system_timer

Typedef for a timer based on the system clock.

typedef basic_waitable_timer< chrono::system_clock > system_timer;

Types

Name	Description
clock_type	The clock type.
duration	The duration type of the clock.
executor_type	The type of the executor associated with the object.
time_point	The time point type of the clock.
traits_type	The wait traits type.

Member Functions

Name	Description
async_wait	Start an asynchronous wait on the timer.
basic_waitable_timer	Constructor. — Constructor to set a particular expiry time as an absolute time. — Constructor to set a particular expiry time relative to now. — Move-construct a basic_waitable_timer from another.
cancel	Cancel any asynchronous operations that are waiting on the timer. — (Deprecated: Use non-error_code overload.) Cancel any asynchronous operations that are waiting on the timer.
cancel_one	Cancels one asynchronous operation that is waiting on the timer. — (Deprecated: Use non-error_code overload.) Cancels one asynchronous operation that is waiting on the timer.
expires_after	Set the timer's expiry time relative to now.
expires_at	(Deprecated: Use expiry().) Get the timer's expiry time as an absolute time. — Set the timer's expiry time as an absolute time. — (Deprecated: Use non-error_code overload.) Set the timer's expiry time as an absolute time.
expires_from_now	(Deprecated: Use expiry().) Get the timer's expiry time relative to now. — (Deprecated: Use expires_after().) Set the timer's expiry time relative to now.
expiry	Get the timer's expiry time as an absolute time.
get_executor	Get the executor associated with the object.
get_io_context	(Deprecated: Use get_executor().) Get the io_context associated with the object.
get_io_service	(Deprecated: Use get_executor().) Get the io_context associated with the object.
operator=	Move-assign a basic_waitable_timer from another.
wait	Perform a blocking wait on the timer.
~basic_waitable_timer	Destroys the timer.

The basic_waitable_timer class template provides the ability to perform a blocking or asynchronous wait for a timer to expire.

A waitable timer is always in one of two states: "expired" or "not expired". If the wait() or async_wait() function is called on an expired timer, the wait operation will complete immediately.

Most applications will use one of the steady_timer, system_timer or high_resolution_timer typedefs.

Remarks

This waitable timer functionality is for use with the C++11 standard library's <chrono> facility, or with the Boost.Chrono library.

Thread Safety

Distinct objects: Safe.

Shared objects: Unsafe.

Examples

Performing a blocking wait (C++11):

// Construct a timer without setting an expiry time.
boost::asio::steady_timer timer(io_context);

// Set an expiry time relative to now.
timer.expires_after(std::chrono::seconds(5));

// Wait for the timer to expire.
timer.wait();

Performing an asynchronous wait (C++11):

void handler(const boost::system::error_code& error)
{
  if (!error)
  {
    // Timer expired.
  }
}

...

// Construct a timer with an absolute expiry time.
boost::asio::steady_timer timer(io_context,
    std::chrono::steady_clock::now() + std::chrono::seconds(60));

// Start an asynchronous wait.
timer.async_wait(handler);

Changing an active waitable timer's expiry time

Changing the expiry time of a timer while there are pending asynchronous waits causes those wait operations to be cancelled. To ensure that the action associated with the timer is performed only once, use something like this: used:

void on_some_event()
{
  if (my_timer.expires_after(seconds(5)) > 0)
  {
    // We managed to cancel the timer. Start new asynchronous wait.
    my_timer.async_wait(on_timeout);
  }
  else
  {
    // Too late, timer has already expired!
  }
}

void on_timeout(const boost::system::error_code& e)
{
  if (e != boost::asio::error::operation_aborted)
  {
    // Timer was not cancelled, take necessary action.
  }
}

The boost::asio::basic_waitable_timer::expires_after() function cancels any pending asynchronous waits, and returns the number of asynchronous waits that were cancelled. If it returns 0 then you were too late and the wait handler has already been executed, or will soon be executed. If it returns 1 then the wait handler was successfully cancelled.
If a wait handler is cancelled, the boost::system::error_code passed to it contains the value boost::asio::error::operation_aborted.

This typedef uses the C++11 <chrono> standard library facility, if available. Otherwise, it may use the Boost.Chrono library. To explicitly utilise Boost.Chrono, use the basic_waitable_timer template directly:

typedef basic_waitable_timer<boost::chrono::system_clock> timer;

Requirements

Header: boost/asio/system_timer.hpp

Convenience header: boost/asio.hpp