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This Library's Relationship to Boost.Iterator

Boost.Iterator is a library that is already in Boost, and it has been around for a long time.

However, it was attempting to solve a lot of problems related to iterators, not just how to write them from scratch. It is also not easy to modernize it for use in C++11 and later. Specifically:

- Boost.Iterator contains a large number of iterator adaptors; those have since been rendered moot by C++20 ranges.

- Boost.Iterator's iterator_facade template is not limited just to the existing standard C++ iterator categories; that was an experiment that never landed in standard C++, so it adds needless complexity.

- Boost.Iterator's iterator_facade was written against C++98, so it is not constexpr- and noexcept-friendly.

- Boost.Iterator's iterator_facade does not support proxy iterators, which are fully supported by the C++20 iterator concepts.

- There is opportunity to reduce the amount of code the user must write in order to use iterator_facade.

- Boost.Iterator contains two templates, iterator_facade and iterator_adaptor, that represent two ways of writing a new iterator while writing as little code as possible. It would be nice to have the functionality for both available in one template, but it is difficult to unify those two templates as written.

For these reasons, it seems more appropriate to introduce a new Boost library than to try and address the shortcomings of Boost.Iterator's iterator_facade and iterator_adaptor templates directly.