title: what does msgpack's object_with_zone do?
body: <p>When writing a custom serializer for <code>msgpack_c</code> one also needs to implement <code>object_with_zone</code>.</p>
  
  <p>The documentation how to implement this is very sparse ( <a href="https://github.com/msgpack/msgpack-c/wiki/v2_0_cpp_adaptor" rel="nofollow noreferrer">https://github.com/msgpack/msgpack-c/wiki/v2_0_cpp_adaptor</a> ).</p>
  
  <p>In what circumstances is this method called?</p>