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 to specify the access to a reference of an element using an underlying 1D index.

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 are the same 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 ; and 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 member function produces a valid reference to an element.

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)