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

 

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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 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.

See also

default_accessor (C++23)

a type for indexed access to elements of mdspan (class template)