boost bind and place holders

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C++

Boost

Hi all,
I am learning boost library for the network programming and i read about boost bind and have some doubts in the following code.

boost::asio::async_read_until(socket_, buf, boost::regex("\r\n\r\n"),
          boost::bind(&connection::handle_read, shared_from_this(),
                      ba::placeholders::error,
                      ba::placeholders::bytes_transferred));

here inside bind it has three arguments

(shared_from_this(),
                      ba::placeholders::error,
                      ba::placeholders::bytes_transferred)

but the member function "handle_read" inside the "connection" class has only two arguments as given below.

void handle_read(const boost::system::error_code& error,
                    size_t bytes_transferred) {
       ba::async_write(socket_, ba::buffer(message_),
                       boost::bind(&connection::handle_write, shared_from_this(),
                                   ba::placeholders::error,
                                   ba::placeholders::bytes_transferred));
   }

i dont understand why bind uses an extra argument shared_from_this(),i also want to know about the placeholders.
can any one please clarify on this .


with regards,
hari .