3320. span::cbegin/cend methods produce different results than std::[ranges::]cbegin/cend

Section: 23.7.2.2.7 [span.iterators] Status: C++20 Submitter: Poland Opened: 2019-11-06 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

Addresses PL 247

span<T> provides a const-qualified begin() method and cbegin() method that produces a different result if T is not const-qualifed:

1. begin() produces mutable iterator over T (as if T*)

2. cbegin() produces const iterator over T (as if T const*)

As consequence for the object s of type span<T>, the call to the std::cbegin(s)/std::ranges::cbegin(s) produces different result than s.cbegin().

Proposed change:

Change span<T> members cbegin()/cend()/crbegin()/crend()/const_iterator to be equivalent to begin()/end()/rbegin()/rend()/iterator respectively.

Tomasz Kamiński:

Per LEWG discussion in Belfast these methods and related typedefs should be removed.

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 23.7.2.2.1 [span.overview], class template span synopsis, as indicated:

   namespace std {
     template<class ElementType, size_t Extent = dynamic_extent>
     class span {
     public:
       // constants and types
       using element_type = ElementType;
       using value_type = remove_cv_t<ElementType>;
       using index_type = size_t;
       using difference_type = ptrdiff_t;
       using pointer = element_type*;
       using const_pointer = const element_type*;
       using reference = element_type&;
       using const_reference = const element_type&;
       using iterator = implementation-defined; // see 23.7.2.2.7 [span.iterators]
       using const_iterator = implementation-defined;
       using reverse_iterator = std::reverse_iterator<iterator>;
       using const_reverse_iterator = std::reverse_iterator<const_iterator>;
       static constexpr index_type extent = Extent;
       
       […]
       // 23.7.2.2.7 [span.iterators], iterator support
       constexpr iterator begin() const noexcept;
       constexpr iterator end() const noexcept;
       constexpr const_iterator cbegin() const noexcept;
       constexpr const_iterator cend() const noexcept;
       constexpr reverse_iterator rbegin() const noexcept;
       constexpr reverse_iterator rend() const noexcept;
       constexpr const_reverse_iterator crbegin() const noexcept;
       constexpr const_reverse_iterator crend() const noexcept;
       friend constexpr iterator begin(span s) noexcept { return s.begin(); }
       friend constexpr iterator end(span s) noexcept { return s.end(); }
       […]
     };
   […]
   }
   

2. Modify 23.7.2.2.7 [span.iterators] as indicated:

   using iterator = implementation-defined;
   using const_iterator = implementation-defined;
   

   -1- The types models contiguous_iterator (24.3.4.14 [iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements (24.3.5.7 [random.access.iterators]), and meets the requirements for constexpr iterators (24.3.1 [iterator.requirements.general]). All requirements on container iterators (23.2 [container.requirements]) apply to span::iterator and span::const_iterator as well.

   […]

   constexpr const_iterator cbegin() const noexcept;
   

   -6- Returns: A constant iterator referring to the first element in the span. If empty() is true, then it returns the same value as cend().

   constexpr const_iterator cend() const noexcept;
   

   -7- Returns: A constant iterator which is the past-the-end value.

   constexpr const_reverse_iterator crbegin() const noexcept;
   

   -8- Effects: Equivalent to: return const_reverse_iterator(cend());

   constexpr const_reverse_iterator crend() const noexcept;
   

   -9- Effects: Equivalent to: return const_reverse_iterator(cbegin());