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4253. basic_const_iterator should provide iterator_type

Section: 24.5.3.3 [const.iterators.iterator] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2025-04-29 Last modified: 2025-06-13

Priority: Not Prioritized

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Discussion:

Currently, iterator adaptors in <iterator> that wrap a single iterator such as reverse_iterator, move_iterator, and counted_iterator all provide a public iterator_type member for users to access the underlying iterator type, except for basic_const_iterator (demo):

#include <iterator>

using I  = int*;
using RI = std::reverse_iterator<I>;
using MI = std::move_iterator<I>;
using CI = std::counted_iterator<I>;
using BI = std::basic_const_iterator<I>;

static_assert(std::same_as<RI::iterator_type, I>);
static_assert(std::same_as<MI::iterator_type, I>);
static_assert(std::same_as<CI::iterator_type, I>);
static_assert(std::same_as<BI::iterator_type, I>); // error

It seems reasonable to add one for basic_const_iterator for consistency.

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 24.5.3.3 [const.iterators.iterator] as indicated:

   namespace std {
     […]
     template<input_iterator Iterator>
     class basic_const_iterator {
       Iterator current_ = Iterator();                             // exposition only
       using reference = iter_const_reference_t<Iterator>;         // exposition only
       using rvalue-reference =                                    // exposition only
         iter-const-rvalue-reference-t<Iterator>;
             
       public:
         using iterator_type = Iterator;
         using iterator_concept = see below;
         using iterator_category = see below;  // not always present
         using value_type = iter_value_t<Iterator>;
         using difference_type = iter_difference_t<Iterator>;
         […]
     };
   }