C++ named requirements: AccessorPolicy (since C++23)

AccessorPolicy defines how elements are accessed from a given data handle to such elements and an index. It is used as a policy in std::mdspan

Requirements

A type A satisfies AccessorPolicy if it models copyable, and the following are true

• std::is_nothrow_move_constructible_v<A>
• std::is_nothrow_move_assignable_v<A>
• std::is_nothrow_swappable_v<A>

And, given the following types and values, the expressions shown in the table below are valid and have the indicated semantics:

Legend

Type	Definition
A	an accessor policy type
OP	an offset policy type
Value	Definition
a	a value of type (possibly const-qualified) A
p	a value of type (possibly const-qualified) A::data_handle_type
i, j, n	values of type std::size_t

Member types

Name	Type	Requirements
A::element_type	(unspecified)	A complete object type that is not an abstract class type
A::data_handle_type	(unspecified) but the type need not be element_type*	Models copyable The following conditions are all true: std::is_nothrow_move_constructible_v < typename A:: data_handle_type > std::is_nothrow_move_assignable_v < typename A:: data_handle_type > std::is_nothrow_swappable_v < typename A:: data_handle_type >
A::reference	(unspecified) but the type need not be element_type&	std::common_reference_with < typename A:: reference &&, typename A:: element_type & > is true
A::offset_policy	OP	Satisfies the requirements of AccessorPolicy std::constructible_from <OP, const A& > is true typename OP​::​element_type and typename A​::​element_type

Member functions

Expression	Return type	Semantics
a.access(p, i)	typename A::reference	The expression is equality-preserving.
a.offset(p, i)	typename A::offset_policy::data_handle_type	A value q such that for b being A::offset_policy(a), and any integer n for which [ ​0​ ,  n ) is an accessible range[note 1] of p and a [ ​0​ ,  n - i ) is an accessible range of q and b 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 ) The expression is equality-preserving.

1. ↑ A range of indices [ ​0​ ,  N ) is an accessible range of a given data handle and an accessor if, for each index in the range, the access

Standard library

The following standard library types satisfy AccessorPolicy requirements:

default_accessor (C++23)	a type for indexed access to elements of mdspan (class template)
aligned_accessor (C++26)	a type for aligned access to elements of mdspan (class template)
Defined in namespace std::linalg
scaled_accessor (C++26)	std::mdspan accessor policy whose reference represents the product of a scaling factor that is fixed and its nested std::mdspan accessor's reference (class template)
conjugated_accessor (C++26)	std::mdspan accessor policy whose reference represents the complex conjugate of its nested std::mdspan accessor's reference (class template)