How to pass c++ objects as a parameter by reference to python functions?

[taguhiM]

I am working on a library written in C++, which uses pybind11 to call python functions. I want to access and modify some objects in C++ (e.g. std::vector) from Python.
So, the question is how to pass c++ objects as a parameter by reference to python functions? As far as I understood the standard way pybind11 does this with the automatic conversion is by copy, which slows down the code.
In the make_opaque section of the documentation only the case of calling c++ from python is documented. However, I need the vice versa.

#include <pybind11/embed.h>
#include <pybind11/pybind11.h>
#include "pybind11/stl.h"

namespace py = pybind11;
using namespace py::literals;

int main(){
    std::vector<double> V;
    py::scoped_interpreter guard{};
    py::module_ py_function = py::module_::import("py_function");
    auto x = py_function.attr("something")(V);
    ...
    return 0;
}

[Holt59]

You need to 1) import the module containing the bindings for std::vector<double> and 2) wrap the value in std::ref (or pass a pointer):

#include <memory>
#include <iostream>
#include <vector>
#include <pybind11/pybind11.h>
#include <pybind11/embed.h>
#include <pybind11/stl.h>
#include <pybind11/stl_bind.h>

namespace py = pybind11;

PYBIND11_MAKE_OPAQUE(std::vector<double>);

PYBIND11_EMBEDDED_MODULE(module, m)
{
    py::bind_vector<std::vector<double>>(m, "VectorDouble");
}

int main() {
    std::vector<double> V{1, 2, 3};
    py::scoped_interpreter guard{};
    py::module_ cpp_module = py::module_::import("module");
    py::module_ py_function = py::module_::import("py_function");
    auto x = py_function.attr("double_all_values")(std::ref(V));  // or &V instead of std::ref(V)

    for (const auto& v : V) {
        std::cout << v << ' ';
    }
    std::cout << '\n';

    return 0;
}