chunk_view::outer-iterator::value_type should provide empty
Hewill Kang

Created on 2023-11-05.00:00:00 last changed 4 months ago


Date: 2024-03-11.21:21:24

Proposed resolution:

This wording is relative to N4964.

  1. Modify [range.chunk.outer.value] as indicated:

      namespace std::ranges {
        template<view V>
          requires input_range<V>
        struct chunk_view<V>::outer-iterator::value_type : view_interface<value_type> {
          chunk_view* parent_;                                        // exposition only
          constexpr explicit value_type(chunk_view& parent);          // exposition only
          constexpr inner-iterator begin() const noexcept;
          constexpr default_sentinel_t end() const noexcept;
          constexpr bool empty() const noexcept;
          constexpr auto size() const
            requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
    constexpr default_sentinel_t end() const noexcept;

    -3- Returns: default_sentinel.

    constexpr bool empty() const noexcept;

    -?- Effects: Equivalent to: return parent_->remainder_ == 0;

Date: 2024-03-15.00:00:00

[ 2024-03-11; Reflector poll ]

Set status to Tentatively NAD after reflector poll in November 2023.

"The example shows you could use it if it existed, but not why that would be useful."

"This is a bad idea - the fact that the chunk 'shrinks' as it is iterated over is an implementation detail and not supposed to be observable."

Date: 2023-11-05.00:00:00

chunk_view::outer-iterator::value_type can determine whether it is empty by simply checking whether the chunk_view's remainder_ is 0, which makes it valuable to explicitly provide a noexcept empty member.

Otherwise, the view_interface::empty is synthesized only through the size member when the original sentinel and iterator type model sized_sentinel_for, which seems overkill:

#include <cassert>
#include <iostream>
#include <sstream>
#include <ranges>

int main() {
  auto ints = std::istringstream{"1 2 3 4 5 6 7 8 9 10"};
  for (auto chunk : std::views::istream<int>(ints) | std::views::chunk(3)) {
    for (auto elem : chunk) {
      assert(!chunk.empty()); // no matching function for call to 'empty()'
      std::cout << elem << " ";
    assert(chunk.empty()); // ditto
    std::cout << "\n";
Date User Action Args
2024-03-11 21:21:24adminsetmessages: + msg13972
2024-03-11 21:21:24adminsetstatus: new -> nad
2023-11-05 10:47:39adminsetmessages: + msg13819
2023-11-05 00:00:00admincreate