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3286. ranges::size is not required to be valid after a call to ranges::begin on an input range

Section: 26.7.10.2 [range.take.view], 26.5.4.2 [range.subrange.ctor] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25

Priority: 0

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Discussion:

On an input (but not forward) range, begin(rng) is not required to be an equality-preserving expression (26.4.2 [range.range]/3.3). If the range is also sized, then it is not valid to call size(rng) after begin(rng) (26.4.3 [range.sized]/2.2). In several places in the ranges clause, this precondition is violated. A trivial re-expression of the effects clause fixes the problem.

[2019-10-12 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 26.7.10.2 [range.take.view], class template take_view synopsis, as indicated:

   namespace std::ranges {
     template<view V>
     class take_view : public view_interface<take_view<V>> {
     private:
       […]
     public:
       […]
       constexpr auto begin() requires (!simple-view<V>) {
         if constexpr (sized_range<V>) {
           if constexpr (random_access_range<V>)
             return ranges::begin(base_);
           else {
             auto sz = size();
             return counted_iterator{ranges::begin(base_), size()sz};
           }
         } else
           return counted_iterator{ranges::begin(base_), count_};
       }
   
       constexpr auto begin() const requires range<const V> {
         if constexpr (sized_range<const V>) {
           if constexpr (random_access_range<const V>)
             return ranges::begin(base_);
           else {
             auto sz = size();
             return counted_iterator{ranges::begin(base_), size()sz};
           }
         } else
           return counted_iterator{ranges::begin(base_), count_};
       }
       
       […]
     };
     […]
   }
   

2. Modify 26.5.4.2 [range.subrange.ctor] as indicated:

   template<not-same-as<subrange> R>
     requires forwarding-range<R> &&
       convertible_to<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S>
   constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);
   

   -6- Effects: Equivalent to:

   1. (6.1) — If StoreSize is true, subrange{ranges::begin(r), ranges::end(r)r, ranges::size(r)}.

   2. (6.2) — Otherwise, subrange{ranges::begin(r), ranges::end(r)}.