title: MongoDB C++ driver BSON() usage to generate BSON with null values
body: <p>Is there a way of using BSON() macro from the MongoDB C++ driver in order to generate a BSON will null value. For examples, in order to generate a BSON to represent this document: <code>{"a": "foo", "b": null}</code>:</p>
  
  <pre><code>BSONObj myObj = BSON("a" &lt;&lt; "foo" &lt;&lt; "b" &lt;&lt; &lt;something that I don't know&gt;);
  </code></pre>
  
  <p>I have tried with this (naïve) approach but it doesn't work:</p>
  
  <pre><code>BSONObj myObj = BSON("a" &lt;&lt; "foo" &lt;&lt; "b" &lt;&lt; NULL);
  </code></pre>