23 Containers library [containers]

23.7 Views [views]

23.7.3 Multidimensional access [views.multidim]

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

[0, N)

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.

In [mdspan.accessor.reqmts],

(3.1)
A denotes an accessor policy.
(3.2)
a denotes a value of type A or const A.
(3.3)
p denotes a value of type A::data_handle_type or const A::data_handle_type.

[Note 1:
The type A::data_handle_type need not be dereferenceable.
— end note]
(3.4)
n, i, and j each denote values of type size_t.

23.7.3.5.2 Requirements [mdspan.accessor.reqmts]

A type A meets the accessor policy requirements if

(1.1)
A models copyable,
(1.2)
is_nothrow_move_constructible_v<A> is true,
(1.3)
is_nothrow_move_assignable_v<A> is true,
(1.4)
is_nothrow_swappable_v<A> is true, and
(1.5)
the following types and expressions are well-formed and have the specified semantics.

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typename A::element_type

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

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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, and is_nothrow_swappable_v<A::data_handle_type> is true.

[Note 1:

The type of data_handle_type need not be element_type*.

— end note]

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typename A::reference

Result: A type that models common_reference_with<A::reference&&, A::element_type&>.

[Note 2:

The type of reference need not be element_type&.

— end note]

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typename A::offset_policy

Result: A type OP such that:

(5.1)
OP meets the accessor policy requirements,
(5.2)
constructible_from<OP, const A&> is modeled, and
(5.3)
is_same_v<typename OP::element_type, typename A::element_type> is true.

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a.access(p, i)

Result: A::reference

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 where p is span<A::element_type, dynamic_extent> and access(p, i) returns p[i % p.size()] does not need to impose a precondition on i.

— end note]

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a.offset(p, i)

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:

(10.1)
$[0, n - i)$ is an accessible range of q and b; and
(10.2)
b.access(q, j) provides access to the same element as a.access(p, i + j), for every j in the range $[0, n - i)$ .

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 .

[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.

23.7.3.5.3.2 Members [mdspan.accessor.default.members]

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

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

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

Effects: Equivalent to: return p[i];

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

Effects: Equivalent to: return p + i;