I'm working with boost example httpserver3 : http://www.boost.org/doc/libs/1_53_0/doc/html/boost_asio/examples.html#boost_asio.examples.http_server_3
I can't clearly understand what happens when the reset method of new_connection_ is called
void server::start_accept()
{
new_connection_.reset(new connection(io_service_, request_handler_));
acceptor_.async_accept(new_connection_->socket(),
boost::bind(&server::handle_accept, this,
boost::asio::placeholders::error));
}
void server::handle_accept(const boost::system::error_code& e)
{
if (!e)
{
new_connection_->start();
}
std::cout<<"handle_accept"<<std::endl;
start_accept();
}
I know that "reset" will cause the previous object to be deleted as the documentation says. what happens if the server object accepts a new connection while previous connection object is handling it's request? will the previous connection be destroyed immediately or it will receive an error and exits?
void connection::handle_read(const boost::system::error_code& e,
std::size_t bytes_transferred)
{
if (!e)
{
boost::tribool result;
boost::tie(result, boost::tuples::ignore) = request_parser_.parse(
request_, buffer_.data(), buffer_.data() + bytes_transferred);
if (result)
{
request_handler_.handle_request(request_, reply_);
boost::asio::async_write(socket_, reply_.to_buffers(),
strand_.wrap(
boost::bind(&connection::handle_write, shared_from_this(),
boost::asio::placeholders::error)));
}
else if (!result)
{
reply_ = reply::stock_reply(reply::bad_request);
boost::asio::async_write(socket_, reply_.to_buffers(),
strand_.wrap(
boost::bind(&connection::handle_write, shared_from_this(),
boost::asio::placeholders::error)));
}
else
{
socket_.async_read_some(boost::asio::buffer(buffer_),
strand_.wrap(
boost::bind(&connection::handle_read, shared_from_this(),
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred)));
}
}
// If an error occurs then no new asynchronous operations are started. This
// means that all shared_ptr references to the connection object will
// disappear and the object will be destroyed automatically after this
// handler returns. The connection class's destructor closes the socket.
}
void connection::handle_write(const boost::system::error_code& e)
{
if (!e)
{
// Initiate graceful connection closure.
boost::system::error_code ignored_ec;
socket_.shutdown(boost::asio::ip::tcp::socket::shutdown_both, ignored_ec);
}
// No new asynchronous operations are started. This means that all shared_ptr
// references to the connection object will disappear and the object will be
// destroyed automatically after this handler returns. The connection class's
// destructor closes the socket.
}
Ah, the magic of shared_from_this()
. You've missed this part of the code:
void connection::start()
{
socket_.async_read_some(boost::asio::buffer(buffer_),
strand_.wrap(
boost::bind(&connection::handle_read, shared_from_this(),
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred)));
}
What happens is this:
server::handle_accept()
calls new_connection_->start()
.connection::start()
schedules a read operation, and by doing this, creates a copy of the original shared pointer with the shared_from_this()
call.server::handle_accept()
calls server::start_accept()
.server::start_accept()
does reset()
the original shared pointer, but the ASIO service has a copy of it, so as long as connection's callbacks keep scheduling more operations using shared_from_this()
, the connection won't be destroyed (because there'll be at least one shared pointer still pointing to the heap-allocated connection
object).