More Exceptional C++: 40 New Engineering Puzzles, Programming Problems, and Solutions

A condition may carry configuration, but must not secretly carry position

Item 2 standalone PDF pp. 2–5; Item 3 standalone PDF pp. 5–9

The real boundary shown by Items 2–3 is that the algorithm owns the traversal strategy while the caller owns the judgment rule. A predicate that captures a threshold and keeps it unchanged has data, yet every copy still answers the same question; a predicate that treats the number of calls as the current position hides control flow inside the condition, so copying or a different call order changes its meaning. To delete a position, use an explicit position operation; to delete elements matching a value condition, let the condition depend only on the element and stable configuration. C++20 erase_if simplifies the erase-remove idiom but does not provide guarantees against this confusion. The handwritten reference counting at the end of the original book is used to illustrate shared state; one cannot infer from it that shared state solves order dependence, nor should old smart pointers be copied into new projects.