Why is a shared pointer used for the string if the function call results in pass by reference?

In this boost async udp server example: http://www.boost.org/doc/libs/1_53_0/doc/html/boost_asio/tutorial/tutdaytime6/src.html

  boost::shared_ptr<std::string> message(
      new std::string(make_daytime_string()));

  socket_.async_send_to(boost::asio::buffer(*message), remote_endpoint_,
      boost::bind(&udp_server::handle_send, this, message,
        boost::asio::placeholders::error,
        boost::asio::placeholders::bytes_transferred));

from http://www.boost.org/doc/libs/1_53_0/doc/html/boost_asio/reference/basic_datagram_socket/async_send_to/overload1.html

the signature of 1st argument is pass by reference

const ConstBufferSequence & buffers 

then why is a shared pointer used for the message to be send?

That is because the string is not just passed as the first argument to async_send_to(), but it is also used in the bind() expression that is being passed to async_send_to() as the third argument.

Function handle_send() expects a shared_ptr to a string. Since the call is asynchronous, a string object with automatic storage duration may have fallen out of scope and get destroyed by the time handle_send() gets executed. Hence, the use of a shared_ptr.