Hello everyone, I have a member function with signature const Matrix2D & foo const (); Which returns an internal data structure. Matrix2D is using Matrix2D = Eigen::Matrix; I have explicitly set a to_python converter for Matrix2D, and I have exported the function with a call policy of return_internal_reference. However, when this function is called from Python I get TypeError: No Python class registered for C++ class Eigen::Matrix This happens also if the reference is not const. At the same time, if I modify the call policy to return_value_policy() Python can decode the matrix just fine. The same holds true if I return the Matrix2D by value. However I really need to have a reference to the matrix in order store it into other classes. How can I make Python see the data structure through the reference? The converter in case it is needed: boost::python::list toPythonList(const double * data, size_t num) { boost::python::list list; for (size_t i = 0; i < num; ++i) list.append(data[i]); return list;} struct Matrix2DToPython{ static PyObject* convert(Matrix2D const & m) { boost::python::list list; for (int i = 0; i < m.rows(); ++i) { list.append(toPythonList(m.row(i).data(), m.cols())); } return boost::python::incref(list.ptr()); }}; Thanks, Eugenio