handler_ptr gives the strong exception guarantee (API Change):

* The handler_ptr constructor now provides the
  strong exception guarantee.

* The signature for managed objects constructed by
  `handler_ptr` receives a const reference to the handler.

Actions required:

* Change the constructor signature for state objects
  used with handler_ptr to receive a const reference to
  the handler.

example/echo-op/echo_op.cpp

@@ -107,7 +107,7 @@ class echo_op
         // contained object constructor is a reference to the
         // managed final completion handler.
         //
-        explicit state(Handler& handler, AsyncStream& stream_)
+        explicit state(Handler const& handler, AsyncStream& stream_)
             : stream(stream_)
             , buffer((std::numeric_limits<std::size_t>::max)(),
                 boost::asio::get_associated_allocator(handler))