[mdspan.layout.right]

# 23 Containers library [[containers]](./#containers)

## 23.7 Views [[views]](views#mdspan.layout.right)

### 23.7.3 Multidimensional access [[views.multidim]](views.multidim#mdspan.layout.right)

#### 23.7.3.4 Layout mapping [[mdspan.layout]](mdspan.layout#right)

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

#### [23.7.3.4.6.1](#overview) Overview [[mdspan.layout.right.overview]](mdspan.layout.right.overview)

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22238)

layout_right provides a layout mapping
where the rightmost extent is stride 1, and
strides increase right-to-left as the product of extents[.](#overview-1.sentence-1)

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; // [[mdspan.layout.right.cons]](#cons "23.7.3.4.6.2 Constructors"), constructorsconstexpr 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; // [[mdspan.layout.right.obs]](#obs "23.7.3.4.6.3 Observers"), observersconstexpr 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_*{}; // *exposition only*// [[mdspan.sub.map]](mdspan.sub.map "23.7.3.7.6 Specializations of submdspan_­mapping"), submdspan mapping specializationtemplate<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...); }};}

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22312)

If Extents is not a specialization of extents,
then the program is ill-formed[.](#overview-2.sentence-1)

[3](#overview-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22316)

layout_right​::​mapping<E> is a trivially copyable type
that models [regular](concepts.object#concept:regular "18.6 Object concepts [concepts.object]") for each E[.](#overview-3.sentence-1)

[4](#overview-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22320)

*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[.](#overview-4.sentence-1)

#### [23.7.3.4.6.2](#cons) Constructors [[mdspan.layout.right.cons]](mdspan.layout.right.cons)

[ð](#lib:layout_right::mapping,constructor)

`constexpr mapping(const extents_type& e) noexcept;
`

[1](#cons-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22334)

*Preconditions*: The size of the multidimensional index space e is representable as
a value of type index_type ([[basic.fundamental]](basic.fundamental "6.9.2 Fundamental types"))[.](#cons-1.sentence-1)

[2](#cons-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22339)

*Effects*: Direct-non-list-initializes *extents_* with e[.](#cons-2.sentence-1)

[ð](#lib:layout_right::mapping,constructor_)

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

[3](#cons-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22352)

*Constraints*: is_constructible_v<extents_type, OtherExtents> is true[.](#cons-3.sentence-1)

[4](#cons-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22356)

*Preconditions*: other.required_span_size() is representable as
a value of type index_type ([[basic.fundamental]](basic.fundamental "6.9.2 Fundamental types"))[.](#cons-4.sentence-1)

[5](#cons-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22361)

*Effects*: Direct-non-list-initializes *extents_* with other.extents()[.](#cons-5.sentence-1)

[ð](#lib:layout_right::mapping,constructor__)

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

[6](#cons-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22374)

*Constraints*: 

- [(6.1)](#cons-6.1)

extents_type​::​rank() <= 1 is true, and

- [(6.2)](#cons-6.2)

is_constructible_v<extents_type, OtherExtents> is true[.](#cons-6.sentence-1)

[7](#cons-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22383)

*Preconditions*: other.required_span_size() is representable as
a value of type index_type ([[basic.fundamental]](basic.fundamental "6.9.2 Fundamental types"))[.](#cons-7.sentence-1)

[8](#cons-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22388)

*Effects*: Direct-non-list-initializes *extents_* with other.extents()[.](#cons-8.sentence-1)

[ð](#lib:layout_right::mapping,constructor___)

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

[9](#cons-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22402)

*Constraints*: 

- [(9.1)](#cons-9.1)

  *is-layout-right-padded-mapping-of*<LayoutRightPaddedMapping> is true[.](#cons-9.1.sentence-1)

- [(9.2)](#cons-9.2)

  is_constructible_v<extents_type, typename LayoutRightPaddedMapping​::​extents_-  
type> is true[.](#cons-9.2.sentence-1)

[10](#cons-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22413)

*Mandates*: If

- [(10.1)](#cons-10.1)

Extents​::​rank() is greater than one,

- [(10.2)](#cons-10.2)

Extents​::​static_extent(Extents​::​rank() - 1) does not equal dynamic_extent, and

- [(10.3)](#cons-10.3)

LayoutRightPaddedMapping​::​*static-padding-stride* does not equal dynamic_extent,

then Extents​::​static_extent(Extents​::​rank() - 1) equalsLayoutRightPaddedMapping​::​*static-padding-stride*[.](#cons-10.sentence-1)

[11](#cons-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22429)

*Preconditions*: 

- [(11.1)](#cons-11.1)

  If extents_type​::​rank() > 1 is true,
then other.stride(extents_type​::​rank() - 2)  
equalsother.extents().extent(extents_type​::​rank() - 1)[.](#cons-11.1.sentence-1)

- [(11.2)](#cons-11.2)

  other.required_span_size() is representable as
a value of type index_type[.](#cons-11.2.sentence-1)

[12](#cons-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22441)

*Effects*: Direct-non-list-initializes extents_ with other.extents()[.](#cons-12.sentence-1)

[ð](#lib:layout_right::mapping,constructor____)

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

[13](#cons-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22454)

*Constraints*: is_constructible_v<extents_type, OtherExtents> is true[.](#cons-13.sentence-1)

[14](#cons-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22458)

*Preconditions*: 

- [(14.1)](#cons-14.1)

  If extents_type​::​rank() > 0 is true,
then for all r in the range [0, extents_type​::​rank()),other.stride(r) equalsother.extents().*rev-prod-of-extents*(r)[.](#cons-14.1.sentence-1)

- [(14.2)](#cons-14.2)

  other.required_span_size() is representable as
a value of type index_type ([[basic.fundamental]](basic.fundamental "6.9.2 Fundamental types"))[.](#cons-14.2.sentence-1)

[15](#cons-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22471)

*Effects*: Direct-non-list-initializes *extents_* with other.extents()[.](#cons-15.sentence-1)

#### [23.7.3.4.6.3](#obs) Observers [[mdspan.layout.right.obs]](mdspan.layout.right.obs)

[ð](#lib:required_span_size,layout_right::mapping)

`constexpr index_type required_span_size() const noexcept;
`

[1](#obs-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22484)

*Returns*: extents().*fwd-prod-of-extents*(extents_type​::​rank())[.](#obs-1.sentence-1)

[ð](#lib:operator(),layout_right::mapping)

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

[2](#obs-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22496)

*Constraints*: 

- [(2.1)](#obs-2.1)

sizeof...(Indices) == extents_type​::​rank() is true,

- [(2.2)](#obs-2.2)

(is_convertible_v<Indices, index_type> && ...) is true, and

- [(2.3)](#obs-2.3)

(is_nothrow_constructible_v<index_type, Indices> && ...) istrue[.](#obs-2.sentence-1)

[3](#obs-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22508)

*Preconditions*: extents_type​::​*index-cast*(i) is
a multidimensional index in *extents_* ([[mdspan.overview]](mdspan.overview "23.7.3.1 Overview"))[.](#obs-3.sentence-1)

[4](#obs-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22513)

*Effects*: Let P be a parameter pack such thatis_same_v<index_sequence_for<Indices...>, index_sequence<P...>> is true[.](#obs-4.sentence-1)

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

[ð](#lib:stride,layout_right::mapping)

`constexpr index_type stride(rank_type i) const noexcept;
`

[5](#obs-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22531)

*Constraints*: extents_type​::​rank() > 0 is true[.](#obs-5.sentence-1)

[6](#obs-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22535)

*Preconditions*: i < extents_type​::​rank() is true[.](#obs-6.sentence-1)

[7](#obs-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22539)

*Returns*: extents().*rev-prod-of-extents*(i)[.](#obs-7.sentence-1)

[ð](#lib:operator==,layout_right::mapping)

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

[8](#obs-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22551)

*Constraints*: extents_type​::​rank() == OtherExtents​::​rank() is true[.](#obs-8.sentence-1)

[9](#obs-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L22555)

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