title: How to create Eigen::VectorXd from an array of double pointers
body: <p>I'm a beginner when it comes to Eigen. I have searched but not found an answer to this question.</p>
  
  <p>I have a an array of double pointers, i.e. <code>double** x</code> which I have no control over. It comes in this format and there is nothing I can do about it. I want to construct an <code>Eigen::VectorXd vec</code> that use the same memory as pointed to by x.</p>
  
  <p>In other words, I would like the following to hold true:</p>
  
  <pre><code>vec(i) == *(x[i]) for each i
  </code></pre>
  
  <p>and</p>
  
  <pre><code>&amp;vec(i) == x[i] for each i
  </code></pre>
  
  <p>I hope this makes sense. How can I achieve this?</p>
  
  <p>Thanks in advance!</p>