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Visitor pattern decouples the data structure from the operations acting on the structure, making the operation set relatively free to evolve. Visitor pattern is suitable for systems with relatively stable data structure and easy change of algorithm. Because visitor mode makes it easy to increase algorithm operation. If system data structure objects are easy to change and new data objects are often added, visitor mode is not suitable. The advantage of visitor mode is that it is easy to add operations, because adding operations means adding new visitors. Visitor mode centralizes the relevant behaviors into one visitor object, and its changes do not affect the system data structure. Its disadvantage is that it is difficult to add new data structures. -From Encyclopedia此时汉王刘邦正坐在宫殿里。对于事情的真相只有领兵的忠诚将校才知道，这会子这小兵说是发现盗匪踪迹。他发现浑身汗透了，喘息不止，头更是疼痛欲裂。　　第四季剧情讲述在9个月后，因为核弹爆炸而引起的核冬天遮掩了太阳，令到陷入长期黑暗的世界受没了活动限制的血族所控制，并让血祖建立了一个极权主义的政权。李左车率领援军赶赴，濮阳渡口却先一步被越军占领，以至于延误了救援时机。