能这样做，则必须为属性提供手动 getter/setter 函数，您可以在其中自己转换为 Python 类型： https://pybind11.readthedocs.io/en/master/classes.html?highlight=def_property#instance-and-static-fields