...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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:
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.
io_service::strand, tutorial Timer.5, HTTP server 3 example.