I'm reading http://eigen.tuxfamily.org/dox-devel/cl ... _1Ref.html. According to this page, I believe that this program [obviously change your eigen path], should work:
**********
#include <iostream>
#include "/Users/rif/tmp/eigen3.2.4/Eigen/Core"

void PrintRowOrColumnConst(const Eigen::Ref<const Eigen::VectorXf>& v) {
std::cout << v << "\n\n";
}

int main(int argc, char** argv) {
Eigen::MatrixXf M(3, 4);
M << 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12;
// 1 2 3 4
// 5 6 7 8
// 9 10 11 12
std::cout << M << "\n\n";

PrintRowOrColumnConst(M.row(1));
}
**********
M.row(1) doesn't reference contiguous data, so it should copy to a temporary. What actually happens is that we get a runtime crash about row and column sizes not matching, I assume because the function expects a VectorXf, which is an n-by-1 structure, and it gets a 1-by-n structure. So I think the webpage should *either* change

* foo2(A.row()); // The row is copied into a contiguous temporary
to
* foo2(A.row().transpose()); // The row is copied into a contiguous temporary

or should define its functions to take Ref<MatrixXf> rather than Ref<VectorXf>. Am I interpreting correctly?

rif

You are perfectly right, we need to add a .transpose().