title: How do I access an JSON array with boost::property_tree？
body: <pre><code>{
      "menu": 
      {
          "foo": true,
          "bar": "true",
          "value": 102.3E+06,
          "popup": 
          [
              {"value": "New", "onclick": "CreateNewDoc()"},
              {"value": "Open", "onclick": "OpenDoc()"},
          ]
      }
  }
  </code></pre>
  
  <p>How can I get the value of onclick?</p>