title: dynamic typedef
body: <p>I have a typedef inside a class from a c++ machine library (dlib) that looks like this:</p>
  
  <pre><code>typedef dlib::matrix&lt;double, 64, 1&gt; sample_type;
  </code></pre>
  
  <p>Except the 64 is actually a variable.  I'd like the typedef to be visible throughout the class, but I want the second parameter (the 64) be an argument when the constructor is called.  How can I achieve this, while keeping the typedef visible everywhere?</p>