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The Visitor [1] design pattern is one of the twenty-three well-known Gang of Four design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.
With this in mind, one ought never to presume the use of agents in the double-dispatch and their application in visitor patterns. If one can clearly see a design limit as to the domain of class types that will be involved in the co-variant interactions, then a direct call is the more efficient solution in terms of computational expense.
As a side note we already have few examples of visitor patterns elsewhere (e.g. C++ example or java one). I have a nice book on OOP programming (1997) I used as reference for Container (abstract data type). It has a dedicated subsection called 15.4 Iteration where a case of visitor pattern use is explained so I'm going to cite it if nobody minds.
Comparison of C Sharp and Java; Class (computer programming) Closure (computer programming) Command pattern; Command-line argument parsing; Comment (computer programming) Comparison of programming languages (algebraic data type) Composite entity pattern; Composite pattern; Conditional operator; Constant (computer programming) Continuation ...
The single-serving visitor pattern should be used when visitors do not need to remain in memory. This is often the case when visiting a hierarchy of objects (such as when the visitor pattern is used together with the composite pattern) to perform a single task on it, for example counting the number of cameras in a 3D scene.
The above example can be realized using the visitor pattern. It would require each animal class to implement an accept method accepting an object implementing the interface AnimalVisitor (adding non-local boilerplate code). The function animalToFood would be realized as the visit method of an implementation of AnimalVisitor.
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Multiple dispatch is used much more heavily in Julia, where multiple dispatch was a central design concept from the origin of the language: collecting the same statistics as Muschevici on the average number of methods per generic function, it was found that the Julia standard library uses more than double the amount of overloading than in the ...