Cancels one asynchronous operation that is waiting on the timer.
This function forces the completion of one pending asynchronous wait
operation against the timer. Handlers are cancelled in FIFO order. The
handler for the cancelled operation will be invoked with the
asio::error::operation_aborted error code.
Cancelling the timer does not change the expiry time.
The number of asynchronous operations that were cancelled. That is, either 0 or 1.
Thrown on failure.
If the timer has already expired when
cancel_one() is called, then the handlers for asynchronous
wait operations will:
These handlers can no longer be cancelled, and therefore are passed an error code that indicates the successful completion of the wait operation.