Nico Josuttis gave a talk recently that included an example and I wanted to explain what’s going on in this example, what the issue is, and what (if anything) is broken.

Mutating Through a Filter

by Barry Revzin

From the article:

As with a lot of my explanations, we have to start from the beginning.

The C++ iterator model has a number of iterator categories: input, forward, bidirectional, random access, and (since C++20) contiguous. This post only needs to consider the first two.

An input range (a range whose iterator is an input iterator) is a single-pass range. You can only ever call begin() one time on it. You can’t have multiple different input iterators into the same range - incrementing one immediately invalidates any existing copies.