Reference on Eigen matrix with pybind11 (read/write)

Question

This is an implementation of a class containing a vector of Eigen Matrix plus the definition of the python binding.

#include <vector>

#include <pybind11/eigen.h>
#include <Eigen/Dense>

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

namespace py = pybind11;

using EMatrixDbl = Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic>;

class MyMap
{

public:
    MyMap(int i, int j)
    {
        m_matrices.resize(1);
        m_matrices[0].resize(i, j);
        m_matrices[0].setZero();
    }
    
    EMatrixDbl& operator[](int k)
    {
        return m_matrices[k];
    }
    
    EMatrixDbl& get(int k)
    {
        return m_matrices[k];
    }
    
    std::vector<EMatrixDbl> m_matrices;
    
    
};

PYBIND11_MODULE(MyMapPy, m) {
    
    py::class_<MyMap> c(m, "MyMap");
    
    c.def(py::init<int, int>());
    
    c.def("get", &MyMap::get, py::return_value_policy::reference_internal);

    c.def("get_lambda", [](MyMap& self, int k){
            EMatrixDbl& m_map = self[k];
            return m_map;
        }, py::return_value_policy::reference_internal);
}

When I use the binding, only the method get allow to edit the matrix.

1. Why get_lambda is not able to do the same?
2. How to bind MyMap::operator[] directly to provide a writeable interface of the binding?

import MyMapPy
m = MyMapPy.MyMap(3, 3)

m.get(0)[0,0] = 1
m.get(0)  # [0,0] is 1

m.get_lambda(0)[0,0] = 2
m.get_lambda(0)  # [0,0] is always 1...

Answer 1

The lambda function return type is deduced to EMatrixDbl not EMatrixDbl&. Easiest fix is to specify the return type:

        [](MyMap& self, int k) -> EMatrixDbl&

Result

>>> m.get_lambda(0)[0,0]=2
>>> m.get(0)
array([[2., 0., 0.],
       [0., 0., 0.],
       [0., 0., 0.]])

A more general approach is to specify the return type as decltytpe(auto) see this answer.