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3102. Clarify span iterator and const_iterator behavior

Section: 23.7.2.2.1 [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 27.3.3.4 [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 23.7.2.2.1 [span.overview] as indicated:

   -4- The iterator types for span is a random access iterator and a contiguous iteratorspan::iterator and span::const_iterator are random access iterators (24.3.5.7 [random.access.iterators]), contiguous iterators (24.3.1 [iterator.requirements.general]), and constexpr iterators (24.3.1 [iterator.requirements.general]). All requirements on container iterators (23.2 [container.requirements]) apply to span::iterator and span::const_iterator as well.