24 Containers library [containers]

24.7 Views [views]

24.7.3 Multidimensional access [views.multidim]

24.7.3.4 Layout mapping [mdspan.layout]

24.7.3.4.9 Class template layout_right_padded::mapping [mdspan.layout.rightpad]

24.7.3.4.9.1 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 to extents_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(); // exposition only static constexpr size_t last-static-extent = // exposition only extents_type::static_extent(rank_ - 1); // [mdspan.layout.rightpad.expo], exposition-only members static constexpr size_t static-padding-stride = see below; // exposition only public: // [mdspan.layout.rightpad.cons], constructors 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; // [mdspan.layout.rightpad.obs], observers 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: // [mdspan.layout.rightpad.expo], exposition-only members index_type stride-rm2 = static-padding-stride; // exposition only extents_type extents_{}; // exposition only // [mdspan.sub.map], submdspan mapping specialization template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const // exposition only -> 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:

(4.1)
the empty parameter pack, if rank_ equals zero;
(4.2)
otherwise, 0zu, if rank_ equals one;
(4.3)
otherwise, the parameter pack 0zu, 1zu, …, rank_- 1.

Mandates:

(5.1)
If rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents() is representable as a value of type index_type.
(5.2)
padding_value is representable as a value of type index_type.
(5.3)
If
- (5.3.1)
  rank_ is greater than one,
- (5.3.2)
  padding_value does not equal dynamic_extent, and
- (5.3.3)
  last-static-extent does not equal dynamic_extent,
then LEAST-MULTIPLE-AT-LEAST(padding_value, last-static-extent) is representable as a value of type size_t, and is representable as a value of type index_type.
(5.4)
If
- (5.4.1)
  rank_ is greater than one,
- (5.4.2)
  padding_value does not equal dynamic_extent, and
- (5.4.3)
  extents_type::static_extent(k) does not equal dynamic_extent for all k in the range [0, rank_),
then the product of LEAST-MULTIPLE-AT-LEAST(padding_value, ext.static_extent(rank_ - 1)) and all values ext.static_extent(k) with k in the range of [0, rank_ - 1) is representable as a value of type size_t, and is representable as a value of type index_type.

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

🔗

static constexpr size_t static-padding-stride = see below;

The value is

(1.1)
0, if rank_ equals zero or one;
(1.2)
otherwise, dynamic_extent, if padding_value or last-static-extent equals dynamic_extent;
(1.3)
otherwise, the size_t value which is LEAST-MULTIPLE-AT-LEAST(padding_value, last-static-extent).

🔗

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]

24.7.3.4.9.3 Constructors [mdspan.layout.rightpad.cons]

🔗

constexpr mapping(const extents_type& ext);

Preconditions:

(1.1)
The size of the multidimensional index space ext is representable as a value of type index_type.
(1.2)
If rank_ is greater than one and padding_value does not equal dynamic_extent, then LEAST-MULTIPLE-AT-LEAST(padding_value, ext.extent(rank_ - 1)) is representable as a value of type index_type.
(1.3)
If rank_ is greater than one and padding_value does not equal dynamic_extent, then the product of LEAST-MULTIPLE-AT-LEAST(padding_value, ext.extent(rank_ - 1)) and all values ext.extent(k) with k in the range of [0, rank_ - 1) is representable as a value of type index_type.

Effects:

(2.1)
Direct-non-list-initializes extents_ with ext; and
(2.2)
if rank_ is greater than one, direct-non-list-initializes stride-rm2
- (2.2.1)
  with ext.extent(rank_ - 1) if padding_value is dynamic_extent,
- (2.2.2)
  otherwise with LEAST-MULTIPLE-AT-LEAST(padding_value, ext.extent(rank_ - 1)).

🔗

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

Constraints:

(3.1)
is_convertible_v<OtherIndexType, index_type> is true.
(3.2)
is_nothrow_constructible_v<index_type, OtherIndexType> is true.

Preconditions:

(4.1)
pad is representable as a value of type index_type.
(4.2)
extents_type::index-cast(pad) is greater than zero.
(4.3)
If rank_ is greater than one, then LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1)) is representable as a value of type index_type.
(4.4)
If rank_ is greater than one, then the product of LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(
rank_ - 1)) and all values ext.extent(k) with k in the range of [0, rank_ - 1) is representable as a value of type index_type.
(4.5)
If padding_value is not equal to dynamic_extent, padding_value equals extents_type::
index-cast(pad).

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)).

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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:

(8.1)
If rank_ > 1 is true and padding_value == dynamic_extent is false, then other.stride(
rank_ - 2) equals LEAST-MULTIPLE-AT-LEAST(padding_value, extents_type::index-cast(other.extents().extent(rank_ - 1))) and
(8.2)
other.required_span_size() is representable as a value of type index_type.

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:

(11.1)
If rank_ is greater than 1 and padding_value does not equal dynamic_extent, then other.
stride(rank_ - 2) equals LEAST-MULTIPLE-AT-LEAST(padding_value, extents_type::index-cast(other.extents().extent(rank_ - 1)))
(11.2)
If rank_ is greater than 0, then other.stride(rank_ - 1) equals 1.
(11.3)
If rank_ is greater than 2, then for all r in the range [0, rank_ - 2), other.stride(r) equals (other.extents().rev-prod-of-extents(r) / other.extents().extent(rank_ - 1)) * other.stride(rank_ - 2)
(11.4)
other.required_span_size() is representable as a value of type index_type.

Effects:

(12.1)
Direct-non-list-initializes extents_ with other.extents(); and
(12.2)
if rank_ is greater than one, direct-non-list-initializes stride-rm2 with other.stride(rank_ - 2).

🔗

template<class LayoutRightPaddedMapping>
  constexpr explicit( see below )
    mapping(const LayoutRightPaddedMapping& other);

Constraints:

(13.1)
is-layout-right-padded-mapping-of<LayoutRightPaddedMapping> is true.
(13.2)
is_constructible_v<extents_type, typename LayoutRightPaddedMapping::extents_-
type> is true.

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

Preconditions:

(15.1)
If rank_ is greater than 1 and padding_value does not equal dynamic_extent, then other.
stride(rank_ - 2) equals LEAST-MULTIPLE-AT-LEAST(padding_value, extents_type::index-cast(other.extent(rank_ - 1)))
(15.2)
other.required_span_size() is representable as a value of type index_type.

Effects:

(16.1)
Direct-non-list-initializes extents_ with other.extents(); and
(16.2)
if rank_ is greater than one, direct-non-list-initializes stride-rm2 with other.stride(rank_ - 2).

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:

(18.1)
is-layout-left-padded-mapping-of<LayoutLeftPaddedMapping> is true or
is-mapping-of<layout_left, LayoutLeftPaddedMapping> is true.
(18.2)
rank_ equals zero or one.
(18.3)
is_constructible_v<extents_type, typename LayoutLeftPaddedMapping::extents_type>
is true.

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]

24.7.3.4.9.4 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:

(3.1)
sizeof...(Indices) == rank_ is true.
(3.2)
(is_convertible_v<Indices, index_type> && ...) is true.
(3.3)
(is_nothrow_constructible_v<index_type, Indices> && ...) is true.

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:

(6.1)
If rank_ equals zero or one, then true;
(6.2)
otherwise, if neither static-padding-stride nor last-static-extent equal dynamic_extent, then static-padding-stride == last-static-extent;
(6.3)
otherwise, false.

🔗

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:

(9.1)
If r equals rank_ - 1: 1;
(9.2)
otherwise, if r equals rank_ - 2: stride-rm2;
(9.3)
otherwise, the product of stride-rm2 and all values extents_.extent(k) with k in the range of [r + 1, rank_ - 1).

🔗

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

Constraints:

(10.1)
is-layout-right-padded-mapping-of<LayoutRightPaddedMapping> is true.
(10.2)
LayoutRightPaddedMapping::extents_type::rank() == rank_ is true.

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

Otherwise, false.

24 Containers library [containers]

24.7 Views [views]

24.7.3 Multidimensional access [views.multidim]

24.7.3.4 Layout mapping [mdspan.layout]

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

24.7.3.4.9.1 Overview [mdspan.layout.rightpad.overview]

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

24.7.3.4.9.3 Constructors [mdspan.layout.rightpad.cons]

24.7.3.4.9.4 Observers [mdspan.layout.rightpad.obs]

24.7.3.4.9 Class template layout_right_padded::mapping [mdspan.layout.rightpad]