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Determine whether the io_service object has been stopped.
        
bool stopped() const;
          This function is used to determine whether an io_service object has been stopped,
          either through an explicit call to stop(), or due to running out of work. When
          an io_service
          object is stopped, calls to run(), run_one(), poll() or poll_one() will return immediately without invoking
          any handlers.
        
          true if the io_service object is stopped,
          otherwise false.