title: Checking if a path exists in a boost property tree
body: <p>How do I check if a path exists in a property tree?</p>
  
  <p>Example:</p>
  
  <pre><code>boost::property_tree::ptree tree;
  
  // if path doesn't exist, put value
  if (/*...*/)
  {
      tree.put("my.path.to.thing", true);
  }
  </code></pre>