3760. cartesian_product_view::iterator's parent_ is never valid

Section: 25.7.33 [range.cartesian] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-27 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

cartesian_product_view::iterator has a pointer member parent_ that points to cartesian_product_view, but its constructor only accepts a tuple of iterators, which makes parent_ always default-initialized to nullptr.

The proposed resolution is to add an aliased Parent parameter to the constructor and initialize parent_ with addressof, as we usually do.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after eight 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 25.7.33.2 [range.cartesian.view] as indicated:

   constexpr iterator<false> begin()
     requires (!simple-view<First> || ... || !simple-view<Vs>);
   

   -2- Effects: Equivalent to: return iterator<false>(*this, tuple-transform(ranges::begin, bases_));

   constexpr iterator<true> begin() const
     requires (range<const First> && ... && range<const Vs>);
   

   -3- Effects: Equivalent to: return iterator<true>(*this, tuple-transform(ranges::begin, bases_));

   constexpr iterator<false> end()
     requires ((!simple-view<First> || ... || !simple-view<Vs>)
       && cartesian-product-is-common<First, Vs...>);
   constexpr iterator<true> end() const
     requires cartesian-product-is-common<const First, const Vs...>;
   

   -4- Let:

   1. (4.1) — is-const be true for the const-qualified overload, and false otherwise;

   2. (4.2) — is-empty be true if the expression ranges::empty(rng) is true for any rng among the underlying ranges except the first one and false otherwise; and

   3. (4.3) — begin-or-first-end(rng) be expression-equivalent to is-empty ? ranges::begin(rng) : cartesian-common-arg-end(rng) if rng is the first underlying range and ranges::begin(rng) otherwise.

   -5- Effects: Equivalent to:

   iterator<is-const> it(*this, tuple-transform(
     [](auto& rng){ return begin-or-first-end(rng); }, bases_));
   return it;
   

2. Modify [ranges.cartesian.iterator] as indicated:

   namespace std::ranges {
     template<input_range First, forward_range... Vs>
       requires (view<First> && ... && view<Vs>)
     template<bool Const>
     class cartesian_product_view<First, Vs...>::iterator {
     public:
       […]
   
     private:
       using Parent = maybe-const<Const, cartesian_product_view>;           // exposition only
       Parentmaybe-const<Const, cartesian_product_view>* parent_ = nullptr; // exposition only
       tuple<iterator_t<maybe-const<Const, First>>,
         iterator_t<maybe-const<Const, Vs>>...> current_;                   // exposition only
       
       template<size_t N = sizeof...(Vs)>
         constexpr void next();                                             // exposition only
       
       template<size_t N = sizeof...(Vs)>
         constexpr void prev();                                             // exposition only
       
       template<class Tuple>
         constexpr difference_type distance-from(Tuple t);                  // exposition only
       
       constexpr explicit iterator(Parent& parent, tuple<iterator_t<maybe-const<Const, First>>,
         iterator_t<maybe-const<Const, Vs>>...> current);                   // exposition only
     };
   }
   

   […]

   constexpr explicit iterator(Parent& parent, tuple<iterator_t<maybe-const<Const, First>>,
     iterator_t<maybe-const<Const, Vs>>...> current);
   

   -10- Effects: Initializes parent_ with addressof(parent) and current_ with std::move(current).

   constexpr iterator(iterator<!Const> i) requires Const &&
     (convertible_to<iterator_t<First>, iterator_t<const First>> &&
       ... && convertible_to<iterator_t<Vs>, iterator_t<const Vs>>);
   

   -11- Effects: Initializes parent_ with i.parent_ and current_ with std::move(i.current_).