title: How to typedef a std::array with a unspecified size?
body: <p>I want to write some variables like</p>
  
  <pre><code>std::array&lt;double, array_num&gt; a;
  </code></pre>
  
  <p>where <code>array_num</code> is a <code>const int</code> representing the length of the array. But it's long and I want to create an alias for it:</p>
  
  <pre><code>typedef std::array&lt;double, array_num&gt; my_array;
  </code></pre>
  
  <p>Is it right? How can I use <code>my_array</code> like <code>my_array&lt;3&gt;</code>?</p>