[views] (original) (raw)

23.7.1 General [views.general]

The header defines the view span.

The header defines the class template mdspan and other facilities for interacting with these multidimensional views.

23.7.2 Contiguous access [views.contiguous]

23.7.2.2 Class template span [views.span]

23.7.2.2.1 Overview [span.overview]

A span is a view over a contiguous sequence of objects, the storage of which is owned by some other object.

All member functions of span have constant time complexity.

namespace std { template<class ElementType, size_t Extent = dynamic_extent> class span { public: using element_type = ElementType;using value_type = remove_cv_t<ElementType>;using size_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; using const_iterator = std::const_iterator<iterator>;using reverse_iterator = std::reverse_iterator<iterator>;using const_reverse_iterator = std::const_iterator<reverse_iterator>;static constexpr size_type extent = Extent;constexpr span() noexcept;template<class It> constexpr explicit(extent != dynamic_extent) span(It first, size_type count);template<class It, class End> constexpr explicit(extent != dynamic_extent) span(It first, End last);template<size_t N> constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;template<class T, size_t N> constexpr span(array<T, N>& arr) noexcept;template<class T, size_t N> constexpr span(const array<T, N>& arr) noexcept;template<class R> constexpr explicit(extent != dynamic_extent) span(R&& r);constexpr explicit(extent != dynamic_extent) span(std::initializer_list<value_type> il);constexpr span(const span& other) noexcept = default;template<class OtherElementType, size_t OtherExtent> constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept;constexpr span& operator=(const span& other) noexcept = default;template<size_t Count> constexpr span<element_type, Count> first() const;template<size_t Count> constexpr span<element_type, Count> last() const;template<size_t Offset, size_t Count = dynamic_extent> constexpr span<element_type, _see below_> subspan() const;constexpr span<element_type, dynamic_extent> first(size_type count) const;constexpr span<element_type, dynamic_extent> last(size_type count) const;constexpr span<element_type, dynamic_extent> subspan( size_type offset, size_type count = dynamic_extent) const;constexpr size_type size() const noexcept;constexpr size_type size_bytes() const noexcept;constexpr bool empty() const noexcept;constexpr reference operator[](size_type idx) const;constexpr reference at(size_type idx) const; constexpr reference front() const;constexpr reference back() const;constexpr pointer data() const noexcept;constexpr iterator begin() const noexcept;constexpr iterator end() const noexcept;constexpr const_iterator cbegin() const noexcept { return begin(); } constexpr const_iterator cend() const noexcept { return end(); } constexpr reverse_iterator rbegin() const noexcept;constexpr reverse_iterator rend() const noexcept;constexpr const_reverse_iterator crbegin() const noexcept { return rbegin(); } constexpr const_reverse_iterator crend() const noexcept { return rend(); } private: pointer data_; size_type size_; };template<class It, class EndOrSize> span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>,maybe-static-ext<EndOrSize>>;template<class T, size_t N> span(T (&)[N]) -> span<T, N>;template<class T, size_t N> span(array<T, N>&) -> span<T, N>;template<class T, size_t N> span(const array<T, N>&) -> span<const T, N>;template<class R> span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;}

ElementType is required to be a complete object type that is not an abstract class type.

For a span s, any operation that invalidates a pointer in the range [s.data(), s.data() + s.size()) invalidates pointers, iterators, and references to elements of s.

23.7.2.2.2 Constructors, copy, and assignment [span.cons]

constexpr span() noexcept;

Constraints: Extent == dynamic_extent || Extent == 0 is true.

Postconditions: size() == 0 && data() == nullptr.

template<class It> constexpr explicit(extent != dynamic_extent) span(It first, size_type count);

Constraints: Let U be remove_reference_t<iter_reference_t<It>>.

Preconditions:

Hardened preconditions: If extent is not equal to dynamic_extent, then count == extent is true.

Effects: Initializes data_ with to_address(first) and_size__ with count.

template<class It, class End> constexpr explicit(extent != dynamic_extent) span(It first, End last);

Constraints: Let U be remove_reference_t<iter_reference_t<It>>.

Preconditions:

Hardened preconditions: If extent is not equal to dynamic_extent, then (last - first) == extent is true.

Effects: Initializes data_ with to_address(first) and_size__ with last - first.

Throws: When and what last - first throws.

template<size_t N> constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;template<class T, size_t N> constexpr span(array<T, N>& arr) noexcept;template<class T, size_t N> constexpr span(const array<T, N>& arr) noexcept;

Constraints: Let U be remove_pointer_t<decltype(std​::​data(arr))>.

Effects: Constructs a span that is a view over the supplied array.

[Note 4:

type_identity_t affects class template argument deduction.

— _end note_]

Postconditions: size() == N && data() == std​::​data(arr) is true.

template<class R> constexpr explicit(extent != dynamic_extent) span(R&& r);

Constraints: Let U be remove_reference_t<ranges​::​range_reference_t<R>>.

Preconditions:

Hardened preconditions: If extent is not equal to dynamic_extent, then ranges​::​size(r) == extent is true.

Effects: Initializes data_ with ranges​::​data(r) and_size__ with ranges​::​size(r).

Throws: What and when ranges​::​data(r) and ranges​::​size(r) throw.

constexpr explicit(extent != dynamic_extent) span(std::initializer_list<value_type> il);

Constraints: is_const_v<element_type> is true.

Hardened preconditions: If extent is not equal to dynamic_extent, then il.size() == extent is true.

Effects: Initializes data_ with il.begin() and_size__ with il.size().

constexpr span(const span& other) noexcept = default;

Postconditions: other.size() == size() && other.data() == data().

template<class OtherElementType, size_t OtherExtent> constexpr explicit(_see below_) span(const span<OtherElementType, OtherExtent>& s) noexcept;

Constraints:

Hardened preconditions: If extent is not equal to dynamic_extent, then s.size() == extent is true.

Effects: Constructs a span that is a view over the range [s.data(), s.data() + s.size()).

Postconditions: size() == s.size() && data() == s.data().

Remarks: The expression inside explicit is equivalent to:extent != dynamic_extent && OtherExtent == dynamic_extent

constexpr span& operator=(const span& other) noexcept = default;

Postconditions: size() == other.size() && data() == other.data().

23.7.2.2.3 Deduction guides [span.deduct]

template<class It, class EndOrSize> span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>,[_maybe-static-ext_](#concept:maybe-static-ext "23.7.2.1 Header <span> synopsis [span.syn]")<EndOrSize>>;

template<class R> span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;

23.7.2.2.4 Subviews [span.sub]

template<size_t Count> constexpr span<element_type, Count> first() const;

Mandates: Count <= Extent is true.

Hardened preconditions: Count <= size() is true.

Effects: Equivalent to: return R{data(), Count};where R is the return type.

template<size_t Count> constexpr span<element_type, Count> last() const;

Mandates: Count <= Extent is true.

Hardened preconditions: Count <= size() is true.

Effects: Equivalent to: return R{data() + (size() - Count), Count};where R is the return type.

template<size_t Offset, size_t Count = dynamic_extent> constexpr span<element_type, _see below_> subspan() const;

Mandates: Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset) is true.

Hardened preconditions: Offset <= size() && (Count == dynamic_extent || Count <= size() - Offset) is true.

Effects: Equivalent to:return span<ElementType, _see below_>( data() + Offset, Count != dynamic_extent ? Count : size() - Offset);

Remarks: The second template argument of the returned span type is:Count != dynamic_extent ? Count: (Extent != dynamic_extent ? Extent - Offset: dynamic_extent)

constexpr span<element_type, dynamic_extent> first(size_type count) const;

Hardened preconditions: count <= size() is true.

Effects: Equivalent to: return {data(), count};

constexpr span<element_type, dynamic_extent> last(size_type count) const;

Hardened preconditions: count <= size() is true.

Effects: Equivalent to: return {data() + (size() - count), count};

constexpr span<element_type, dynamic_extent> subspan( size_type offset, size_type count = dynamic_extent) const;

Hardened preconditions: offset <= size() && (count == dynamic_extent || count <= size() - offset) is true.

Effects: Equivalent to:return {data() + offset, count == dynamic_extent ? size() - offset : count};

23.7.2.2.5 Observers [span.obs]

constexpr size_type size() const noexcept;

Effects: Equivalent to: return size_;

constexpr size_type size_bytes() const noexcept;

Effects: Equivalent to: return size() * sizeof(element_type);

constexpr bool empty() const noexcept;

Effects: Equivalent to: return size() == 0;

23.7.2.2.6 Element access [span.elem]

constexpr reference operator[](size_type idx) const;

Hardened preconditions: idx < size() is true.

Returns: *(data() + idx).

constexpr reference at(size_type idx) const;

Returns: *(data() + idx).

Throws: out_of_range if idx >= size() is true.

constexpr reference front() const;

Hardened preconditions: empty() is false.

constexpr reference back() const;

Hardened preconditions: empty() is false.

Returns: *(data() + (size() - 1)).

constexpr pointer data() const noexcept;

23.7.2.2.7 Iterator support [span.iterators]

using iterator = _implementation-defined_;

All requirements on container iterators ([container.reqmts]) apply tospan​::​iterator as well.

constexpr iterator begin() const noexcept;

Returns: An iterator referring to the first element in the span.

If empty() is true, then it returns the same value as end().

constexpr iterator end() const noexcept;

Returns: An iterator which is the past-the-end value.

constexpr reverse_iterator rbegin() const noexcept;

Effects: Equivalent to: return reverse_iterator(end());

constexpr reverse_iterator rend() const noexcept;

Effects: Equivalent to: return reverse_iterator(begin());

23.7.2.3 Views of object representation [span.objectrep]

template<class ElementType, size_t Extent> span<const byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent> as_bytes(span<ElementType, Extent> s) noexcept;

Effects: Equivalent to: return R{reinterpret_cast<const byte*>(s.data()), s.size_bytes()};where R is the return type.

template<class ElementType, size_t Extent> span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent> as_writable_bytes(span<ElementType, Extent> s) noexcept;

Constraints: is_const_v<ElementType> is false.

Effects: Equivalent to: return R{reinterpret_cast<byte*>(s.data()), s.size_bytes()};where R is the return type.

23.7.3 Multidimensional access [views.multidim]

23.7.3.1 Overview [mdspan.overview]

A multidimensional index space is a Cartesian product of integer intervals.

Each interval can be represented by a half-open range , where and are the lower and upper bounds of the dimension.

The rank of a multidimensional index space is the number of intervals it represents.

The size of a multidimensional index space is the product of for each dimension iif its rank is greater than 0, and 1 otherwise.

An integer r is a rank index of an index space Sif r is in the range .

A pack of integers idx is a multidimensional index in a multidimensional index space S(or representation thereof) if both of the following are true:

23.7.3.3 Class template extents [mdspan.extents]

23.7.3.3.1 Overview [mdspan.extents.overview]

The class template extents represents a multidimensional index space of rank equal to sizeof...(Extents).

In ([views]),extents is used synonymously with multidimensional index space.

namespace std { template<class IndexType, size_t... Extents> class extents { public: using index_type = IndexType;using size_type = make_unsigned_t<index_type>;using rank_type = size_t;static constexpr rank_type rank() noexcept { return sizeof...(Extents); } static constexpr rank_type rank_dynamic() noexcept { return dynamic-index(rank()); } static constexpr size_t static_extent(rank_type) noexcept;constexpr index_type extent(rank_type) const noexcept;constexpr extents() noexcept = default;template<class OtherIndexType, size_t... OtherExtents> constexpr explicit(see below) extents(const extents<OtherIndexType, OtherExtents...>&) noexcept;template<class... OtherIndexTypes> constexpr explicit extents(OtherIndexTypes...) noexcept;template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) extents(span<OtherIndexType, N>) noexcept;template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) extents(const array<OtherIndexType, N>&) noexcept;template<class OtherIndexType, size_t... OtherExtents> friend constexpr bool operator==(const extents&,const extents<OtherIndexType, OtherExtents...>&) noexcept;constexpr size_t fwd-prod-of-extents(rank_type) const noexcept; constexpr size_t rev-prod-of-extents(rank_type) const noexcept; template<class OtherIndexType> static constexpr auto index-cast(OtherIndexType&&) noexcept; private: static constexpr rank_type dynamic-index(rank_type) noexcept; static constexpr rank_type dynamic-index-inv(rank_type) noexcept; array<index_type, rank_dynamic()> dynamic-extents{}; };template<class... Integrals> explicit extents(Integrals...) -> see below;}

Mandates:

Each specialization of extents models regular and is trivially copyable.

Let be the element of Extents.

is a dynamic extent if it is equal to dynamic_extent, otherwise is a static extent.

Let be the value of _dynamic-extents_[dynamic-index(r)]if is a dynamic extent, otherwise .

The interval of the multidimensional index space represented by an extents object is .

23.7.3.3.2 Exposition-only helpers [mdspan.extents.expo]

static constexpr rank_type _dynamic-index_(rank_type i) noexcept;

Preconditions: i <= rank() is true.

Returns: The number of with for which is a dynamic extent.

static constexpr rank_type _dynamic-index-inv_(rank_type i) noexcept;

Preconditions: i < rank_dynamic() is true.

Returns: The minimum value of rsuch that dynamic-index(r + 1) == i + 1 is true.

constexpr size_t _fwd-prod-of-extents_(rank_type i) const noexcept;

Preconditions: i <= rank() is true.

Returns: If i > 0 is true, the product of extent(k) for all k in the range [0, i), otherwise 1.

constexpr size_t _rev-prod-of-extents_(rank_type i) const noexcept;

Preconditions: i < rank() is true.

Returns: If i + 1 < rank() is true, the product of extent(k)for all k in the range [i + 1, rank()), otherwise 1.

template<class OtherIndexType> static constexpr auto _index-cast_(OtherIndexType&& i) noexcept;

Effects:

[Note 1:

This function will always return an integral type other than bool.

Since this function's call sites are constrained on convertibility of OtherIndexType to index_type, integer-class types can use the static_cast branch without loss of precision.

— _end note_]

23.7.3.3.3 Constructors [mdspan.extents.cons]

template<class OtherIndexType, size_t... OtherExtents> constexpr explicit(_see below_) extents(const extents<OtherIndexType, OtherExtents...>& other) noexcept;

Constraints:

Preconditions:

Postconditions: *this == other is true.

Remarks: The expression inside explicit is equivalent to:(((Extents != dynamic_extent) && (OtherExtents == dynamic_extent)) || ... ) || (numeric_limits<index_type>::max() < numeric_limits<OtherIndexType>::max())

template<class... OtherIndexTypes> constexpr explicit extents(OtherIndexTypes... exts) noexcept;

Let N be sizeof...(OtherIndexTypes), and let exts_arr bearray<index_type, N>{static_cast<
index_type>(std​::​move(exts))...}.

Constraints:

Preconditions:

Postconditions: *this == extents(exts_arr) is true.

template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) extents(span<OtherIndexType, N> exts) noexcept;template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) extents(const array<OtherIndexType, N>& exts) noexcept;

Constraints:

Preconditions:

Effects:

template<class... Integrals> explicit extents(Integrals...) -> _see below_;

Constraints: (is_convertible_v<Integrals, size_t> && ...) is true.

Remarks: The deduced type is extents<size_t, maybe-static-ext<Integrals>...>.

23.7.3.3.4 Observers of the multidimensional index space [mdspan.extents.obs]

static constexpr size_t static_extent(rank_type i) noexcept;

Preconditions: i < rank() is true.

constexpr index_type extent(rank_type i) const noexcept;

Preconditions: i < rank() is true.

23.7.3.3.5 Comparison operators [mdspan.extents.cmp]

template<class OtherIndexType, size_t... OtherExtents> friend constexpr bool operator==(const extents& lhs,const extents<OtherIndexType, OtherExtents...>& rhs) noexcept;

Returns: true if lhs.rank() equals rhs.rank() and if lhs.extent(r) equals rhs.extent(r)for every rank index r of rhs, otherwise false.

23.7.3.3.6 Alias template dextents [mdspan.extents.dextents]

template<class IndexType, size_t Rank> using dextents = _see below_;

Result: A type E that is a specialization of extentssuch that E​::​rank() == Rank && E​::​rank() == E​::​rank_dynamic() is true, andE​::​index_type denotes IndexType.

23.7.3.3.7 Alias template dims [mdspan.extents.dims]

template<size_t Rank, class IndexType = size_t> using dims = _see below_;

Result: A type E that is a specialization of extentssuch that E​::​rank() == Rank && E​::​rank() == E​::​rank_dynamic() is true, andE​::​index_type denotes IndexType.

23.7.3.4 Layout mapping [mdspan.layout]

23.7.3.4.1 General [mdspan.layout.general]

In [mdspan.layout.reqmts] through [mdspan.layout.stride]:

23.7.3.4.2 Requirements [mdspan.layout.reqmts]

A type M meets the layout mapping requirements if

Result: A type that is a specialization of extents.

Result: typename M​::​extents_type​::​index_type.

Result: typename M​::​extents_type​::​rank_type.

Result: A type MP that meets the layout mapping policy requirements ([mdspan.layout.policy.reqmts]) and for which is-mapping-of<MP, M> is true.

Result: const typename M​::​extents_type&

Result: typename M​::​index_type

Returns: A nonnegative integer less than numeric_limits<typename M​::​index_type>​::​max() and less than or equal to numeric_limits<size_t>​::​max().

m(i...) == m(static_cast<typename M::index_type>(i)...)

Result: typename M​::​index_type

Returns: If the size of the multidimensional index space m.extents() is 0, then 0, else 1 plus the maximum value of m(i...) for all i.

Returns: true only if for every i and j where (i != j || ...) is true,m(i...) != m(j...) is true.

[Note 1:

A mapping can return false even if the condition is met.

For certain layouts, it is possibly not feasible to determine efficiently whether the layout is unique.

— _end note_]

Returns: true only if for all k in the range [0, m.required_span_size())there exists an i such that m(i...) equals k.

[Note 2:

A mapping can return false even if the condition is met.

For certain layouts, it is possibly not feasible to determine efficiently whether the layout is exhaustive.

— _end note_]

Returns: true only if for every rank index r of m.extents() there exists an integer such that, for all i where is a multidimensional index in m.extents() ([mdspan.overview]),m((i + )...) - m(i...) equals .

[Note 3:

This implies that for a strided layout.

— _end note_]

[Note 4:

A mapping can return false even if the condition is met.

For certain layouts, it is possibly not feasible to determine efficiently whether the layout is strided.

— _end note_]

Preconditions: m.is_strided() is true.

Result: typename M​::​index_type

Returns: as defined in m.is_strided() above.

Returns: true only if m.is_unique() is truefor all possible objects m of type M.

[Note 5:

A mapping can return false even if the above condition is met.

For certain layout mappings, it is possibly not feasible to determine whether every instance is unique.

— _end note_]

M::is_always_exhaustive()

Returns: true only if m.is_exhaustive() is truefor all possible objects m of type M.

[Note 6:

A mapping can return false even if the above condition is met.

For certain layout mappings, it is possibly not feasible to determine whether every instance is exhaustive.

— _end note_]

Returns: true only if m.is_strided() is truefor all possible objects m of type M.

[Note 7:

A mapping can return false even if the above condition is met.

For certain layout mappings, it is possibly not feasible to determine whether every instance is strided.

— _end note_]

23.7.3.4.3 Layout mapping policy requirements [mdspan.layout.policy.reqmts]

A type MP meets the layout mapping policy requirements if for a type E that is a specialization of extents,MP​::​mapping<E> is valid and denotes a type Xthat meets the layout mapping requirements ([mdspan.layout.reqmts]), and for which the qualified-id X​::​layout_type is valid and denotes the type MP and the qualified-id X​::​extents_type denotes E.

23.7.3.4.4 Layout mapping policies [mdspan.layout.policy.overview]

namespace std { struct layout_left { template<class Extents> class mapping;};struct layout_right { template<class Extents> class mapping;};struct layout_stride { template<class Extents> class mapping;};template<size_t PaddingValue> struct layout_left_padded { template<class Extents> class mapping;};template<size_t PaddingValue> struct layout_right_padded { template<class Extents> class mapping;};}

Each of layout_left, layout_right, and layout_stride, as well as each specialization oflayout_left_padded and layout_right_padded, meets the layout mapping policy requirements and is a trivially copyable type.

Furthermore,is_trivially_default_constructible_v<T> is truefor any such type T.

23.7.3.4.5 Class template layout_left​::​mapping [mdspan.layout.left]

23.7.3.4.5.1 Overview [mdspan.layout.left.overview]

layout_left provides a layout mapping where the leftmost extent has stride 1, and strides increase left-to-right as the product of extents.

namespace std { template<class Extents> class layout_left::mapping { public: using extents_type = Extents;using index_type = typename extents_type::index_type;using size_type = typename extents_type::size_type;using rank_type = typename extents_type::rank_type;using layout_type = layout_left;constexpr mapping() noexcept = default;constexpr mapping(const mapping&) noexcept = default;constexpr mapping(const extents_type&) noexcept;template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const mapping<OtherExtents>&) noexcept;template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_right::mapping<OtherExtents>&) noexcept;template<class LayoutLeftPaddedMapping> constexpr explicit(!is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type>) mapping(const LayoutLeftPaddedMapping&) noexcept;template<class OtherExtents> constexpr explicit(extents_type::rank() > 0) mapping(const layout_stride::mapping<OtherExtents>&);constexpr mapping& operator=(const mapping&) noexcept = default;constexpr const extents_type& extents() const noexcept { return extents_; } constexpr index_type required_span_size() const noexcept;template<class... Indices> constexpr index_type operator()(Indices...) const noexcept;static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept { return true; } static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } static constexpr bool is_exhaustive() noexcept { return true; } static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type) const noexcept;template<class OtherExtents> friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept;private: extents_type extents_{}; template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const -> see below;template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping( const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...);} };}

If Extents is not a specialization of extents, then the program is ill-formed.

layout_left​::​mapping<E> is a trivially copyable type that models regular for each E.

Mandates: If Extents​::​rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents()is representable as a value of type typename Extents​::​index_type.

23.7.3.4.5.2 Constructors [mdspan.layout.left.cons]

constexpr mapping(const extents_type& e) noexcept;

Preconditions: The size of the multidimensional index space eis representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with e.

template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const mapping<OtherExtents>& other) noexcept;

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions: other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_right::mapping<OtherExtents>& other) noexcept;

Constraints:

Preconditions: other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

template<class LayoutLeftPaddedMapping> constexpr explicit(!is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type>) mapping(const LayoutLeftPaddedMapping&) noexcept;

Constraints:

Mandates: If

then Extents​::​static_extent(0) equalsLayoutLeftPaddedMapping​::​static-padding-stride.

Preconditions:

Effects: Direct-non-list-initializes extents_ with other.extents().

template<class OtherExtents> constexpr explicit(extents_type::rank() > 0) mapping(const layout_stride::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions:

Effects: Direct-non-list-initializes extents_ with other.extents().

23.7.3.4.5.3 Observers [mdspan.layout.left.obs]

constexpr index_type required_span_size() const noexcept;

Returns: extents().fwd-prod-of-extents(extents_type​::​rank()).

template<class... Indices> constexpr index_type operator()(Indices... i) const noexcept;

Constraints:

Preconditions: extents_type​::​index-cast(i) is a multidimensional index in extents_ ([mdspan.overview]).

Effects: Let P be a parameter pack such thatis_same_v<index_sequence_for<Indices...>, index_sequence<P...>> is true.

Equivalent to:return ((static_cast<index_type>(i) * stride(P)) + ... + 0);

constexpr index_type stride(rank_type i) const;

Constraints: extents_type​::​rank() > 0 is true.

Preconditions: i < extents_type​::​rank() is true.

Returns: extents().fwd-prod-of-extents(i).

template<class OtherExtents> friend constexpr bool operator==(const mapping& x, const mapping<OtherExtents>& y) noexcept;

Constraints: extents_type​::​rank() == OtherExtents​::​rank() is true.

Effects: Equivalent to: return x.extents() == y.extents();

23.7.3.4.6 Class template layout_right​::​mapping [mdspan.layout.right]

23.7.3.4.6.1 Overview [mdspan.layout.right.overview]

layout_right provides a layout mapping where the rightmost extent is stride 1, and strides increase right-to-left as the product of extents.

namespace std { template<class Extents> class layout_right::mapping { public: using extents_type = Extents;using index_type = typename extents_type::index_type;using size_type = typename extents_type::size_type;using rank_type = typename extents_type::rank_type;using layout_type = layout_right;constexpr mapping() noexcept = default;constexpr mapping(const mapping&) noexcept = default;constexpr mapping(const extents_type&) noexcept;template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const mapping<OtherExtents>&) noexcept;template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_left::mapping<OtherExtents>&) noexcept;template<class LayoutRightPaddedMapping> constexpr explicit(!is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type>) mapping(const LayoutRightPaddedMapping&) noexcept;template<class OtherExtents> constexpr explicit(extents_type::rank() > 0) mapping(const layout_stride::mapping<OtherExtents>&) noexcept;constexpr mapping& operator=(const mapping&) noexcept = default;constexpr const extents_type& extents() const noexcept { return extents_; } constexpr index_type required_span_size() const noexcept;template<class... Indices> constexpr index_type operator()(Indices...) const noexcept;static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept { return true; } static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } static constexpr bool is_exhaustive() noexcept { return true; } static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type) const noexcept;template<class OtherExtents> friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept;private: extents_type extents_{}; template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const -> see below;template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping( const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...);} };}

If Extents is not a specialization of extents, then the program is ill-formed.

layout_right​::​mapping<E> is a trivially copyable type that models regular for each E.

Mandates: If Extents​::​rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents()is representable as a value of type typename Extents​::​index_type.

23.7.3.4.6.2 Constructors [mdspan.layout.right.cons]

constexpr mapping(const extents_type& e) noexcept;

Preconditions: The size of the multidimensional index space e is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with e.

template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const mapping<OtherExtents>& other) noexcept;

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions: other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_left::mapping<OtherExtents>& other) noexcept;

Constraints:

Preconditions: other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

template<class LayoutRightPaddedMapping> constexpr explicit(!is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type>) mapping(const LayoutRightPaddedMapping&) noexcept;

Constraints:

Mandates: If

then Extents​::​static_extent(Extents​::​rank() - 1) equalsLayoutRightPaddedMapping​::​static-padding-stride.

Preconditions:

Effects: Direct-non-list-initializes extents_ with other.extents().

template<class OtherExtents> constexpr explicit(extents_type::rank() > 0) mapping(const layout_stride::mapping<OtherExtents>& other) noexcept;

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions:

Effects: Direct-non-list-initializes extents_ with other.extents().

23.7.3.4.6.3 Observers [mdspan.layout.right.obs]

index_type required_span_size() const noexcept;

Returns: extents().fwd-prod-of-extents(extents_type​::​rank()).

template<class... Indices> constexpr index_type operator()(Indices... i) const noexcept;

Constraints:

Preconditions: extents_type​::​index-cast(i) is a multidimensional index in extents_ ([mdspan.overview]).

Effects: Let P be a parameter pack such thatis_same_v<index_sequence_for<Indices...>, index_sequence<P...>> is true.

Equivalent to:return ((static_cast<index_type>(i) * stride(P)) + ... + 0);

constexpr index_type stride(rank_type i) const noexcept;

Constraints: extents_type​::​rank() > 0 is true.

Preconditions: i < extents_type​::​rank() is true.

Returns: extents().rev-prod-of-extents(i).

template<class OtherExtents> friend constexpr bool operator==(const mapping& x, const mapping<OtherExtents>& y) noexcept;

Constraints: extents_type​::​rank() == OtherExtents​::​rank() is true.

Effects: Equivalent to: return x.extents() == y.extents();

23.7.3.4.7 Class template layout_stride​::​mapping [mdspan.layout.stride]

23.7.3.4.7.1 Overview [mdspan.layout.stride.overview]

layout_stride provides a layout mapping where the strides are user-defined.

namespace std { template<class Extents> class layout_stride::mapping { public: using extents_type = Extents;using index_type = typename extents_type::index_type;using size_type = typename extents_type::size_type;using rank_type = typename extents_type::rank_type;using layout_type = layout_stride;private: static constexpr rank_type rank_ = extents_type::rank(); public: constexpr mapping() noexcept;constexpr mapping(const mapping&) noexcept = default;template<class OtherIndexType> constexpr mapping(const extents_type&, span<OtherIndexType, _rank__>) noexcept;template<class OtherIndexType> constexpr mapping(const extents_type&, const array<OtherIndexType, _rank__>&) noexcept;template<class StridedLayoutMapping> constexpr explicit(see below) mapping(const StridedLayoutMapping&) noexcept;constexpr mapping& operator=(const mapping&) noexcept = default;constexpr const extents_type& extents() const noexcept { return extents_; } constexpr array<index_type, _rank__> strides() const noexcept { return strides_; } constexpr index_type required_span_size() const noexcept;template<class... Indices> constexpr index_type operator()(Indices...) const noexcept;static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept { return false; } static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } constexpr bool is_exhaustive() const noexcept;static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type i) const noexcept { return _strides__[i]; } template<class OtherMapping> friend constexpr bool operator==(const mapping&, const OtherMapping&) noexcept;private: extents_type extents_{}; array<index_type, _rank__> strides_{}; template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const -> see below;template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping( const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...);} };}

If Extents is not a specialization of extents, then the program is ill-formed.

layout_stride​::​mapping<E> is a trivially copyable type that models regular for each E.

Mandates: If Extents​::​rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents()is representable as a value of type typename Extents​::​index_type.

23.7.3.4.7.2 Exposition-only helpers [mdspan.layout.stride.expo]

Let REQUIRED-SPAN-SIZE(e, strides) be:

Let OFFSET(m) be:

Let is-extents be the exposition-only variable template defined as follows:template<class T> constexpr bool is-extents = false; template<class IndexType, size_t... Args> constexpr bool is-extents<extents<IndexType, Args...>> = true;

Let layout-mapping-alike be the exposition-only concept defined as follows:template<class M> concept layout-mapping-alike = requires { requires is-extents<typename M::extents_type>;{ M::is_always_strided() } -> same_as<bool>;{ M::is_always_exhaustive() } -> same_as<bool>;{ M::is_always_unique() } -> same_as<bool>; bool_constant<M::is_always_strided()>::value; bool_constant<M::is_always_exhaustive()>::value; bool_constant<M::is_always_unique()>::value;};

[Note 1:

This concept checks that the functionsM​::​is_always_strided(),M​::​is_always_exhaustive(), andM​::​is_always_unique() exist, are constant expressions, and have a return type of bool.

— _end note_]

23.7.3.4.7.3 Constructors [mdspan.layout.stride.cons]

constexpr mapping() noexcept;

Preconditions: layout_right​::​mapping<extents_type>().required_span_size()is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with extents_type(), and for all d in the range [0, rank_), direct-non-list-initializes _strides__[d] withlayout_right​::​mapping<extents_type>().stride(d).

template<class OtherIndexType> constexpr mapping(const extents_type& e, span<OtherIndexType, _rank_> s) noexcept;template<class OtherIndexType> constexpr mapping(const extents_type& e, const array<OtherIndexType, _rank_>& s) noexcept;

Constraints:

Preconditions:

Effects: Direct-non-list-initializes extents_ with e, and for all d in the range [0, rank_), direct-non-list-initializes strides_[d] with as_const(s[d]).

template<class StridedLayoutMapping> constexpr explicit(_see below_) mapping(const StridedLayoutMapping& other) noexcept;

Constraints:

Preconditions:

Effects: Direct-non-list-initializes extents_ with other.extents(), and for all d in the range [0, rank_), direct-non-list-initializes _strides__[d]with other.stride(d).

Remarks: The expression inside explicit is equivalent to:!(is_convertible_v<typename StridedLayoutMapping::extents_type, extents_type> && (is-mapping-of<layout_left, StridedLayoutMapping> || is-mapping-of<layout_right, StridedLayoutMapping> || is-layout-left-padded-mapping-of<StridedLayoutMapping> || is-layout-right-padded-mapping-of<StridedLayoutMapping> || is-mapping-of<layout_stride, StridedLayoutMapping>))

23.7.3.4.7.4 Observers [mdspan.layout.stride.obs]

constexpr index_type required_span_size() const noexcept;

Returns: REQUIRED-SPAN-SIZE(extents(), strides_).

template<class... Indices> constexpr index_type operator()(Indices... i) const noexcept;

Constraints:

Preconditions: extents_type​::​index-cast(i) is a multidimensional index in extents_ ([mdspan.overview]).

Effects: Let P be a parameter pack such thatis_same_v<index_sequence_for<Indices...>, index_sequence<P...>> is true.

Equivalent to:return ((static_cast<index_type>(i) * stride(P)) + ... + 0);

constexpr bool is_exhaustive() const noexcept;

Returns:

template<class OtherMapping> friend constexpr bool operator==(const mapping& x, const OtherMapping& y) noexcept;

Constraints:

Preconditions: OtherMapping meets the layout mapping requirements ([mdspan.layout.policy.reqmts]).

Returns: true if x.extents() == y.extents() is true,OFFSET(y) == 0 is true, and each of x.stride(r) == y.stride(r) is truefor r in the range [0, x.extents().rank()).

Otherwise, false.

23.7.3.4.8 Class template layout_left_padded​::​mapping [mdspan.layout.leftpad]

23.7.3.4.8.1 Overview [mdspan.layout.leftpad.overview]

layout_left_padded provides a layout mapping that behaves like layout_left​::​mapping, except that the padding stride stride(1)can be greater than or equal to extent(0).

namespace std { template<size_t PaddingValue> template<class Extents> class layout_left_padded<PaddingValue>::mapping { public: static constexpr size_t padding_value = PaddingValue;using extents_type = Extents;using index_type = typename extents_type::index_type;using size_type = typename extents_type::size_type;using rank_type = typename extents_type::rank_type;using layout_type = layout_left_padded<PaddingValue>;private: static constexpr size_t rank_ = extents_type::rank(); static constexpr size_t first-static-extent = extents_type::static_extent(0);static constexpr size_t static-padding-stride = see below; public: constexpr mapping() noexcept : mapping(extents_type{}) {} constexpr mapping(const mapping&) noexcept = default;constexpr mapping(const extents_type&);template<class OtherIndexType> constexpr mapping(const extents_type&, OtherIndexType);template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_left::mapping<OtherExtents>&);template<class OtherExtents> constexpr explicit(extents_type::rank() > 0) mapping(const layout_stride::mapping<OtherExtents>&);template<class LayoutLeftPaddedMapping> constexpr explicit(see below) mapping(const LayoutLeftPaddedMapping&);template<class LayoutRightPaddedMapping> constexpr explicit(see below) mapping(const LayoutRightPaddedMapping&) noexcept;constexpr mapping& operator=(const mapping&) noexcept = default;constexpr const extents_type& extents() const noexcept { return extents_; } constexpr array<index_type, rank_> strides() const noexcept;constexpr index_type required_span_size() const noexcept;template<class... Indices> constexpr index_type operator()(Indices...) const noexcept;static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept;static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } constexpr bool is_exhaustive() const noexcept;static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type) const noexcept;template<class LayoutLeftPaddedMapping> friend constexpr bool operator==(const mapping&, const LayoutLeftPaddedMapping&) noexcept;private: index_type stride-1 = static-padding-stride; extents_type extents_{}; template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const -> see below;template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping(const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...);} };}

If Extents is not a specialization of extents, then the program is ill-formed.

layout_left_padded​::​mapping<E> is a trivially copyable type that models regular for each E.

Throughout [mdspan.layout.leftpad], let P_rank be the following size rank_ parameter pack of size_t values:

Mandates:

23.7.3.4.8.2 Exposition-only members [mdspan.layout.leftpad.expo]

static constexpr size_t _static-padding-stride_ = _see below_;

The value is

index_type _stride-1_ = _static-padding-stride_;

Recommended practice: Implementations should not store this value if static-padding-stride is not dynamic_extent.

[Note 1:

Using extents<index_type, _static-padding-stride_> instead ofindex_type as the type of stride-1 would achieve this.

— _end note_]

23.7.3.4.8.3 Constructors [mdspan.layout.leftpad.cons]

constexpr mapping(const extents_type& ext);

Preconditions:

Effects:

template<class OtherIndexType> constexpr mapping(const extents_type& ext, OtherIndexType pad);

Constraints:

Preconditions:

Effects: Direct-non-list-initializes extents_ with ext, and if rank_ is greater than one, direct-non-list-initializes stride-1 with_LEAST-MULTIPLE-AT-LEAST_(pad, ext.extent(0)).

template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_left::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Mandates: If OtherExtents​::​rank() is greater than 1, then(static-padding-stride == dynamic_extent) || (OtherExtents::static_extent(0) == dynamic_extent) || (static-padding-stride == OtherExtents::static_extent(0)) is true.

Preconditions:

Effects: Equivalent to mapping(other.extents()).

template<class OtherExtents> constexpr explicit(rank_ > 0) mapping(const layout_stride::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions:

Effects:

template<class LayoutLeftPaddedMapping> constexpr explicit(_see below_) mapping(const LayoutLeftPaddedMapping& other);

Constraints:

Mandates: If rank_ is greater than 1, thenpadding_value == dynamic_extent ||LayoutLeftPaddedMapping::padding_value == dynamic_extent ||padding_value == LayoutLeftPaddedMapping::padding_valueis true.

Effects:

Remarks: The expression inside explicit is equivalent to:rank_> 1 && (padding_value != dynamic_extent || LayoutLeftPaddedMapping::padding_value == dynamic_extent)

template<class LayoutRightPaddedMapping> constexpr explicit(_see below_) mapping(const LayoutRightPaddedMapping& other) noexcept;

Constraints:

Preconditions: other.required_span_size() is representable as a value of type index_type.

Effects: Direct-non-list-initializes extents_ with other.extents().

Remarks: The expression inside explicit is equivalent to:!is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type>

[Note 1:

Neither the input mapping nor the mapping to be constructed uses the padding stride in the rank-0 or rank-1 case, so the padding stride does not affect either the constraints or the preconditions.

— _end note_]

23.7.3.4.8.4 Observers [mdspan.layout.leftpad.obs]

constexpr array<index_type, _rank_> strides() const noexcept;

Returns: array<index_type, _rank__>({stride(P_rank)...}).

constexpr index_type required_span_size() const noexcept;

Returns:

template<class... Indices> constexpr size_t operator()(Indices... idxs) const noexcept;

Constraints:

Preconditions: extents_type​::​index-cast(idxs) is a multidimensional index in extents() ([mdspan.overview]).

Returns: ((static_cast<index_type>(idxs) * stride(P_rank)) + ... + 0).

static constexpr bool is_always_exhaustive() noexcept;

Returns:

constexpr bool is_exhaustive() const noexcept;

Returns: true if rank_ equals zero or one; otherwise, extents_.extent(0) == stride(1).

constexpr index_type stride(rank_type r) const noexcept;

Preconditions: r is smaller than rank_.

Returns:

template<class LayoutLeftPaddedMapping> friend constexpr bool operator==(const mapping& x, const LayoutLeftPaddedMapping& y) noexcept;

Constraints:

Returns: true if x.extents() == y.extents() is true and_rank__ < 2 || x.stride(1) == y.
stride(1) is true.

Otherwise, false.

23.7.3.4.9 Class template layout_right_padded​::​mapping [mdspan.layout.rightpad]

23.7.3.4.9.1 Overview [mdspan.layout.rightpad.overview]

layout_right_padded provides a layout mapping that behaves like layout_right​::​mapping, except that the padding stride stride(extents_type​::​rank() - 2)can be greater than or equal toextents_type​::​extent(extents_type​::​rank() - 1).

namespace std { template<size_t PaddingValue> template<class Extents> class layout_right_padded<PaddingValue>::mapping { public: static constexpr size_t padding_value = PaddingValue;using extents_type = Extents;using index_type = typename extents_type::index_type;using size_type = typename extents_type::size_type;using rank_type = typename extents_type::rank_type;using layout_type = layout_right_padded<PaddingValue>;private: static constexpr size_t rank_ = extents_type::rank(); static constexpr size_t last-static-extent = extents_type::static_extent(rank_ - 1);static constexpr size_t static-padding-stride = see below; public: constexpr mapping() noexcept : mapping(extents_type{}) {} constexpr mapping(const mapping&) noexcept = default;constexpr mapping(const extents_type&);template<class OtherIndexType> constexpr mapping(const extents_type&, OtherIndexType);template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_right::mapping<OtherExtents>&);template<class OtherExtents> constexpr explicit(rank_ > 0) mapping(const layout_stride::mapping<OtherExtents>&);template<class LayoutRightPaddedMapping> constexpr explicit(see below) mapping(const LayoutRightPaddedMapping&);template<class LayoutLeftPaddedMapping> constexpr explicit(see below) mapping(const LayoutLeftPaddedMapping&) noexcept;constexpr mapping& operator=(const mapping&) noexcept = default;constexpr const extents_type& extents() const noexcept { return extents_; } constexpr array<index_type, rank_> strides() const noexcept;constexpr index_type required_span_size() const noexcept;template<class... Indices> constexpr index_type operator()(Indices...) const noexcept;static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept;static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } constexpr bool is_exhaustive() const noexcept;static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type) const noexcept;template<class LayoutRightPaddedMapping> friend constexpr bool operator==(const mapping&, const LayoutRightPaddedMapping&) noexcept;private: index_type stride-rm2 = static-padding-stride; extents_type extents_{}; template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const -> see below;template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping(const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...);} };}

If Extents is not a specialization of extents, then the program is ill-formed.

layout_right_padded​::​mapping<E> is a trivially copyable type that models regular for each E.

Throughout [mdspan.layout.rightpad], let P_rank be the following size rank_ parameter pack of size_t values:

Mandates:

23.7.3.4.9.2 Exposition-only members [mdspan.layout.rightpad.expo]

static constexpr size_t _static-padding-stride_ = _see below_;

The value is

index_type _stride-rm2_ = _static-padding-stride_;

Recommended practice: Implementations should not store this value if static-padding-stride is not dynamic_extent.

[Note 1:

Using extents<index_type, _static-padding-stride_>instead of index_type as the type of _stride-rm2_would achieve this.

— _end note_]

23.7.3.4.9.3 Constructors [mdspan.layout.rightpad.cons]

constexpr mapping(const extents_type& ext);

Preconditions:

Effects:

template<class OtherIndexType> constexpr mapping(const extents_type& ext, OtherIndexType pad);

Constraints:

Preconditions:

Effects: Direct-non-list-initializes extents_ with ext, and if rank_ is greater than one, direct-non-list-initializes stride-rm2 with_LEAST-MULTIPLE-AT-LEAST_(pad, ext.extent(rank_ - 1)).

template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_right::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Mandates: If OtherExtents​::​rank() is greater than 1, then(static-padding-stride == dynamic_extent) || (OtherExtents::static_extent(rank_ - 1) == dynamic_extent) || (static-padding-stride == OtherExtents::static_extent(rank_ - 1)) is true.

Preconditions:

Effects: Equivalent to mapping(other.extents()).

template<class OtherExtents> constexpr explicit(rank_ > 0) mapping(const layout_stride::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions:

Effects:

template<class LayoutRightPaddedMapping> constexpr explicit(_see below_) mapping(const LayoutRightPaddedMapping& other);

Constraints:

Mandates: If rank_ is greater than 1, thenpadding_value == dynamic_extent ||LayoutRightPaddedMapping::padding_value == dynamic_extent ||padding_value == LayoutRightPaddedMapping::padding_valueis true.

Preconditions:

Effects:

Remarks: The expression inside explicit is equivalent to:rank_ > 1 && (padding_value != dynamic_extent || LayoutRightPaddedMapping::padding_value == dynamic_extent)

template<class LayoutLeftPaddedMapping> constexpr explicit(_see below_) mapping(const LayoutLeftPaddedMapping& other) noexcept;

Constraints:

Preconditions: other.required_span_size() is representable as a value of type index_type.

Effects: Direct-non-list-initializes extents_ with other.extents().

Remarks: The expression inside explicit is equivalent to:!is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type>

[Note 1:

Neither the input mapping nor the mapping to be constructed uses the padding stride in the rank-0 or rank-1 case, so the padding stride affects neither the constraints nor the preconditions.

— _end note_]

23.7.3.4.9.4 Observers [mdspan.layout.rightpad.obs]

constexpr array<index_type, _rank_> strides() const noexcept;

Returns: array<index_type, _rank__>(stride(P_rank)...).

constexpr index_type required_span_size() const noexcept;

Returns: 0 if the multidimensional index space extents_ is empty, otherwise *this(((extents_(P_rank) - index_type(1))...)) + 1.

template<class... Indices> constexpr size_t operator()(Indices... idxs) const noexcept;

Constraints:

Preconditions: extents_type​::​index-cast(idxs) is a multidimensional index in extents() ([mdspan.overview]).

Returns: ((static_cast<index_type>(idxs) * stride(P_rank)) + ... + 0).

static constexpr bool is_always_exhaustive() noexcept;

Returns:

constexpr bool is_exhaustive() const noexcept;

Returns: true if rank_ equals zero or one; otherwise,extents_.extent(rank_ - 1) == stride(rank_ - 2)

constexpr index_type stride(rank_type r) const noexcept;

Preconditions: r is smaller than rank_.

Returns:

template<class LayoutRightPaddedMapping> friend constexpr bool operator==(const mapping& x, const LayoutRightPaddedMapping& y) noexcept;

Constraints:

Returns: true if x.extents() == y.extents() is true and_rank__ < 2 || x.stride(rank_ - 2) == y.stride(rank_ - 2) is true.

Otherwise, false.

23.7.3.5 Accessor policy [mdspan.accessor]

23.7.3.5.1 General [mdspan.accessor.general]

An accessor policy defines types and operations by which a reference to a single object is created from an abstract data handle to a number of such objects and an index.

A range of indices is an accessible range of a given data handle and an accessor if, for each i in the range, the accessor policy's access function produces a valid reference to an object.

23.7.3.5.2 Requirements [mdspan.accessor.reqmts]

A type A meets the accessor policy requirements if

Result: A complete object type that is not an abstract class type.

typename A::data_handle_type

Result: A type that models copyable, and for which is_nothrow_move_constructible_v<A​::​data_handle_type> is true,is_nothrow_move_assignable_v<A​::​data_handle_type> is true, andis_nothrow_swappable_v<A​::​data_handle_type> is true.

[Note 1:

The type of data_handle_type need not be element_type*.

— _end note_]

Result: A type that modelscommon_reference_with<A​::​reference&&, A​::​element_type&>.

[Note 2:

The type of reference need not be element_type&.

— _end note_]

typename A::offset_policy

Result: A type OP such that:

Remarks: The expression is equality preserving.

[Note 3:

Concrete accessor policies can impose preconditions for their access function.

However, they might not.

For example, an accessor wherep is span<A​::​element_type, dynamic_extent> andaccess(p, i) returns p[i % p.size()]does not need to impose a precondition on i.

— _end note_]

Result: A​::​offset_policy​::​data_handle_type

Returns: q such that for b being A​::​offset_policy(a), and any integer n for which [0, n) is an accessible range of p and a:

Remarks: The expression is equality-preserving.

23.7.3.5.3 Class template default_accessor [mdspan.accessor.default]

23.7.3.5.3.1 Overview [mdspan.accessor.default.overview]

namespace std { template<class ElementType> struct default_accessor { using offset_policy = default_accessor;using element_type = ElementType;using reference = ElementType&;using data_handle_type = ElementType*;constexpr default_accessor() noexcept = default;template<class OtherElementType> constexpr default_accessor(default_accessor<OtherElementType>) noexcept;constexpr reference access(data_handle_type p, size_t i) const noexcept;constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;};}

default_accessor meets the accessor policy requirements.

ElementType is required to be a complete object type that is neither an abstract class type nor an array type.

Each specialization of default_accessor is a trivially copyable type that models semiregular.

is an accessible range for an object p of type data_handle_type and an object of type default_accessorif and only if [p, p + n) is a valid range.

23.7.3.5.3.2 Members [mdspan.accessor.default.members]

template<class OtherElementType> constexpr default_accessor(default_accessor<OtherElementType>) noexcept {}

Constraints: is_convertible_v<OtherElementType(*)[], element_type(*)[]>is true.

constexpr reference access(data_handle_type p, size_t i) const noexcept;

Effects: Equivalent to: return p[i];

constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;

Effects: Equivalent to: return p + i;

23.7.3.5.4 Class template aligned_accessor [mdspan.accessor.aligned]

23.7.3.5.4.1 Overview [mdspan.accessor.aligned.overview]

namespace std { template<class ElementType, size_t ByteAlignment> struct aligned_accessor { using offset_policy = default_accessor<ElementType>;using element_type = ElementType;using reference = ElementType&;using data_handle_type = ElementType*;static constexpr size_t byte_alignment = ByteAlignment;constexpr aligned_accessor() noexcept = default;template<class OtherElementType, size_t OtherByteAlignment> constexpr aligned_accessor( aligned_accessor<OtherElementType, OtherByteAlignment>) noexcept;template<class OtherElementType> constexpr explicit aligned_accessor(default_accessor<OtherElementType>) noexcept;template<class OtherElementType> constexpr operator default_accessor<OtherElementType>() const noexcept;constexpr reference access(data_handle_type p, size_t i) const noexcept;constexpr typename offset_policy::data_handle_type offset( data_handle_type p, size_t i) const noexcept;};}

Mandates:

aligned_accessor meets the accessor policy requirements.

ElementType is required to be a complete object type that is neither an abstract class type nor an array type.

Each specialization of aligned_accessor is a trivially copyable type that models semiregular.

[0, n) is an accessible range for an object p of type data_handle_type and an object of type aligned_accessor if and only if

[Example 1:

The following function computeuses is_sufficiently_aligned to check whether a given mdspan with default_accessor has a data handle with sufficient alignment to be used with aligned_accessor<float, 4 * sizeof(float)>.

If so, the function dispatches to a function compute_using_fourfold_overalignmentthat requires fourfold over-alignment of arrays, but can therefore use hardware-specific instructions, such as four-wide SIMD (Single Instruction Multiple Data) instructions.

Otherwise, compute dispatches to a possibly less optimized function compute_without_requiring_overalignmentthat has no over-alignment requirement.

void compute_using_fourfold_overalignment( std::mdspan<float, std::dims<1>, std::layout_right, std::aligned_accessor<float, 4 * alignof(float)>> x);void compute_without_requiring_overalignment( std::mdspan<float, std::dims<1>, std::layout_right> x);void compute(std::mdspan<float, std::dims<1>> x) { constexpr auto byte_alignment = 4 * sizeof(float);auto accessor = std::aligned_accessor<float, byte_alignment>{};auto x_handle = x.data_handle();if (std::is_sufficiently_aligned<byte_alignment>(x_handle)) { compute_using_fourfold_overalignment(std::mdspan{x_handle, x.mapping(), accessor});} else { compute_without_requiring_overalignment(x);} } — _end example_]

23.7.3.5.4.2 Members [mdspan.accessor.aligned.members]

template<class OtherElementType, size_t OtherByteAlignment> constexpr aligned_accessor(aligned_accessor<OtherElementType, OtherByteAlignment>) noexcept;

Constraints:

template<class OtherElementType> constexpr explicit aligned_accessor(default_accessor<OtherElementType>) noexcept;

Constraints: is_convertible_v<OtherElementType(*)[], element_type(*)[]>is true.

constexpr reference access(data_handle_type p, size_t i) const noexcept;

Preconditions: [0, i + 1) is an accessible range for p and *this.

Effects: Equivalent to: return assume_aligned<byte_alignment>(p)[i];

template<class OtherElementType> constexpr operator default_accessor<OtherElementType>() const noexcept;

Constraints: is_convertible_v<element_type(*)[], OtherElementType(*)[]>is true.

Effects: Equivalent to: return {};

constexpr typename offset_policy::data_handle_type offset(data_handle_type p, size_t i) const noexcept;

Preconditions: [0, i + 1) is an accessible range for p and *this.

Effects: Equivalent to: return assume_aligned<byte_alignment>(p) + i;

23.7.3.6 Class template mdspan [mdspan.mdspan]

23.7.3.6.1 Overview [mdspan.mdspan.overview]

mdspan is a view of a multidimensional array of elements.

namespace std { template<class ElementType, class Extents, class LayoutPolicy = layout_right,class AccessorPolicy = default_accessor<ElementType>> class mdspan { public: using extents_type = Extents;using layout_type = LayoutPolicy;using accessor_type = AccessorPolicy;using mapping_type = typename layout_type::template mapping<extents_type>;using element_type = ElementType;using value_type = remove_cv_t<element_type>;using index_type = typename extents_type::index_type;using size_type = typename extents_type::size_type;using rank_type = typename extents_type::rank_type;using data_handle_type = typename accessor_type::data_handle_type;using reference = typename accessor_type::reference;static constexpr rank_type rank() noexcept { return extents_type::rank(); } static constexpr rank_type rank_dynamic() noexcept { return extents_type::rank_dynamic(); } static constexpr size_t static_extent(rank_type r) noexcept { return extents_type::static_extent(r); } constexpr index_type extent(rank_type r) const noexcept { return extents().extent(r); } constexpr mdspan();constexpr mdspan(const mdspan& rhs) = default;constexpr mdspan(mdspan&& rhs) = default;template<class... OtherIndexTypes> constexpr explicit mdspan(data_handle_type ptr, OtherIndexTypes... exts);template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) mdspan(data_handle_type p, span<OtherIndexType, N> exts);template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) mdspan(data_handle_type p, const array<OtherIndexType, N>& exts);constexpr mdspan(data_handle_type p, const extents_type& ext);constexpr mdspan(data_handle_type p, const mapping_type& m);constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a);template<class OtherElementType, class OtherExtents,class OtherLayoutPolicy, class OtherAccessorPolicy> constexpr explicit(see below) mdspan(const mdspan<OtherElementType, OtherExtents, OtherLayoutPolicy, OtherAccessorPolicy>& other);constexpr mdspan& operator=(const mdspan& rhs) = default;constexpr mdspan& operator=(mdspan&& rhs) = default;template<class... OtherIndexTypes> constexpr reference operator[](OtherIndexTypes... indices) const;template<class OtherIndexType> constexpr reference operator[](span<OtherIndexType, rank()> indices) const;template<class OtherIndexType> constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;constexpr size_type size() const noexcept;constexpr bool empty() const noexcept;friend constexpr void swap(mdspan& x, mdspan& y) noexcept;constexpr const extents_type& extents() const noexcept { return map_.extents(); } constexpr const data_handle_type& data_handle() const noexcept { return ptr_; } constexpr const mapping_type& mapping() const noexcept { return map_; } constexpr const accessor_type& accessor() const noexcept { return acc_; } static constexpr bool is_always_unique() { return mapping_type::is_always_unique(); } static constexpr bool is_always_exhaustive() { return mapping_type::is_always_exhaustive(); } static constexpr bool is_always_strided() { return mapping_type::is_always_strided(); } constexpr bool is_unique() const { return map_.is_unique(); } constexpr bool is_exhaustive() const { return map_.is_exhaustive(); } constexpr bool is_strided() const { return map_.is_strided(); } constexpr index_type stride(rank_type r) const { return map_.stride(r); } private: accessor_type acc_; mapping_type map_; data_handle_type ptr_; };template<class CArray> requires (is_array_v<CArray> && rank_v<CArray> == 1) mdspan(CArray&) -> mdspan<remove_all_extents_t<CArray>, extents<size_t, extent_v<CArray, 0>>>;template<class Pointer> requires (is_pointer_v<remove_reference_t<Pointer>>) mdspan(Pointer&&) -> mdspan<remove_pointer_t<remove_reference_t<Pointer>>, extents<size_t>>;template<class ElementType, class... Integrals> requires ((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0) explicit mdspan(ElementType*, Integrals...) -> mdspan<ElementType, extents<size_t, maybe-static-ext<Integrals>...>>;template<class ElementType, class OtherIndexType, size_t N> mdspan(ElementType*, span<OtherIndexType, N>) -> mdspan<ElementType, dextents<size_t, N>>;template<class ElementType, class OtherIndexType, size_t N> mdspan(ElementType*, const array<OtherIndexType, N>&) -> mdspan<ElementType, dextents<size_t, N>>;template<class ElementType, class IndexType, size_t... ExtentsPack> mdspan(ElementType*, const extents<IndexType, ExtentsPack...>&) -> mdspan<ElementType, extents<IndexType, ExtentsPack...>>;template<class ElementType, class MappingType> mdspan(ElementType*, const MappingType&) -> mdspan<ElementType, typename MappingType::extents_type,typename MappingType::layout_type>;template<class MappingType, class AccessorType> mdspan(const typename AccessorType::data_handle_type&, const MappingType&,const AccessorType&) -> mdspan<typename AccessorType::element_type, typename MappingType::extents_type,typename MappingType::layout_type, AccessorType>;}

Mandates:

LayoutPolicy shall meet the layout mapping policy requirements ([mdspan.layout.policy.reqmts]), andAccessorPolicy shall meet the accessor policy requirements ([mdspan.accessor.reqmts]).

Each specialization MDS of mdspan models copyable and

A specialization of mdspan is a trivially copyable type if its accessor_type, mapping_type, and data_handle_typeare trivially copyable types.

23.7.3.6.2 Constructors [mdspan.mdspan.cons]

Constraints:

Preconditions: [0, _map__.required_span_size()) is an accessible range of _ptr__ and _acc__for the values of _map__ and _acc__after the invocation of this constructor.

Effects: Value-initializes ptr_, map_, and acc_.

template<class... OtherIndexTypes> constexpr explicit mdspan(data_handle_type p, OtherIndexTypes... exts);

Let N be sizeof...(OtherIndexTypes).

Constraints:

Preconditions: [0, _map__.required_span_size()) is an accessible range of p and _acc__for the values of _map__ and _acc__after the invocation of this constructor.

Effects:

template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) mdspan(data_handle_type p, span<OtherIndexType, N> exts);template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) mdspan(data_handle_type p, const array<OtherIndexType, N>& exts);

Constraints:

Preconditions: [0, _map__.required_span_size()) is an accessible range of p and _acc__for the values of _map__ and _acc__after the invocation of this constructor.

Effects:

constexpr mdspan(data_handle_type p, const extents_type& ext);

Constraints:

Preconditions: [0, _map__.required_span_size()) is an accessible range of p and _acc__for the values of _map__ and _acc__after the invocation of this constructor.

Effects:

constexpr mdspan(data_handle_type p, const mapping_type& m);

Constraints: is_default_constructible_v<accessor_type> is true.

Preconditions: [0, m.required_span_size()) is an accessible range of p and _acc__for the value of _acc__ after the invocation of this constructor.

Effects:

constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a);

Preconditions: [0, m.required_span_size()) is an accessible range of p and a.

Effects:

template<class OtherElementType, class OtherExtents,class OtherLayoutPolicy, class OtherAccessor> constexpr explicit(_see below_) mdspan(const mdspan<OtherElementType, OtherExtents, OtherLayoutPolicy, OtherAccessor>& other);

Constraints:

Mandates:

Preconditions: [0, _map__.required_span_size()) is an accessible range of _ptr__ and _acc__for values of _ptr__, _map__, and _acc__after the invocation of this constructor.

Hardened preconditions: For each rank index r of extents_type,static_extent(r) == dynamic_extent || static_extent(r) == other.extent(r)is true.

Effects:

Remarks: The expression inside explicit is equivalent to:!is_convertible_v<const OtherLayoutPolicy::template mapping<OtherExtents>&, mapping_type> || !is_convertible_v<const OtherAccessor&, accessor_type>

23.7.3.6.3 Members [mdspan.mdspan.members]

template<class... OtherIndexTypes> constexpr reference operator[](OtherIndexTypes... indices) const;

Constraints:

Let I be extents_type​::​index-cast(std​::​move(indices)).

Hardened preconditions: I is a multidimensional index in extents().

[Note 1:

This implies that_map__(I) < map_.required_span_size()is true.

— _end note_]

Effects: Equivalent to:return acc_.access(ptr_, map_(static_cast<index_type>(std::move(indices))...));

template<class OtherIndexType> constexpr reference operator[](span<OtherIndexType, rank()> indices) const;template<class OtherIndexType> constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;

Constraints:

Effects: Let P be a parameter pack such thatis_same_v<make_index_sequence<rank()>, index_sequence<P...>> is true.

Equivalent to:return operator[](extents_type::_index-cast_(as_const(indices[P]))...);

constexpr size_type size() const noexcept;

Preconditions: The size of the multidimensional index space extents()is representable as a value of type size_type ([basic.fundamental]).

Returns: extents().fwd-prod-of-extents(rank()).

constexpr bool empty() const noexcept;

Returns: trueif the size of the multidimensional index space extents() is 0, otherwise false.

friend constexpr void swap(mdspan& x, mdspan& y) noexcept;

Effects: Equivalent to:swap(x.ptr_, y.ptr_); swap(x.map_, y.map_); swap(x.acc_, y.acc_);

23.7.3.7 submdspan [mdspan.sub]

23.7.3.7.1 Overview [mdspan.sub.overview]

The submdspan facilities create a new mdspanviewing a subset of elements of an existing input mdspan.

The subset viewed by the created mdspan is determined by the SliceSpecifier arguments.

For each function defined in [mdspan.sub] that takes a parameter pack named slices as an argument:

23.7.3.7.2 strided_slice [mdspan.sub.strided.slice]

strided_slice represents a set ofextent regularly spaced integer indices.

The indices start at offset, and increase by increments of stride.

namespace std { template<class OffsetType, class ExtentType, class StrideType> struct strided_slice { using offset_type = OffsetType;using extent_type = ExtentType;using stride_type = StrideType;[[no_unique_address]] offset_type offset{};[[no_unique_address]] extent_type extent{};[[no_unique_address]] stride_type stride{};};}

strided_slice has the data members and special members specified above.

It has no base classes or members other than those specified.

Mandates: OffsetType, ExtentType, and StrideTypeare signed or unsigned integer types, or model integral-constant-like.

[Note 1:

strided_slice{.offset = 1, .extent = 10, .stride = 3}indicates the indices 1, 4, 7, and 10.

Indices are selected from the half-open interval [1, 1 + 10).

— _end note_]

23.7.3.7.3 submdspan_mapping_result [mdspan.sub.map.result]

Specializations of submdspan_mapping_resultare returned by overloads of submdspan_mapping.

namespace std { template<class LayoutMapping> struct submdspan_mapping_result { [[no_unique_address]] LayoutMapping mapping = LayoutMapping(); size_t offset{};};}

submdspan_mapping_result has the data members and special members specified above.

It has no base classes or members other than those specified.

LayoutMapping shall meet the layout mapping requirements ([mdspan.layout.policy.reqmts]).

23.7.3.7.4 Exposition-only helpers [mdspan.sub.helpers]

template<class T> constexpr T _de-ice_(T val) { return val; } template<[_integral-constant-like_](#concept:integral-constant-like "23.7.2.1 Header <span> synopsis [span.syn]") T> constexpr auto _de-ice_(T) { return T::value; } template<class IndexType, size_t k, class... SliceSpecifiers> constexpr IndexType _first_(SliceSpecifiers... slices);

Mandates: IndexType is a signed or unsigned integer type.

Let denote the following value:

Preconditions: is representable as a value of type IndexType.

Returns: extents<IndexType>​::​index-cast().

template<size_t k, class Extents, class... SliceSpecifiers> constexpr auto _last_(const Extents& src, SliceSpecifiers... slices);

Mandates: Extents is a specialization of extents.

Let index_type be typename Extents​::​index_type.

Let denote the following value:

Preconditions: is representable as a value of type index_type.

Returns: Extents​::​index-cast().

template<class IndexType, size_t N, class... SliceSpecifiers> constexpr array<IndexType, sizeof...(SliceSpecifiers)> _src-indices_(const array<IndexType, N>& indices, SliceSpecifiers... slices);

Mandates: IndexType is a signed or unsigned integer type.

Returns: An array<IndexType, sizeof...(SliceSpecifiers)> src_idx such that for each k in the range [0, sizeof...(SliceSpecifiers)),src_idx[k] equals

23.7.3.7.5 submdspan_extents function [mdspan.sub.extents]

template<class IndexType, class... Extents, class... SliceSpecifiers> constexpr auto submdspan_extents(const extents<IndexType, Extents...>& src, SliceSpecifiers... slices);

Constraints: sizeof...(slices) equals Extents​::​rank().

Mandates: For each rank index k of src.extents(), exactly one of the following is true:

Preconditions: For each rank index k of src.extents(), all of the following are true:

Let SubExtents be a specialization of extents such that:

Returns: A value ext of type SubExtents such that for each kfor which _map-rank_[k] != dynamic_extent is true,ext.extent(_map-rank_[k]) equals:

23.7.3.7.6 Specializations of submdspan_mapping [mdspan.sub.map]

23.7.3.7.6.1 Common [mdspan.sub.map.common]

The following elements apply to all functions in [mdspan.sub.map].

Constraints: sizeof...(slices) equals extents_type​::​rank().

Mandates: For each rank index k of extents(), exactly one of the following is true:

Preconditions: For each rank index k of extents(), all of the following are true:

Let sub_ext be the result of submdspan_extents(extents(), slices...) and let SubExtents be decltype(sub_ext).

Let sub_strides be an array<SubExtents​::​index_type, SubExtents​::​rank()>such that for each rank index k of extents()for which _map-rank_[k] is not dynamic_extent,sub_strides[_map-rank_[k]] equals:

Let P be a parameter pack such that is_same_v<make_index_sequence<rank()>, index_sequence<P...>>is true.

If first_<index_type, k>(slices...)equals extents().extent(k)for any rank index k of extents(), then let offset be a value of type size_t equal to(*this).required_span_size().

Otherwise, let offset be a value of type size_t equal to(*this)(first_<index_type, P>(slices...)...).

Given a layout mapping type M, a type S is aunit-stride slice for M if

23.7.3.7.6.2 layout_left specialization of submdspan_mapping [mdspan.sub.map.left]

template<class Extents> template<class... SliceSpecifiers> constexpr auto layout_left::mapping<Extents>::_submdspan-mapping-impl_( SliceSpecifiers... slices) const -> _see below_;

Returns:

23.7.3.7.6.3 layout_right specialization of submdspan_mapping [mdspan.sub.map.right]

template<class Extents> template<class... SliceSpecifiers> constexpr auto layout_right::mapping<Extents>::_submdspan-mapping-impl_( SliceSpecifiers... slices) const -> _see below_;

Returns:

23.7.3.7.6.4 layout_stride specialization of submdspan_mapping [mdspan.sub.map.stride]

template<class Extents> template<class... SliceSpecifiers> constexpr auto layout_stride::mapping<Extents>::_submdspan-mapping-impl_( SliceSpecifiers... slices) const -> _see below_;

Returns:

23.7.3.7.6.5 layout_left_padded specialization of submdspan_mapping [mdspan.sub.map.leftpad]

template<class Extents> template<class... SliceSpecifiers> constexpr auto layout_left_padded::mapping<Extents>::_submdspan-mapping-impl_( SliceSpecifiers... slices) const -> _see below_;

Returns:

23.7.3.7.6.6 layout_right_padded specialization of submdspan_mapping [mdspan.sub.map.rightpad]

template<class Extents> template<class... SliceSpecifiers> constexpr auto layout_right_padded::mapping<Extents>::submdspan-mapping-impl( SliceSpecifiers... slices) const -> _see below_;

Returns:

23.7.3.7.7 submdspan function template [mdspan.sub.sub]

template<class ElementType, class Extents, class LayoutPolicy,class AccessorPolicy, class... SliceSpecifiers> constexpr auto submdspan( const mdspan<ElementType, Extents, LayoutPolicy, AccessorPolicy>& src, SliceSpecifiers... slices) -> _see below_;

Let index_type be typename Extents​::​index_type.

Let sub_map_offset be the result ofsubmdspan_mapping(src.mapping(), slices...).

[Note 1:

This invocation of submdspan_mappingselects a function call via overload resolution on a candidate set that includes the lookup set found by argument-dependent lookup ([basic.lookup.argdep]).

— _end note_]

Constraints:

Mandates:

Preconditions:

[Note 2:

These conditions ensure that the mapping returned by submdspan_mappingmatches the algorithmically expected index-mapping given the slice specifiers.

— _end note_]

Effects: Equivalent to:auto sub_map_result = submdspan_mapping(src.mapping(), slices...);return mdspan(src.accessor().offset(src.data(), sub_map_result.offset), sub_map_result.mapping, AccessorPolicy::offset_policy(src.accessor()));

[Example 1:

Given a rank-3 mdspan grid3d representing a three-dimensional grid of regularly spaced points in a rectangular prism, the function zero_surface sets all elements on the surface of the 3-dimensional shape to zero.

It does so by reusing a function zero_2dthat takes a rank-2 mdspan.

template<class T, class E, class L, class A> void zero_2d(mdspan<T, E, L, A> a) { static_assert(a.rank() == 2);for (int i = 0; i < a.extent(0); i++) for (int j = 0; j < a.extent(1); j++) a[i, j] = 0;} template<class T, class E, class L, class A> void zero_surface(mdspan<T, E, L, A> grid3d) { static_assert(grid3d.rank() == 3); zero_2d(submdspan(grid3d, 0, full_extent, full_extent)); zero_2d(submdspan(grid3d, full_extent, 0, full_extent)); zero_2d(submdspan(grid3d, full_extent, full_extent, 0)); zero_2d(submdspan(grid3d, grid3d.extent(0) - 1, full_extent, full_extent)); zero_2d(submdspan(grid3d, full_extent, grid3d.extent(1) - 1, full_extent)); zero_2d(submdspan(grid3d, full_extent, full_extent, grid3d.extent(2) - 1));} — _end example_]