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3796. movable-box as member should use default-initialization instead of copy-initialization

Section: 25.6.5.2 [range.repeat.view], 25.7.31.2 [range.chunk.by.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-10-20 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

Currently, the member variable value_ of repeat_view is initialized with the following expression:

movable-box<W> value_ = W();

which will result in the following unexpected error in libstdc++:

#include <ranges>

int main() {
  std::ranges::repeat_view<int> r; // error: could not convert '0' from 'int' to 'std::ranges::__detail::__box<int>'
}

This is because the single-argument constructors of the simple version of movable-box in libstdc++ are declared as explicit, which is different from the conditional explicit declared by the optional's constructor, resulting in no visible conversion between those two types.

For MSVC-STL, the simple version of movable-box does not provide a single-argument constructor, so this form of initialization will also produce a hard error.

This may be a bug of existing implementations, but we don't need such "copy-initialization", the default-constructed movable-box already value-initializes the underlying type.

Simply using default initialization, as most other range adaptors do, guarantees consistency, which also eliminates extra move construction.

[2022-11-01; Reflector poll]

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

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.6.5.2 [range.repeat.view] as indicated:

   namespace std::ranges {
     template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
       requires (is_object_v<W> && same_as<W, remove_cv_t<W>> &&
                 (is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>))
      class repeat_view : public view_interface<repeat_view<W, Bound>> {
      private:
        // 25.6.5.3 [range.repeat.iterator], class repeat_view::iterator
        struct iterator;                            // exposition only
   
        movable-box<W> value_ = W();                // exposition only, see 25.7.3 [range.move.wrap]
        Bound bound_ = Bound();                     // exposition only
        […]
      };
      […]
   }
   

2. Modify 25.7.31.2 [range.chunk.by.view] as indicated:

   namespace std::ranges {
     template<forward_range V, indirect_binary_predicate<iterator_t<V>, iterator_t<V>> Pred>
       requires view<V> && is_object_v<Pred>
     class chunk_by_view : public view_interface<chunk_by_view<V, Pred>> {
       V base_ = V();                                          // exposition only
       movable-box<Pred> pred_ = Pred();                       // exposition only
   
       // 25.7.31.3 [range.chunk.by.iter], class chunk_by_view::iterator
       class iterator;                                         // exposition only
       […]
     };
     […]
   }