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### 3623. Uses of `std::reverse_iterator` with containers should not require manually including `<iterator>`

Section: 25.5.1.1 [reverse.iterators.general] Status: New Submitter: Jiang An Opened: 2021-10-23 Last modified: 2022-01-29

Priority: 3

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Discussion:

Currently it is unspecified whether the definitions of `std::reverse_iterator` and its related operators are available in `<vector>`, `<array>`, etc. So, it's unspecified now whether the following program is well-formed because it's unspecified whether the equality operator is available:

>     #include <vector>

>     int main()
>     {
>       auto v = std::vector<int>(42);
>       for (auto it = v.rbegin(); it != v.rend(); ++it);
>       for (auto it = std::rbegin(v); it != std::rend(v); ++it);
>     }

Such underspecification also leaves the guarantee that `std::rbegin`, `std::rend`, `std::crbegin`, and `std::crend` are available in some other headers seems not so meaningful. In order to guarantee these function templates can be used meaningfully with containers, users are still required to include `<iterator>` manually.

I think the standard should guarantee that wherever the member `rbegin` (that returns `std::reverse_iterator`) or `std::rbegin` is provided, the definitions of `std::reverse_iterator` and its related operators are also provided. This strategy is already implemented by libc++, libstdc++, and MSVC STL, and thus I believe we should standardize it to reduce uncertainty for users.

Note that the situation for `std::reverse_iterator` is different from LWG 1361, because every operation on `std::size_t` is still valid when the typedef-name itself is absent, but `==` and `!=` on `std::reverse_iterator` fail if the corresponding declarations are unavailable.

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll.

Proposed resolution:

This wording is relative to N4901.

1. Modify 25.5.1.1 [reverse.iterators.general] as indicated:

   > -1- Class template `reverse_iterator` is an iterator adaptor that iterates from the end of the sequence defined by its underlying iterator to the beginning of that sequence.

   > -?- In addition to being available via inclusion of the `<iterator>` header, class template `reverse_iterator` and function templates in 25.5.1.8 [reverse.iter.cmp] and 25.5.1.9 [reverse.iter.nonmember] are available when any of the following headers are included: `<array>` (24.3.2 [array.syn]), `<deque>` (24.3.3 [deque.syn]), `<forward_list>` (24.3.4 [forward.list.syn]), `<list>` (24.3.5 [list.syn]), `<map>` (24.4.2 [associative.map.syn]), `<regex>` (32.3 [re.syn]), `<set>` (24.4.3 [associative.set.syn]), `<span>` (24.7.2 [span.syn]), `<stacktrace>` (19.6.2 [stacktrace.syn]), `<string>` (23.4.2 [string.syn]), `<string_view>` (23.3.2 [string.view.synop]), `<unordered_map>` (24.5.2 [unord.map.syn]), `<unordered_set>` (24.5.3 [unord.set.syn]), and `<vector>` (24.3.6 [vector.syn]).