title: What does BOOST_SERIALIZATION_NVP do when serializing object?
body: <p>I am using boost.serialization. some sample code use BOOST_SERIALIZATION_NVP in serialize method:</p>
  
  <pre><code>template&lt;class Archive&gt;
  void save(Archive &amp; ar, const unsigned int version) const
  {
     ar &amp; BOOST_SERIALIZATION_NVP(_from_prop);
  }
  </code></pre>
  
  <p>I tried to google its functionality but nothing useful is found. what is the diff between</p>
  
  <pre><code> ar &amp; BOOST_SERIALIZATION_NVP(_from_prop)
  </code></pre>
  
  <p>and </p>
  
  <pre><code> ar &amp; _from_prop? 
  </code></pre>