io_context

Like traditional Unix network programming, Boost.Asioalso has "socket" concept, but that's not enough, an io_context object (io_service class is deprecated now) is needed to communicate with Operating System's I/O services. The architecture is like following:

io_context derives from execution_context:

class io_context
  : public execution_context
{
......
}

While execution_context derives from noncopyable:

class execution_context
  : private noncopyable
{
......
}

Check noncopyable class definition:

class noncopyable
{
protected:
  noncopyable() {}
  ~noncopyable() {}
private:
  noncopyable(const noncopyable&);
  const noncopyable& operator=(const noncopyable&);
};

It means the io_context object can't be copy constructed/copy assignment/move constructed/move assignment, So during initialization of socket, i.e., associate socket with io_context, the io_context should be passed as a reference. E.g.:

template <typename Protocol
    BOOST_ASIO_SVC_TPARAM_DEF1(= datagram_socket_service<Protocol>)>
class basic_datagram_socket
  : public basic_socket<Protocol BOOST_ASIO_SVC_TARG>
{
public:
......
  explicit basic_datagram_socket(boost::asio::io_context& io_context)
    : basic_socket<Protocol BOOST_ASIO_SVC_TARG>(io_context)
  {
  }
......
}