Re: [boost] [asio] Tutorial 7 using strdup, new

From: Christopher Kohlhoff (chris_at_[hidden])
Date: 2006-02-08 07:51:59

Hi Peter,

--- Peter Dimov <pdimov_at_[hidden]> wrote:
> Can you explain the "exactly once" principle in more detail, or point
> me to the documentation that discusses it?

Basically if:

- the function used to initiate the operation does not throw;
  and

- the demuxer::run() call is run until there is no work left.

the handlers for the operations are guaranteed to be called.

See:

 
http://asio.sourceforge.net/boost-asio-proposal-0.3.6/libs/asio/doc/reference/demuxer_handler_exception.html

for example code showing how to ensure that demuxer::run() keeps
going until all work is completed even if exceptions are thrown.

>     boost::asio::socket_acceptor tcp_acceptor(demuxer,
>         boost::asio::ipv4::tcp::endpoint(13));
> 
>     boost::asio::stream_socket* tcp_socket
>       = new boost::asio::stream_socket(demuxer);
> 
>     tcp_acceptor.async_accept(*tcp_socket,
>         boost::bind(handle_tcp_accept, &tcp_acceptor, tcp_socket,
>           boost::asio::placeholders::error));
>
> For example, what happens here if async_accept fails, perhaps
> because of insufficient memory? Does it throw bad_alloc?

It could.

> Is the handler executed?

No (according to above rules).

This particular code doesn't seem to be exception safe as it
stands :o) It should be more like:
  
    std::auto_ptr<boost::asio::stream_socket> tcp_socket(
        new boost::asio::stream_socket(demuxer));

    tcp_acceptor.async_accept(*tcp_socket,
        boost::bind(handle_tcp_accept, &tcp_acceptor, tcp_socket,
          boost::asio::placeholders::error));

    tcp_socket.release();

However, this is probably all too complicated for a tutorial.
I'm now leaning towards just going with the shared_from_this()
style for the tutorial (as used in the HTTP server example),
with an appropriate link into the shared_ptr docs explaining it.
If a developer is sufficiently motivated to want to eliminate
the costs of using shared_ptr, I'll point them to other examples
that showcase alternative styles.

Cheers,
Chris