title: Boost::shared_ptr and its attributes
body: <p>Under the boost library I often see objects like</p>
  
  <pre><code>   boost::shared_ptr&lt;std::string&gt;
     boost:shared_ptr&lt;tcp_connection&gt;
  
     boost::enable_shared_from_this&lt;tcp_connection&gt;
  </code></pre>
  
  <p>What type of attributes does shared_ptr has?  and how does enable_shared_from_this relate to shared_ptr?</p>