[boost] [asio] How to initialize a strand

Hello, I am just curious - how you can you initialize an strand via assignment? I am passing an strand by reference to an object so I have a reference variable declared: boost::asio::io_service::strand & strand_; In the constructor I would like to assign the reference variable an io_service (which has also been passed by reference). I would like to initialize strand_ in a way such as this ( I know it doesn't work) strand_ = boost::asio::io_service::strand(io_service_); Any ideas? Thanks