io_service::strand

Provides serialised handler execution.

class strand

Member Functions

Name	Description
dispatch	Request the strand to invoke the given handler.
get_io_service	Get the io_service associated with the strand.
post	Request the strand to invoke the given handler and return immediately.
strand	Constructor.
wrap	Create a new handler that automatically dispatches the wrapped handler on the strand.
~strand	Destructor.

The io_service::strand class provides the ability to post and dispatch handlers with the guarantee that none of those handlers will execute concurrently.

Order of handler invocation

Given:

a strand object s
an object a meeting completion handler requirements
an object a1 which is an arbitrary copy of a made by the implementation
an object b meeting completion handler requirements
an object b1 which is an arbitrary copy of b made by the implementation

if any of the following conditions are true:

s.post(a) happens-before s.post(b)
s.post(a) happens-before s.dispatch(b), where the latter is performed outside the strand
s.dispatch(a) happens-before s.post(b), where the former is performed outside the strand
s.dispatch(a) happens-before s.dispatch(b), where both are performed outside the strand

then asio_handler_invoke(a1, &a1) happens-before asio_handler_invoke(b1, &b1).

Note that in the following case:

async_op_1(..., s.wrap(a));
async_op_2(..., s.wrap(b));

the completion of the first async operation will perform s.dispatch(a), and the second will perform s.dispatch(b), but the order in which those are performed is unspecified. That is, you cannot state whether one happens-before the other. Therefore none of the above conditions are met and no ordering guarantee is made.

Thread Safety

Distinct objects: Safe.

Shared objects: Safe.

Requirements

Header: boost/asio/strand.hpp

Convenience header: boost/asio.hpp