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### 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.
implementation_type | The underlying implementation type of I/O object.
service_type | The type of the service that will be used to provide I/O operations.
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.
cancel | Cancel any asynchronous operations that are waiting on the timer.
cancel_one | Cancels one asynchronous operation that is waiting on the timer.
expires_at | Get the timer's expiry time as an absolute time. Set the timer's expiry time as an absolute time.
expires_from_now | Get the timer's expiry time relative to now. Set the timer's expiry time relative to now.
get_io_service | Get the io_service associated with the object.
wait | Perform a blocking wait on the timer.

##### Protected Member Functions

Name | Description
--- | ---
get_implementation | Get the underlying implementation of the I/O object.
get_service | Get the service associated with the I/O object.

##### Protected Data Members

Name | Description
--- | ---
implementation | (Deprecated: Use get_implementation().) The underlying implementation of the I/O object.
service | (Deprecated: Use get_service().) The service associated with the I/O object.

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_service);

    // Set an expiry time relative to now.
    timer.expires_from_now(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_service,
        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_from_now(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_from_now()` 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: None