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### 3102. Clarify `span iterator` and `const_iterator` behavior

Section: 22.7.3.1 [span.overview] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-04-12 Last modified: 2021-02-25

Priority: 0

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Discussion:

There are multiple issues with how span specifies its iterators:

- `const_iterator` isn't mentioned.

- The relationship between `iterator` and `const_iterator` isn't specified. (`span` isn't a container, so it doesn't receive this automatically.)

- The iterators should be specified to be constexpr.

By imitating 21.4.3.3 [string.view.iterators]/3 "All requirements on container iterators ([container.requirements]) apply to `basic_string_view::const_iterator` as well.", we can specify that `iterator` is convertible to `const_iterator`.

[ 2018-04-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4741.

> 1. Edit 22.7.3.1 [span.overview] as indicated:

>    > -4- The iterator types for span is a random access iterator and a contiguous iterator `span::iterator` and `span::const_iterator` are random access iterators (23.3.5.7 [random.access.iterators]), contiguous iterators (23.3.1 [iterator.requirements.general]), and constexpr iterators (23.3.1 [iterator.requirements.general]). All requirements on container iterators (22.2 [container.requirements]) apply to `span::iterator` and `span::const_iterator` as well.