23 Containers library [containers]

23.7 Views [views]

23.7.3 Multidimensional access [views.multidim]

23.7.3.5 Accessor policy [mdspan.accessor]

23.7.3.5.2 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.

typename A::element_type

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, 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.