This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++20 status.
span::cbegin/cend
methods produce different results than std::[ranges::]cbegin/cend
Section: 24.7.2.2.7 [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:
begin()
produces mutable iterator over T
(as if T*
)
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()
.
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.
Modify 24.7.2.2.1 [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 24.7.2.2.7 [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; […] // 24.7.2.2.7 [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(); } […] }; […] }
Modify 24.7.2.2.7 [span.iterators] as indicated:
using iterator = implementation-defined;using const_iterator = implementation-defined;[…]-1- The type
smodelscontiguous_iterator
(25.3.4.14 [iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements (25.3.5.7 [random.access.iterators]), and meets the requirements for constexpr iterators (25.3.1 [iterator.requirements.general]). All requirements on container iterators (24.2 [container.requirements]) apply tospan::iterator
andas well.span::const_iterator
constexpr const_iterator cbegin() const noexcept;
-6- Returns: A constant iterator referring to the first element in the span. Ifempty()
istrue
, then it returns the same value ascend()
.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());