title: Copy or reference when accessing row vectors of Eigen matrices
body: <p>I'm working with code that uses the <a href="http://eigen.tuxfamily.org/index.php?title=Main_Page" rel="nofollow">Eigen</a> matrix library. I've noticed that all through that code, there are accessors as follows: </p>
  
  <pre><code>RowVector3f V(size_t vertex_index) const
  {
      return mymatrix.row(vertex_index);
  }
  </code></pre>
  
  <p>Wouldn't it be better to use an accessor that returns a const ref? Is that possible with Eigen? Or maybe just return the pointer to internal float to the row-begin and cast it to a vector (of course assuming matching row-major layout)?</p>