title: Create a boost::shared_ptr to an existing variable
body: <p>I have an existing variable, e.g.</p>
  
  <pre><code>int a = 3;
  </code></pre>
  
  <p>How can I now create a <code>boost::shared_ptr</code> to <code>a</code>? For example:</p>
  
  <pre><code>boost::shared_ptr&lt; int &gt; a_ptr = &amp;a; // this doesn't work
  </code></pre>