如何在 pybind11 和 c++ 中通過引用傳遞向量

Question

我嘗試通過引用從 python 通過 pybind11 將向量/數組傳遞到 C++ 庫。 C++ 庫可以填寫數據。 調用C++后，希望python端能拿到數據。

這是簡化的 C++ 代碼：

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

class Setup
{
public:
    Setup(int version) : _version(version) {}
    int _version;
};

class Calculator
{
public:
    Calculator() {}
    static void calc(const Setup& setup, std::vector<double>& results) { ... }
}

namespace py = pybind11;

PYBIND11_MODULE(one_calculator, m) {
    // optional module docstring
    m.doc() = "pybind11 one_calculator plugin";

    py::class_<Setup>(m, "Setup")
        .def(py::init<int>());

    py::class_<Calculator>(m, "Calculator")
        .def(py::init<>())
        .def("calc", &Calculator::calc);
}

在 python 方面，我打算：

import os
import sys
import numpy as np
import pandas as pd
sys.path.append(os.path.realpath('...'))
from one_calculator import Setup, Calculator

a_setup = Setup(1)
a_calculator = Calculator()

results = []
a_calculator.calc(a_setup, results)

results

顯然結果沒有傳回。 有沒有一種巧妙的方法來做到這一點？

Answer 1

想出了一個辦法：

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

#include "Calculator.h" // where run_calculator is

namespace py = pybind11;

// wrap c++ function with Numpy array IO
int wrapper(const std::string& input_file, py::array_t<double>& in_results) {
    if (in_results.ndim() != 2)
        throw std::runtime_error("Results should be a 2-D Numpy array");

    auto buf = in_results.request();
    double* ptr = (double*)buf.ptr;

    size_t N = in_results.shape()[0];
    size_t M = in_results.shape()[1];

    std::vector<std::vector<double> > results;

    run_calculator(input_file, results);

    size_t pos = 0;
    for (size_t i = 0; i < results.size(); i++) {
        const std::vector<double>& line_data = results[i];
        for (size_t j = 0; j < line_data.size(); j++) {
            ptr[pos] = line_data[j];
            pos++;
        }
    }
}

PYBIND11_MODULE(calculator, m) {
    // optional module docstring
    m.doc() = "pybind11 calculator plugin";

    m.def("run_calculator", &wrapper, "Run the calculator");
}

Python側

results= np.zeros((N, M))
run_calculator(input_file, results)

這樣，我也不會將類 Setup 和 Calculator 暴露給 python 端。