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            (Deprecated: Use non-error_code overload.) Run the io_context object's event processing
            loop to execute ready handlers.
          
count_type poll( asio::error_code & ec);
            The poll()
            function runs handlers that are ready to run, without blocking, until
            the io_context
            has been stopped or there are no more ready handlers.
          
Set to indicate what error occurred, if any.
The number of handlers that were executed.