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3502. elements_view should not be allowed to return dangling references

Section: 26.7.22.3 [range.elements.iterator] Status: WP Submitter: Tim Song Opened: 2020-11-18 Last modified: 2021-02-26

Priority: 2

View all other issues in [range.elements.iterator].

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Discussion:

This compiles but the resulting view is full of dangling references:

std::vector<int> vec = {42};
auto r = vec | std::views::transform([](auto c) { return std::make_tuple(c, c); })
             | std::views::keys;

This is because elements_view::iterator::operator* is specified as

constexpr decltype(auto) operator*() const { return get<N>(*current_); }

Here *current_ is a prvalue, and so the get<N> produces a reference into the materialized temporary that becomes dangling as soon as operator* returns.

We should either ban this case altogether, or make operator* (and operator[]) return by value when *current_ is a prvalue and the corresponding tuple element is not a reference (since this get is std::get, we need not worry about weird user-defined overloads.)

[2020-11-29; Reflector prioritization]

Set priority to 2 during reflector discussions.

[2021-01-31 Tim adds PR]

[2021-02-08; Reflector poll]

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

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 26.7.22.2 [range.elements.view], as indicated:

   namespace std::ranges {
     template<class T, size_t N>
       concept has-tuple-element =                   // exposition only
         requires(T t) {
           typename tuple_size<T>::type;
           requires N < tuple_size_v<T>;
           typename tuple_element_t<N, T>;
           { get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
         };
       
     template<class T, size_t N>
     concept returnable-element = is_reference_v<T> || move_­constructible<tuple_element_t<N, T>>;
   
     template<input_range V, size_t N>
       requires view<V> && has-tuple-element<range_value_t<V>, N> &&
                has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
                returnable-element<range_reference_t<V>, N>
     class elements_view : public view_interface<elements_view<V, N>> {
       […]
     };
   }
   

2. Modify 26.7.22.3 [range.elements.iterator] as indicated:

   namespace std::ranges {
     template<input_range V, size_t N>
       requires view<V> && has-tuple-element<range_value_t<V>, N> &&
                has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
                returnable-element<range_reference_t<V>, N>
     template<bool Const>
     class elements_view<V, N>::iterator {                 // exposition only
       using Base = maybe-const<Const, V>;                 // exposition only
   
       iterator_t<Base> current_ = iterator_t<Base>();     // exposition only
      
       static constexpr decltype(auto) get-element(const iterator_t<Base>& i);    // exposition only
     public:
       […]
       constexpr decltype(auto) operator*() const
       { return get<N>get-element(*current_); }
   
       […]
       constexpr decltype(auto) operator[](difference_type n) const
       requires random_­access_­range<Base>
       { return get<N>get-element(*(current_ + n)); }
     };
   }
   

   static constexpr decltype(auto) get-element(const iterator_t<Base>& i);    // exposition only
   

   -?- Effects: Equivalent to:

   
   if constexpr (is_reference_v<range_reference_t<Base>>) {
     return get<N>(*i);
   }
   else {
     using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>;
     return static_cast<E>(get<N>(*i));
   }
   
   

3. Modify 26.7.22.4 [range.elements.sentinel] as indicated:

   namespace std::ranges {
     template<input_range V, size_t N>
       requires view<V> && has-tuple-element<range_value_t<V>, N> &&
                has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
                returnable-element<range_reference_t<V>, N>
     template<bool Const>
     class elements_view<V, N>::sentinel {                 // exposition only
       […]
     };
   }