Eh sometimes you gotta use it, not here ig. This can be done in O(1) without any loops

In C++, you can perfectly do the some_name(parameter)(parameter)(parameter) thing. Whole problem in the kata is to figure out how to do this. Mind, this is a 5 kyu kata, and this means it's not necessarily easy. You need to be aware of a couple of language constructs to implement solution.

Sample tests show how it is called: Assert::That(add(1)(2)(3), Equals(6));.

There are plenty of examples in description, no?

it modifies v, but v isn't reference, so caller won't know.