pybind11 with "auto" keyword for overloaded function

Question

I would like help in wrapping an overloaded function that uses the return type of "auto". For example, the functions on lines 699 and 708 at https://github.com/microsoft/SEAL/blob/master/native/src/seal/ciphertext.h

        SEAL_NODISCARD inline auto &scale() noexcept
        {
            return scale_;
        }
        SEAL_NODISCARD inline auto &scale() const noexcept
        {
            return scale_;
        }

When I try to bind as follows,

py::class_<Ciphertext>(m, "Ciphertext")  
     .def("scale", (auto  (Ciphertext::*)() const)&Ciphertext::scale, "returns a constant reference to the scale")   

I see this error

...mseal.cpp:223:18: error: invalid use of ‘auto’
   .def("scale", (auto (Ciphertext::*)() const)&Ciphertext::scale, "returns a constant reference to the scale")

I am using C++17 and python3. I don't want to modify the C++ SEAL library. Thank you.

Answer 1

EDIT: I just found out that pybind11 has a helper construct to do the same thing, leading to much simpler/cleaner code. Replace the PYBIND11_MODULE block in the original answer with:

PYBIND11_MODULE(mseal, m) {
    py::class_<Ciphertext>(m, "Ciphertext")
        .def(py::init<>())
        .def("scale",
             py::overload_cast<>(&Ciphertext::scale, py::const_),
             "returns a constant reference to the scale")
        .def("scale_nonconst",
             py::overload_cast<>(&Ciphertext::scale),
             "returns a reference to the scale");
}

Original answer: You will need decltype to get to the return type and std::declval to disambiguate the overload for decltype. A full working example follows (C++14 minimum), where I've added the non-const version just to show that you have full control over the selection of either:

#include <pybind11/pybind11.h>
#include <pybind11/pytypes.h>

#include <iostream>
#include <utility>

namespace py = pybind11;

class Ciphertext {
public:
    inline auto &scale() noexcept {
        std::cerr << "non-const called" << std::endl;
        return scale_;
    }
    inline auto &scale() const noexcept {
         std::cerr << "const called" << std::endl;
       return scale_;
    }

private:
    int scale_;
};


PYBIND11_MODULE(mseal, m) {
    py::class_<Ciphertext>(m, "Ciphertext")
        .def(py::init<>())
        .def("scale",
             static_cast<decltype(std::declval<Ciphertext const&>().scale()) (Ciphertext::*)() const>(&Ciphertext::scale),
             "returns a constant reference to the scale")
        .def("scale_nonconst",
             static_cast<decltype(std::declval<Ciphertext&>().scale()) (Ciphertext::*)()>(&Ciphertext::scale),
             "returns a reference to the scale");
}

Which, when compiled into mseal.so works as expected:

>>> import mseal
>>> mseal.Ciphertext().scale()
const called
0
>>> mseal.Ciphertext().scale_nonconst()
non-const called
0
>>>