visitor-pattern – Tarik Billa

How to make a safer C++ variant visitor, similar to switch statements?

September 23, 2023 by Tarik

If you want to only allow a subset of types, then you can use a static_assert at the beginning of the lambda, e.g.: template <typename T, typename… Args> struct is_one_of: std::disjunction<std::is_same<std::decay_t<T>, Args>…> {}; std::visit([](auto&& arg) { static_assert(is_one_of<decltype(arg), int, long, double, std::string>{}, “Non matching type.”); using T = std::decay_t<decltype(arg)>; if constexpr (std::is_same_v<T, int>) std::cout << “int … Read more

How to write the Visitor Pattern for Abstract Syntax Tree in Python?

September 1, 2023 by Tarik

Wikipedia has a great overview of how the Visitor pattern works, although the sample implementation that they use is in Java. You can easily port that to Python, though, no? Basically, you want to implement a mechanism for double dispatch. Each node in your AST would need to implement an accept() method (NOT a visit() … Read more

Confused about the Visitor Design Pattern

July 31, 2023 by Tarik

The code in the OP resembles a well-known variation of the Visitor design pattern known as an Internal Visitor (see e.g. Extensibility for the Masses. Practical Extensibility with Object Algebras by Bruno C. d. S. Oliveira and William R. Cook). That variation, however, uses generics and return values (instead of void) to solve some of … Read more

Difference betwen Visitor pattern & Double Dispatch

May 3, 2023 by Tarik

In short they come from to different conceptualizations that, in some languages where double dispatch is not natively supported, lead to the visitor pattern as a way to concatenate two (or more) single dispatch in order to have a multi-dispatch surrogate. In long The idea of multiple dispatch is – essentially – allow a call … Read more

What is the difference between Strategy pattern and Visitor Pattern?

April 30, 2023 by Tarik

The strategy pattern is like a 1:many relationship. When there is one type of object and I want to apply multiple operations to it, I use the strategy pattern. For example, if I have a Video class that encapsulates a video clip, I might want to compress it in different ways. So I create a … Read more

Alternative to the visitor pattern?

April 13, 2023 by Tarik

You might want to have a look at the Strategy pattern. This still gives you a separation of concerns while still being able to add new functionality without having to change each class in your hierarchy. class AbstractShape { IXmlWriter _xmlWriter = null; IShapeDrawer _shapeDrawer = null; public AbstractShape(IXmlWriter xmlWriter, IShapeDrawer drawer) { _xmlWriter = … Read more

Visitor pattern’s purpose with examples [duplicate]

January 15, 2023 by Tarik

So you’ve probably read a bajillion different explanations of the visitor pattern, and you’re probably still saying “but when would you use it!” Traditionally, visitors are used to implement type-testing without sacrificing type-safety, so long as your types are well-defined up front and known in advance. Let’s say we have a few classes as follows: … Read more

What is the point of accept() method in Visitor pattern?

January 13, 2023 by Tarik

The visitor pattern’s visit/accept constructs are a necessary evil due to C-like languages’ (C#, Java, etc.) semantics. The goal of the visitor pattern is to use double-dispatch to route your call as you’d expect from reading the code. Normally when the visitor pattern is used, an object hierarchy is involved where all the nodes are … Read more

When should I use the Visitor Design Pattern? [closed]

September 26, 2022 by Tarik

I’m not very familiar with the Visitor pattern. Let’s see if I got it right. Suppose you have a hierarchy of animals class Animal { }; class Dog: public Animal { }; class Cat: public Animal { }; (Suppose it is a complex hierarchy with a well-established interface.) Now we want to add a new … Read more