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[mdspan.accessor.aligned]
# 23 Containers library [[containers]](./#containers)
## 23.7 Views [[views]](views#mdspan.accessor.aligned)
### 23.7.3 Multidimensional access [[views.multidim]](views.multidim#mdspan.accessor.aligned)
#### 23.7.3.5 Accessor policy [[mdspan.accessor]](mdspan.accessor#aligned)
#### 23.7.3.5.4 Class template aligned_accessor [mdspan.accessor.aligned]
#### [23.7.3.5.4.1](#overview) Overview [[mdspan.accessor.aligned.overview]](mdspan.accessor.aligned.overview)
namespace std {template<class ElementType, size_t ByteAlignment>struct [aligned_accessor](#lib:aligned_accessor "23.7.3.5.4.1 Overview [mdspan.accessor.aligned.overview]") {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; };}
[1](#overview-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24469)
*Mandates*:
- [(1.1)](#overview-1.1)
byte_alignment is a power of two, and
- [(1.2)](#overview-1.2)
byte_alignment >= alignof(ElementType) is true[.](#overview-1.sentence-1)
[2](#overview-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24476)
aligned_accessor meets the accessor policy requirements[.](#overview-2.sentence-1)
[3](#overview-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24479)
ElementType is required to be a complete object type
that is neither an abstract class type nor an array type[.](#overview-3.sentence-1)
[4](#overview-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24483)
Each specialization of aligned_accessor is
a trivially copyable type that models [semiregular](concepts.object#concept:semiregular "18.6 Object concepts [concepts.object]")[.](#overview-4.sentence-1)
[5](#overview-5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24487)
[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
- [(5.1)](#overview-5.1)
[p, p + n) is a valid range, and,
- [(5.2)](#overview-5.2)
if n is greater than zero,
then is_sufficiently_aligned<byte_alignment>(p) is true[.](#overview-5.sentence-1)
[6](#overview-6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24499)
[*Example [1](#overview-example-1)*:
The following function compute uses 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)>[.](#overview-6.sentence-1)
If so, the function dispatches to
a function compute_using_fourfold_overalignment that requires fourfold over-alignment of arrays,
but can therefore use hardware-specific instructions,
such as four-wide SIMD (Single Instruction Multiple Data) instructions[.](#overview-6.sentence-2)
Otherwise, compute dispatches to a
possibly less optimized function compute_without_requiring_overalignment that has no over-alignment requirement[.](#overview-6.sentence-3)
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) Members [[mdspan.accessor.aligned.members]](mdspan.accessor.aligned.members)
[🔗](#lib:aligned_accessor,constructor)
`template<class OtherElementType, size_t OtherByteAlignment>
constexpr aligned_accessor(aligned_accessor<OtherElementType, OtherByteAlignment>) noexcept;
`
[1](#members-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24545)
*Constraints*:
- [(1.1)](#members-1.1)
is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true[.](#members-1.1.sentence-1)
- [(1.2)](#members-1.2)
OtherByteAlignment >= byte_alignment is true[.](#members-1.2.sentence-1)
[2](#members-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24555)
*Effects*: None[.](#members-2.sentence-1)
[🔗](#lib:aligned_accessor,constructor_)
`template<class OtherElementType>
constexpr explicit aligned_accessor(default_accessor<OtherElementType>) noexcept;
`
[3](#members-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24567)
*Constraints*: is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true[.](#members-3.sentence-1)
[4](#members-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24572)
*Effects*: None[.](#members-4.sentence-1)
[🔗](#lib:access,aligned_accessor)
`constexpr reference access(data_handle_type p, size_t i) const noexcept;
`
[5](#members-5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24583)
*Preconditions*: [0, i + 1) is an accessible range for p and *this[.](#members-5.sentence-1)
[6](#members-6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24587)
*Effects*: Equivalent to: return assume_aligned<byte_alignment>(p)[i];
[🔗](#lib:operator_default_accessor,aligned_accessor)
`template<class OtherElementType>
constexpr operator default_accessor<OtherElementType>() const noexcept;
`
[7](#members-7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24599)
*Constraints*: is_convertible_v<element_type(*)[], OtherElementType(*)[]> is true[.](#members-7.sentence-1)
[8](#members-8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24604)
*Effects*: Equivalent to: return {};
[🔗](#lib:offset,aligned_accessor)
`constexpr typename offset_policy::data_handle_type
offset(data_handle_type p, size_t i) const noexcept;
`
[9](#members-9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24616)
*Preconditions*: [0, i + 1) is an accessible range for p and *this[.](#members-9.sentence-1)
[10](#members-10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24620)
*Effects*: Equivalent to: return assume_aligned<byte_alignment>(p) + i;

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[mdspan.accessor.aligned.members]
# 23 Containers library [[containers]](./#containers)
## 23.7 Views [[views]](views#mdspan.accessor.aligned.members)
### 23.7.3 Multidimensional access [[views.multidim]](views.multidim#mdspan.accessor.aligned.members)
#### 23.7.3.5 Accessor policy [[mdspan.accessor]](mdspan.accessor#aligned.members)
#### 23.7.3.5.4 Class template aligned_accessor [[mdspan.accessor.aligned]](mdspan.accessor.aligned#members)
#### 23.7.3.5.4.2 Members [mdspan.accessor.aligned.members]
[🔗](#lib:aligned_accessor,constructor)
`template<class OtherElementType, size_t OtherByteAlignment>
constexpr aligned_accessor(aligned_accessor<OtherElementType, OtherByteAlignment>) noexcept;
`
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24545)
*Constraints*:
- [(1.1)](#1.1)
is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true[.](#1.1.sentence-1)
- [(1.2)](#1.2)
OtherByteAlignment >= byte_alignment is true[.](#1.2.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24555)
*Effects*: None[.](#2.sentence-1)
[🔗](#lib:aligned_accessor,constructor_)
`template<class OtherElementType>
constexpr explicit aligned_accessor(default_accessor<OtherElementType>) noexcept;
`
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24567)
*Constraints*: is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true[.](#3.sentence-1)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24572)
*Effects*: None[.](#4.sentence-1)
[🔗](#lib:access,aligned_accessor)
`constexpr reference access(data_handle_type p, size_t i) const noexcept;
`
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24583)
*Preconditions*: [0, i + 1) is an accessible range for p and *this[.](#5.sentence-1)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24587)
*Effects*: Equivalent to: return assume_aligned<byte_alignment>(p)[i];
[🔗](#lib:operator_default_accessor,aligned_accessor)
`template<class OtherElementType>
constexpr operator default_accessor<OtherElementType>() const noexcept;
`
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24599)
*Constraints*: is_convertible_v<element_type(*)[], OtherElementType(*)[]> is true[.](#7.sentence-1)
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24604)
*Effects*: Equivalent to: return {};
[🔗](#lib:offset,aligned_accessor)
`constexpr typename offset_policy::data_handle_type
offset(data_handle_type p, size_t i) const noexcept;
`
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24616)
*Preconditions*: [0, i + 1) is an accessible range for p and *this[.](#9.sentence-1)
[10](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24620)
*Effects*: Equivalent to: return assume_aligned<byte_alignment>(p) + i;

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[mdspan.accessor.aligned.overview]
# 23 Containers library [[containers]](./#containers)
## 23.7 Views [[views]](views#mdspan.accessor.aligned.overview)
### 23.7.3 Multidimensional access [[views.multidim]](views.multidim#mdspan.accessor.aligned.overview)
#### 23.7.3.5 Accessor policy [[mdspan.accessor]](mdspan.accessor#aligned.overview)
#### 23.7.3.5.4 Class template aligned_accessor [[mdspan.accessor.aligned]](mdspan.accessor.aligned#overview)
#### 23.7.3.5.4.1 Overview [mdspan.accessor.aligned.overview]
namespace std {template<class ElementType, size_t ByteAlignment>struct [aligned_accessor](#lib:aligned_accessor "23.7.3.5.4.1 Overview [mdspan.accessor.aligned.overview]") {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; };}
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24469)
*Mandates*:
- [(1.1)](#1.1)
byte_alignment is a power of two, and
- [(1.2)](#1.2)
byte_alignment >= alignof(ElementType) is true[.](#1.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24476)
aligned_accessor meets the accessor policy requirements[.](#2.sentence-1)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24479)
ElementType is required to be a complete object type
that is neither an abstract class type nor an array type[.](#3.sentence-1)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24483)
Each specialization of aligned_accessor is
a trivially copyable type that models [semiregular](concepts.object#concept:semiregular "18.6 Object concepts [concepts.object]")[.](#4.sentence-1)
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24487)
[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
- [(5.1)](#5.1)
[p, p + n) is a valid range, and,
- [(5.2)](#5.2)
if n is greater than zero,
then is_sufficiently_aligned<byte_alignment>(p) is true[.](#5.sentence-1)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24499)
[*Example [1](#example-1)*:
The following function compute uses 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)>[.](#6.sentence-1)
If so, the function dispatches to
a function compute_using_fourfold_overalignment that requires fourfold over-alignment of arrays,
but can therefore use hardware-specific instructions,
such as four-wide SIMD (Single Instruction Multiple Data) instructions[.](#6.sentence-2)
Otherwise, compute dispatches to a
possibly less optimized function compute_without_requiring_overalignment that has no over-alignment requirement[.](#6.sentence-3)
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*]

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[mdspan.accessor.default]
# 23 Containers library [[containers]](./#containers)
## 23.7 Views [[views]](views#mdspan.accessor.default)
### 23.7.3 Multidimensional access [[views.multidim]](views.multidim#mdspan.accessor.default)
#### 23.7.3.5 Accessor policy [[mdspan.accessor]](mdspan.accessor#default)
#### 23.7.3.5.3 Class template default_accessor [mdspan.accessor.default]
#### [23.7.3.5.3.1](#overview) Overview [[mdspan.accessor.default.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; };}
[1](#overview-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24382)
default_accessor meets the accessor policy requirements[.](#overview-1.sentence-1)
[2](#overview-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24385)
ElementType is required to be a complete object type
that is neither an abstract class type nor an array type[.](#overview-2.sentence-1)
[3](#overview-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24389)
Each specialization of default_accessor is
a trivially copyable type that models [semiregular](concepts.object#concept:semiregular "18.6 Object concepts [concepts.object]")[.](#overview-3.sentence-1)
[4](#overview-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24393)
[0,n) is an accessible range for
an object p of type data_handle_type and
an object of type default_accessor if and only if [p, p + n) is a valid range[.](#overview-4.sentence-1)
#### [23.7.3.5.3.2](#members) Members [[mdspan.accessor.default.members]](mdspan.accessor.default.members)
[🔗](#lib:default_accessor,constructor)
`template<class OtherElementType>
constexpr default_accessor(default_accessor<OtherElementType>) noexcept {}
`
[1](#members-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24408)
*Constraints*: is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true[.](#members-1.sentence-1)
[🔗](#lib:access,default_accessor)
`constexpr reference access(data_handle_type p, size_t i) const noexcept;
`
[2](#members-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24420)
*Effects*: Equivalent to: return p[i];
[🔗](#lib:offset,default_accessor)
`constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;
`
[3](#members-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24431)
*Effects*: Equivalent to: return p + i;

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[mdspan.accessor.default.members]
# 23 Containers library [[containers]](./#containers)
## 23.7 Views [[views]](views#mdspan.accessor.default.members)
### 23.7.3 Multidimensional access [[views.multidim]](views.multidim#mdspan.accessor.default.members)
#### 23.7.3.5 Accessor policy [[mdspan.accessor]](mdspan.accessor#default.members)
#### 23.7.3.5.3 Class template default_accessor [[mdspan.accessor.default]](mdspan.accessor.default#members)
#### 23.7.3.5.3.2 Members [mdspan.accessor.default.members]
[🔗](#lib:default_accessor,constructor)
`template<class OtherElementType>
constexpr default_accessor(default_accessor<OtherElementType>) noexcept {}
`
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24408)
*Constraints*: is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true[.](#1.sentence-1)
[🔗](#lib:access,default_accessor)
`constexpr reference access(data_handle_type p, size_t i) const noexcept;
`
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24420)
*Effects*: Equivalent to: return p[i];
[🔗](#lib:offset,default_accessor)
`constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;
`
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24431)
*Effects*: Equivalent to: return p + i;

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[mdspan.accessor.default.overview]
# 23 Containers library [[containers]](./#containers)
## 23.7 Views [[views]](views#mdspan.accessor.default.overview)
### 23.7.3 Multidimensional access [[views.multidim]](views.multidim#mdspan.accessor.default.overview)
#### 23.7.3.5 Accessor policy [[mdspan.accessor]](mdspan.accessor#default.overview)
#### 23.7.3.5.3 Class template default_accessor [[mdspan.accessor.default]](mdspan.accessor.default#overview)
#### 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; };}
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24382)
default_accessor meets the accessor policy requirements[.](#1.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24385)
ElementType is required to be a complete object type
that is neither an abstract class type nor an array type[.](#2.sentence-1)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24389)
Each specialization of default_accessor is
a trivially copyable type that models [semiregular](concepts.object#concept:semiregular "18.6 Object concepts [concepts.object]")[.](#3.sentence-1)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24393)
[0,n) is an accessible range for
an object p of type data_handle_type and
an object of type default_accessor if and only if [p, p + n) is a valid range[.](#4.sentence-1)

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[mdspan.accessor.general]
# 23 Containers library [[containers]](./#containers)
## 23.7 Views [[views]](views#mdspan.accessor.general)
### 23.7.3 Multidimensional access [[views.multidim]](views.multidim#mdspan.accessor.general)
#### 23.7.3.5 Accessor policy [[mdspan.accessor]](mdspan.accessor#general)
#### 23.7.3.5.1 General [mdspan.accessor.general]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24204)
An [*accessor policy*](#def:accessor_policy "23.7.3.5.1 General [mdspan.accessor.general]") 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[.](#1.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24209)
A range of indices [0,N) is an [*accessible range*](#def:range,accessible "23.7.3.5.1 General [mdspan.accessor.general]") 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[.](#2.sentence-1)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24215)
In [[mdspan.accessor.reqmts]](mdspan.accessor.reqmts "23.7.3.5.2 Requirements"),
- [(3.1)](#3.1)
A denotes an accessor policy[.](#3.1.sentence-1)
- [(3.2)](#3.2)
a denotes a value of type A or const A[.](#3.2.sentence-1)
- [(3.3)](#3.3)
p denotes a value of type A​::​data_handle_type or const A​::​data_handle_type[.](#3.3.sentence-1)
[*Note [1](#note-1)*:
The type A​::​data_handle_type need not be dereferenceable[.](#3.3.sentence-2)
— *end note*]
- [(3.4)](#3.4)
n, i, and j each denote values of type size_t[.](#3.4.sentence-1)

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[mdspan.accessor.reqmts]
# 23 Containers library [[containers]](./#containers)
## 23.7 Views [[views]](views#mdspan.accessor.reqmts)
### 23.7.3 Multidimensional access [[views.multidim]](views.multidim#mdspan.accessor.reqmts)
#### 23.7.3.5 Accessor policy [[mdspan.accessor]](mdspan.accessor#reqmts)
#### 23.7.3.5.2 Requirements [mdspan.accessor.reqmts]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24234)
A type A meets the accessor policy requirements if
- [(1.1)](#1.1)
A models [copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]"),
- [(1.2)](#1.2)
is_nothrow_move_constructible_v<A> is true,
- [(1.3)](#1.3)
is_nothrow_move_assignable_v<A> is true,
- [(1.4)](#1.4)
is_nothrow_swappable_v<A> is true, and
- [(1.5)](#1.5)
the following types and expressions
are well-formed and have the specified semantics[.](#1.sentence-1)
[🔗](#itemdecl:1)
`typename A::element_type
`
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24255)
*Result*: A complete object type that is not an abstract class type[.](#2.sentence-1)
[🔗](#itemdecl:2)
`typename A::data_handle_type
`
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24265)
*Result*: A type that models [copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]"), 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[.](#3.sentence-1)
[*Note [1](#note-1)*:
The type of data_handle_type need not be element_type*[.](#3.sentence-2)
— *end note*]
[🔗](#itemdecl:3)
`typename A::reference
`
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24281)
*Result*: A type that models[common_reference_with](concept.commonref#concept:common_reference_with "18.4.5 Concept common_­reference_­with [concept.commonref]")<A​::​reference&&, A​::​element_type&>[.](#4.sentence-1)
[*Note [2](#note-2)*:
The type of reference need not be element_type&[.](#4.sentence-2)
— *end note*]
[🔗](#itemdecl:4)
`typename A::offset_policy
`
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24295)
*Result*: A type OP such that:
- [(5.1)](#5.1)
OP meets the accessor policy requirements,
- [(5.2)](#5.2)
[constructible_from](concept.constructible#concept:constructible_from "18.4.11 Concept constructible_­from [concept.constructible]")<OP, const A&> is modeled, and
- [(5.3)](#5.3)
is_same_v<typename OP​::​element_type, typename A​::​element_type> is true[.](#5.sentence-1)
[🔗](#itemdecl:5)
`a.access(p, i)
`
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24313)
*Result*: A​::​reference
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24317)
*Remarks*: The expression is equality preserving[.](#7.sentence-1)
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24321)
[*Note [3](#note-3)*:
Concrete accessor policies can impose preconditions for their access function[.](#8.sentence-1)
However, they might not[.](#8.sentence-2)
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[.](#8.sentence-3)
— *end note*]
[🔗](#itemdecl:6)
`a.offset(p, i)
`
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24337)
*Result*: A​::​offset_policy​::​data_handle_type
[10](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24341)
*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:
- [(10.1)](#10.1)
[0,n−i) is an accessible range of q and b; and
- [(10.2)](#10.2)
b.access(q, j) provides access to
the same element as a.access(p, i + j),
for every j in the range [0,n−i)[.](#10.sentence-1)
[11](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L24355)
*Remarks*: The expression is equality-preserving[.](#11.sentence-1)