Visitor pattern
The visitor takes the instance reference as input, and implements the goal through double dispatch.
The Gang of Four defines the Visitor as: Represent[ing] an operation to be performed on elements of an object structure.
At its core, there are several types to represent basic geometric shapes like circles, lines, and arcs.
A fundamental operation on this type hierarchy is saving a drawing to the system's native file format.
Adding ever more methods for saving into many different file formats soon clutters the relatively pure original geometric data structure.
Such a save function would take a drawing as input, traverse it, and encode into that specific file format.
In the case of the CAD example, such format specific behaviors would be implemented by a subclass of Visitor (i.e. SaverPNG).
Additionally, the compiler now complains if a shape is omitted since it is now expected by the common base traversal/save function.
[3]: 288 For example, iteration over a directory structure could be implemented by a function class instead of more conventional loop pattern.
This would allow deriving various useful information from directories content by implementing a visitor functionality for every item while reusing the iteration code.
[3]: 289 A drawback of this approach, however, is that you can't break out of the loop easily or iterate concurrently (in parallel i.e. traversing two containers at the same time by a single i variable).
The client creates the object structure, directly or indirectly, and instantiates the concrete visitors.
When an operation is to be performed which is implemented using the Visitor pattern, it calls the accept method of the top-level element(s).
(As a bonus, if the visitor can't handle an argument of the given element's type, then the compiler will catch the error.)
Dynamic Visitor) by allowing use of simple function overloading to cover all the cases being visited.
A typical visitor interface might be The following example is in the language Java, and shows how the contents of a tree of nodes (in this case describing the components of a car) can be printed.
Thus all traces of the Visitor Pattern disappear, except for the mapping function, in which there is no evidence of two objects being involved.
