Why? The main advantage of passing a state as an argument is letting the user control the state. If you need an array as state, the user might want to put the a

Why? The main advantage of passing a state as an argument is letting the user control the state. If you need an array as state, the user might want to put the array on the stack, or the heap, or in static memory, or protected behind a mutex somewhere... It doesn't really matter that much how things are actually implemented: most languages are flexible enough that you can do this with a class or a function or even implement the algorithm as a generic interface. But if you want the algorithm implementation to be reusable, you have to remove as many assumptions about the state as possible because otherwise some users won't be able to use your algorithm. If you pick up a class, and the class owns the state, now you need to make the class generic to allow the user to customize the state, you need to allow the user to break the state invariants, to be able to unsafely read and write from it, because otherwise you are forcing the user to read into a separate buffer, and then make a copy to your class, etc. Somebody that knows how to avoid all these issues when writing their algorithm as a class, probably also knows that by just using a function most of these issues just cannot happen anymore (or are much harder to introduce). You can also write the algorithm as a function, that takes some generic state, and provide a "class wrapper" for convenience, so that those who don't want to customize anything don't have to. But then your class doesn't implement the algorithm anymore, it just wraps it.