### system_timer

Typedef for a timer based on the system clock.

    typedef basic_waitable_timer< chrono::system_clock > system_timer;

##### Types

Name | Description
--- | ---
rebind_executor | Rebinds the timer type to another executor.
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. — 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.
cancel_one | Cancels one asynchronous operation that is waiting on the timer.
expires_after | Set the timer's expiry time relative to now.
expires_at | Set the timer's expiry time as an absolute time.
expiry | Get the timer's expiry time as an absolute time.
get_executor | Get the executor associated with the object.
operator= | Move-assign a basic_waitable_timer from another.
wait | Perform a blocking wait on the timer.
~basic_waitable_timer [destructor] | 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.
    asio::steady_timer timer(my_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 asio::error_code& error)
    {
      if (!error)
      {
        // Timer expired.
      }
    }

    ...

    // Construct a timer with an absolute expiry time.
    asio::steady_timer timer(my_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 asio::error_code& e)
    {
      if (e != asio::error::operation_aborted)
      {
        // Timer was not cancelled, take necessary action.
      }
    }

- The `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 `error_code` passed to it contains the value `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: `asio/system_timer.hpp`

Convenience header: `asio.hpp`