CODEWITHSUNDEEP
pythonc++pass-by-referencepybind11
xycs
xycs

Reputation: 131

How to pass a vector by reference in pybind11 & c++

I try to pass vector/array by reference from python through pybind11 to a C++ library. The C++ library may fill in data. After the call to C++, I hope the python side will get the data.

Here is the simplified C++ code:

#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);
}

On the python side, I intend to:

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

Apparently the results are not passed back. Is there a neat way to do it?

Upvotes: 3

Views: 5556

Answers (1)

xycs
xycs

Reputation: 131

Figured out a way:

#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 side

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

This way I also do not expose classes Setup and Calculator to the python side.

Upvotes: 3

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