#### basic_waitable_timer::async_wait

Start an asynchronous wait on the timer.

    template<
        typename WaitToken = default_completion_token_t<executor_type>>
    DEDUCED async_wait(
        WaitToken && token = default_completion_token_t< executor_type >());

This function may be used to initiate an asynchronous wait against the timer. It is an initiating function for an asynchronous operation, and always returns immediately.

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

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

###### Parameters

token

  The completion token that will be used to produce a completion handler, which will be called when the timer expires. Potential completion tokens include `use_future`, `use_awaitable`, `yield_context`, or a function object with the correct completion signature. The function signature of the completion handler must be:

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

  Regardless of whether the asynchronous operation completes immediately or not, the completion 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 `async_immediate`.

###### Completion Signature

    void(asio::error_code)

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following `cancellation_type` values:

- `cancellation_type::terminal`
- `cancellation_type::partial`
- `cancellation_type::total`