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[Music] what's up guys welcome to another video brought to you by the simple engineer today we're going to be covering a cool topic known as the visitor design pattern and the visitor design pattern is probably one of the most complex and underrated design patterns from the original Gang of Four book in my opinion so in this video what I'd like to uncover is first of all talk about what exactly does this pattern solve what's the issue that it solves and then we'll dive into the theory as to why this pattern came to fruition and that revolves around some object-oriented programming principles centered around polymorphism and then number three I've devised a an example problem that will actually go over and then we'll implement it using the visitor design pattern so without further ado let's just dive right into it one of the things that I actually like is the description that wikipedia gives the visitor design pattern and what it says is it allows us to separate new operations to existing object structures without having to modify those original object structures so let's say that I have an object and it has a pond a bunch of methods on it that are defined from some interface so let's say I have an animal interface and I have dog cats parrots etc anytime that I add a new operation to one of those interfaces to like the animal interface I have to go and I have to update every single subtype to account for this new operation so what I'm doing is I'm tightly coupling the operations that I'm performing on these different animals to each individual animal object and that becomes very very hard to maintain as we grow the number of operations on our classes and in addition to that we are violating the open-closed principle and the open-closed principle says we need to keep our objects we need to keep our AP eyes open for extensibility but closed for modification in other words we don't want have to modify the original API just to add new operations that interact with these objects we should be able to do it in a very extensible way so if I want to create you know an evaluator for an animal or I want to create a new one you know a new method that says hey give me the noise this animal makes I don't want to have to modify the each individual animal class to be able to do that so this is what the visitor design pattern solves so to better understand what it looks like from a hierarchical point of view let's look at a UML diagram so in the example that we gave this would be essentially equivalent to the animal interface here so let's say that this is an animal interface and concrete element a would be something like a dog and concrete element would be would be something like a...