Eigen block not lvalue?

Question

I am using Eigen library and its block operations, according to https://eigen.tuxfamily.org/dox/group__TutorialBlockOperations.html the blocks are both lvalues and rvalues. But the following program does not compile:

#include "Eigen/Dense"
#include <iostream>

int main() {
  Eigen::Matrix<float, Eigen::Dynamic, Eigen::Dynamic> m(4, 4); 
  m << 1, 2, 3, 4,
       5, 6, 7, 8,
       9, 10, 11, 12, 
       13, 14, 15, 16; 
  Eigen::Matrix<float, Eigen::Dynamic, Eigen::Dynamic> &column = m.col(0);
} 

In this program, I want to use column as a reference to the matrix m so I could alter the elements in-place. The error message is:

test_eigen_rvalue.cc: In function ‘int main()’:
test_eigen_rvalue.cc:11:73: error: invalid initialization of non-const reference of type ‘Eigen::Matrix<float, -1, -1>&’ from an rvalue of type ‘Eigen::DenseBase<Eigen::Matrix<float, -1, -1> >::ColXpr {aka Eigen::Block<Eigen::Matrix<float, -1, -1>, -1, 1, true>}’
   Eigen::Matrix<float, Eigen::Dynamic, Eigen::Dynamic> &column = m.col(0);

It seems the issue is that m.col(0) is not an lvalue.

Answer 1

The block method does not return a Matrix object but a Block<> expression. In this case, it is a good idea to use the auto keyword (but be careful):

auto column = m.col(0);

Or use the convenient typedefs:

MatrixXf::ColXpr column = m.col(0);

Or even a Ref<>:

Ref<VectorXf> column = m.col(0);