Visitor is a design pattern that allow the execution of operations on elements of a data structure without changing them. In general, the operation to be performed depends on the type of element. To solve this problem, visitors use a double-dispatch approach. As such, it comes as no surprise that external dispatch is an alternative (simpler and more modular) solution.
In the standard approach to visitors, a visitor class is implemented with a (visit) routine for each element type (see Eiffel visitor, and ED-Eiffel visitor). External dispatch allows two alternative visitor alike approaches: The dual approach of abstracting the element type instead of the visitor (ED-Eiffel element oriented alike visitor); and abstracting both the element type and visitor (ED-Eiffel element-visitor oriented alike visitor).
- A standard visitor implementation in Eiffel (no External Dispatch involved);
- The standard approach abstracting the visitor class (a routine for each element);
- Dual approach of abstracting elements instead of visitors (a routine for each visitor);
- Approach abstracting both the visitor and the element types (a class for each element-visitor combination).
(mos at ua.pt) University of Aveiro, DETI/IEETA