title: Faster JsonCpp alternative that allows copying/mutability of Json objects?
body: <p>JsonCpp is slow. And the code is pretty messy.</p>
  
  <p>Is there any alternative that is faster, cleaner and supports stuff like:</p>
  
  <pre><code>Json::Value val, copy;
  val["newMember"] = 100;
  val["newMember2"] = "hello";
  copy = val;
  val["newMember2"] = "bye";
  assert(val["newMember"] == copy["newMember"]);
  assert(val["newMember2"] != copy["newMember2"]);
  </code></pre>
  
  <p>JsonCpp supports code like the one above.</p>
  
  <p>I've tried <strong>rapidjson</strong>, which is <strong>very</strong> fast, but unfortunately it does not support copying Json values.</p>
  
  <p>Any alternative? Bonus point for benchmarks.</p>