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4006. chunk_view::outer-iterator::value_type should provide empty

Section: 25.7.29.4 [range.chunk.outer.value] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2023-11-05 Last modified: 2024-03-11

Priority: Not Prioritized

View all other issues in [range.chunk.outer.value].

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Discussion:

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";
  }
}

[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."

Proposed resolution:

This wording is relative to N4964.

1. Modify 25.7.29.4 [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> {
       private:
         chunk_view* parent_;                                        // exposition only
   
         constexpr explicit value_type(chunk_view& parent);          // exposition only
   
       public:
         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;