The visitor design pattern
The visitor pattern is used to represent a user operation to be performed on elements of a graph structure, without modifying classes of these elements. The visitor class implements all of the appropriate virtual functions to be applied to various element types, and archive the goal through the mechanism of double dispatch, that is, the dispatch of certain virtual function calls, depending on the runtime types of both the receiver element and the visitor itself.
Based on the theory of double dispatch, developers can customize their visitors with special operation requests, and bind the visitor to different types of elements at runtime without changing the element interfaces. This is a great way to extend element functionalities ...
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