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Inherited from basic_socket.
Close the socket.
void close();
            This function is used to close the socket. Any asynchronous send, receive
            or connect operations will be cancelled immediately, and will complete
            with the asio::error::operation_aborted error.
          
Thrown on failure. Note that, even if the function indicates an error, the underlying descriptor is closed.
            For portable behaviour with respect to graceful closure of a connected
            socket, call shutdown() before closing the socket.