#### basic_signal_set::async_wait

Start an asynchronous operation to wait for a signal to be delivered.

    template<
        typename SignalHandler>
    DEDUCED async_wait(
        SignalHandler && handler);

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

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

- One of the registered signals in the signal set occurs; or
- The signal set was cancelled, in which case the handler is passed the error code `asio::error::operation_aborted`.

###### Parameters

handler

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

      void handler(
        const asio::error_code& error, // Result of operation.
        int signal_number // Indicates which signal occurred.
      );

  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`.