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4236. chunk_view::outer-iterator::value_type should provide reserve_hint

Section: 25.7.29.4 [range.chunk.outer.value] Status: New Submitter: Hewill Kang Opened: 2025-03-26 Last modified: 2025-03-29

Priority: Not Prioritized

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Discussion:

Consider:

views::istream<int>(is) | views::chunk(N) | ranges::to<std::list<std::vector<int>>>();

When the stream is large enough, each chunk will be of size N in most cases, except it is the last chunk.

In this case, there is no reason not to provide a reserve_hint as this can be easily done by just return remainder_. Otherwise, when N is large, each vector will be reallocated multiple times unnecessarily.

This is also consistent with the forward_range version, since its value type is views::take(subrange(current_, end_), n_), which always has a reserve_hint as take_view unconditionally provides it.

Proposed resolution:

This wording is relative to N5008.

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> {
       […]
       constexpr auto size() const
         requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
   
       constexpr auto reserve_hint() const noexcept;
     };
   }  
   

   […]

   constexpr auto size() const
     requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
   

   -4- Effects: Equivalent to:

   return to-unsigned-like(ranges::min(parent_->remainder_,
                                       ranges::end(parent_->base_) - *parent_->current_));
   

   constexpr auto reserve_hint() const noexcept;
   

   -?- Effects: Equivalent to:

   return to-unsigned-like(parent_->remainder_);