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#### basic_deadline_timer::async_wait

Start an asynchronous wait on the timer.

    template<
        typename WaitHandler = DEFAULT>
    DEDUCED async_wait(
        WaitHandler && handler = DEFAULT);

This function may be used to initiate an asynchronous wait against the timer. It always returns immediately.

For each call to `async_wait()`, the supplied handler will be called exactly once. The handler will be called when:

- The timer has expired.
- The timer was cancelled, in which case the handler is passed the error code `boost::asio::error::operation_aborted`.

###### Parameters

handler

  The handler to be called when the timer expires. Copies will be made of the handler as required. The function signature of the handler must be:

      void handler(
        const boost::system::error_code& error // Result of operation.
      );

  Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `post`.