[/
 / Copyright (c) 2003-2008 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:strands Strands]

A strand is defined as a strictly sequential invocation of event handlers (i.e.
no concurrent invocation). Use of strands allows execution of code in a
multithreaded program without the need for explicit locking (e.g. using
mutexes).

Strands may be either implicit or explicit, as illustrated by the following
alternative approaches:

* Calling io_service::run() from only one thread means all event handlers
  execute in an implicit strand, due to the io_service's guarantee that handlers
  are only invoked from inside run().

* Where there is a single chain of asynchronous operations associated with a
  connection (e.g. in a half duplex protocol implementation like HTTP) there is
  no possibility of concurrent execution of the handlers. This is an implicit
  strand.

* An explicit strand is an instance of `io_service::strand`. All event handler
  function objects need to be wrapped using `io_service::strand::wrap()` or
  otherwise posted/dispatched through the `io_service::strand` object.

In the case of composed asynchronous operations, such as `async_read()` or
`async_read_until()`, if a completion handler goes through a strand, then all
intermediate handlers should also go through the same strand. This is needed to
ensure thread safe access for any objects that are shared between the caller
and the composed operation (in the case of `async_read()` it's the socket,
which the caller can close() to cancel the operation). This is done by having
hook functions for all intermediate handlers which forward the calls to the
customisable hook associated with the final handler:

  struct my_handler
  {
    void operator()() { ... }
  };

  template<class F>
  void asio_handler_invoke(F f, my_handler*)
  {
    // Do custom invocation here.
    // Default implementation calls f();
  }

The `io_service::strand::wrap()` function creates a new completion handler that
defines `asio_handler_invoke` so that the function object is executed through the
strand.

[endsect]
