[Boost-users] Handler requirement for Asynchronous Operations in boost::asio

Subject: [Boost-users] Handler requirement for Asynchronous Operations in boost::asio
From: Tu Hoang Do (solidius4747_at_[hidden])
Date: 2011-12-19 05:02:51

It is specified in
http://www.boost.org/doc/libs/1_47_0/doc/html/boost_asio/reference/AcceptHandler.html
that a handler for `async_accept()` must satisfy the following function
signature:

    void accept_handler(
        const boost::system::error_code& ec)
    {
      ...
    }

However, in the `Daytime.3` example, using boost::bind, the handler can be
specified as much parameter as desired, as long as it does not exceed the
limit of `boost::bind` (which is 9 arguments at maximum):

    class tcp_server
    {
    public:
      tcp_server(boost::asio::io_service& io_service)
        : acceptor_(io_service, tcp::endpoint(tcp::v4(), 13))
      {
        start_accept();
      }

    private:
      void start_accept()
      {
        tcp_connection::pointer new_connection =
          tcp_connection::create(acceptor_.get_io_service());

        acceptor_.async_accept(new_connection->socket(),
            boost::bind(&tcp_server::handle_accept, this, new_connection,
              boost::asio::placeholders::error));
      }

      void handle_accept(tcp_connection::pointer new_connection,
          const boost::system::error_code& error)
      {
        if (!error)
        {
          new_connection->start();
        }

        start_accept();
      }

      tcp::acceptor acceptor_;
    };

Why is that possible? I thought even with `boost::bind`, the exact function
signature still has to be matched.

Notice the `handle_accept()` funciton and how it is used in
`async_accept()`. The full code listing is here:
http://www.boost.org/doc/libs/1_48_0/doc/html/boost_asio/tutorial/tutdaytime3/src.html