title: boost::variant get last accessed type
body: <p>This is what I want to do:</p>
  
  <pre><code>boost::variant a&lt;int, string&gt;;
  int b;
  a=4;
  b=a; //doesn't work. What is the easiest way to make b=4?
  </code></pre>
  
  <p>I know I can use get, but I want to be able to do this without specifying the type. I can do it with apply_visitor and a visitor object, but I was wondering if there is a simpler way.</p>