title: Creating JSON arrays in Boost using Property Trees
body: <p>I'm trying to create a JSON array using boost property trees. </p>
  
  <p>The <a href="http://www.boost.org/doc/libs/1_49_0/doc/html/boost_propertytree/parsers.html#boost_propertytree.parsers.json_parser" rel="noreferrer">documentation</a> says: "JSON arrays are mapped to nodes. Each element is a child node with an empty name."</p>
  
  <p>So I'd like to create a property tree with empty names, then call <code>write_json(...)</code> to get the array out. However, the documentation doesn't tell me how to create unnamed child nodes. I tried <code>ptree.add_child("", value)</code>, but this yields:</p>
  
  <pre><code>Assertion `!p.empty() &amp;&amp; "Empty path not allowed for put_child."' failed
  </code></pre>
  
  <p>The documentation doesn't seem to address this point, at least not in any way I can figure out. Can anyone help?</p>