@bartfer
If I go this way with exactly the described problem of “?” (and solved it like described)
I still don't get the values of the matrix.

Eigen::MatrixXd A(3,5);
A<< 1, 2, 3, 4, 5,
    2, 3, 4, 5, 6,
    3, 4, 6, 7, 8;

(lldb) p A
(Eigen::Matrix<double,-1,-1,0,-1,-1>) $6 = [3, 5] (dynamic matrix) {
  Eigen::PlainObjectBase<Eigen::Matrix<double,-1,-1,0,-1,-1> > = {...} {
    m_storage = {m_data=0x00000212c03ec980 {1}, m_rows=3, m_cols=5} {
      m_data = 0x00000212c03ec980 {1}
      m_rows = 3
      m_cols = 5
    }
  }
}