title: Map two-dimensional array to Eigen::Matrix
body: <p>I can't figure out if and how it's possible to map a two-dimensional double array to an Eigen::Matrix.
  Is it possible to map an array <code>double d[][]</code> which I receive as <code>double** p</code> to an Eigen::Matrix?</p>
  
  <p>While one-dimensinal arrays work fine, I wasn't able to map <code>p</code> to <code>Eigen::Map&lt;Eigen::Matrix&lt;double, n, n&gt;&gt;</code>. Is that possible and how could it be done? The size <code>n</code> is not really constant, but I could accept a hard coded size.</p>
  
  <hr>
  
  <p>I tried several versions, but none worked. I thought the following should work (assume the size <code>n</code> would be 4).</p>
  
  <pre><code>Eigen::Map&lt;Eigen::Matrix&lt;double, 4, 4&gt;&gt; p_OUTPUT(&amp;p[0][0]);
  </code></pre>
  
  <p>The code compiles and runs, but only the elements of the first column and the first element of the second column map the correct values. Using <code>p[0]</code> as argument yields the same result.
  Other versions I tried (for example without the <code>&amp;</code>) did not compile.</p>