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如何使用pybind11将原始指针转换为轻量级python数据类型?

pybind11 wrapper ctypes c++ python

BPL · 技术社区 · 3 年前

考虑一下这个小小的pybind11包装器+测试:

setup.py

from pybind11.setup_helpers import Pybind11Extension
from pybind11.setup_helpers import build_ext
from setuptools import setup

setup(
    name="wrapper",
    ext_modules=[Pybind11Extension("wrapper", ["wrapper.cpp"])],
    cmdclass={"build_ext": build_ext},
)

wrapper.cpp

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

namespace py = pybind11;

struct Mesh2d {
public:
  Mesh2d(int numVerts, int numTris)
      : numVertices(numVerts), numTriangles(numTris) {
    vertices = new float[numVerts * 2];
    indices = new int[numTris * 3];
  }

  ~Mesh2d() {
    delete vertices;
    delete indices;
  }

  void makeVertex(int i, float x, float y) {
    vertices[i * 2 + 0] = x;
    vertices[i * 2 + 1] = y;
  }

  void makeTriangle(int i, int a, int b, int c) {
    indices[i * 3 + 0] = a;
    indices[i * 3 + 1] = b;
    indices[i * 3 + 2] = c;
  }

  float *vertices;
  int *indices;
  int numVertices;
  int numTriangles;
};

PYBIND11_MODULE(wrapper, m) {
  m.doc() = "mcve";
  py::class_<Mesh2d>(m, "Mesh2d")
      .def(py::init<int, int>())
      .def("make_vertex", &Mesh2d::makeVertex)
      .def("make_triangle", &Mesh2d::makeTriangle)
      .def_readonly("vertices", &Mesh2d::vertices)
      .def_readonly("indices", &Mesh2d::indices);
}

test.py

from wrapper import Mesh2d

def test():
    m = Mesh2d(3, 1)
    m.make_vertex(0, -1.0, -1.0)
    m.make_vertex(1, 1.0, -1.0)
    m.make_vertex(2, 0.0, 1.0)
    m.make_triangle(0, 0, 1, 2)
    print(m.vertices.__class__)
    print(m.indices.__class__)

test()

提问

现在,因为我对顶点和索引都没有做任何特别的事情 __class__ 我即将进入python世界 <class 'float'> 和 <class 'int'> 分别。将这些原始指针转换为合适对象的正确pybind11方法是什么,例如:

array.array("f", vertices)
array.array("I", indices)

(ctypes.c_float * 6)(*vertices)
(ctypes.c_uint * 3)(*indices)

memoryview(struct.pack("6f", *vertices))
memoryview(struct.pack("3I", *indices))

struct.pack("6f", *vertices)
struct.pack("3I", *indices)

我在这里的最终目标是能够在c++端生成非常复杂的重数据(将实时修改的重数据)。因此,包装器上的开销很小(理想情况下,不需要对python数据结构进行任何不必要的复制,只需要提供原始指针)。

参考资料

https://pybind11.readthedocs.io/en/stable/advanced/functions.html#return-value-policies

0 回复 | 直到 3 年前

jlucier 3 年前

如果我没记错的话,你应该能够使用 numpy 内存的数组视图非常便宜(如果不是几乎没有开销的话)。这里有一个完整的例子。

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

namespace py = pybind11;

struct Mesh2d {
public:
  Mesh2d(int numVerts, int numTris)
      : numVertices(numVerts), numTriangles(numTris) {
    vertices = new float[numVerts * 2];
    indices = new int[numTris * 3];

    // these constructors are just views of the memory and will not copy the data
    // (nor take ownership of the pointer)
    // You could also make these shapes {numVerts, 2} and {numTris, 3} or whatever else is helpful to you.
    pyVertices = py::array_t<float>({numVerts * 2}, vertices);
    pyIndices = py::array_t<int>({numTris * 3}, indices);
  }

  ~Mesh2d() {
    delete vertices;
    delete indices;
  }

  void makeVertex(int i, float x, float y) {
    vertices[i * 2 + 0] = x;
    vertices[i * 2 + 1] = y;
  }

  void makeTriangle(int i, int a, int b, int c) {
    indices[i * 3 + 0] = a;
    indices[i * 3 + 1] = b;
    indices[i * 3 + 2] = c;
  }

  float *vertices;
  int *indices;
  py::array_t<float> pyVertices;
  py::array_t<int> pyIndices;
  int numVertices;
  int numTriangles;
};

PYBIND11_MODULE(wrapper, m) {
  m.doc() = "mcve";
  py::class_<Mesh2d>(m, "Mesh2d")
      .def(py::init<int, int>())
      .def("make_vertex", &Mesh2d::makeVertex)
      .def("make_triangle", &Mesh2d::makeTriangle)
      .def_readonly("vertices", &Mesh2d::pyVertices)
      .def_readonly("indices", &Mesh2d::pyIndices);
}

构造函数用于 py::array_t 是 here . 我还想指出 xtensor 如果你正在寻找类似C++中的numpy。