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

 

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C++ named requirements

Basic DefaultConstructible MoveConstructible (C++11) CopyConstructible CopyAssignable MoveAssignable (C++11) Destructible Type properties ScalarType PODType TriviallyCopyable (C++11) TrivialType (C++11) StandardLayoutType (C++11)   ImplicitLifetimeType Library-wide BooleanTestable EqualityComparable LessThanComparable Swappable ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11) Allocator FunctionObject Callable Predicate BinaryPredicate Compare

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Random Numbers SeedSequence (C++11) RandomNumberEngine (C++11) RandomNumberDistribution (C++11) UniformRandomBitGenerator (C++11) RandomNumberEngineAdaptor (C++11) Concurrency BasicLockable (C++11) Lockable (C++11) TimedLockable (C++11) SharedLockable (C++14) SharedTimedLockable (C++14) Mutex (C++11) TimedMutex (C++11) SharedMutex (C++17) SharedTimedMutex (C++14) Ranges RangeAdaptorObject (C++20) RangeAdaptorClosureObject (C++20) Multidimensional View LayoutMapping (C++23) LayoutMappingPolicy (C++23) AccessorPolicy (C++23) Other CharTraits RegexTraits (C++11)   BitmaskType LiteralType (C++11) NumericType UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11) Clock (C++11) TrivialClock (C++11)

 

LayoutMapping controls the mapping of a multidimensional index to a one-dimensional offset to data handle in std::mdspan.

Requirements

A type M satisfies LayoutMapping if it models copyable and equality_comparable, and the following are true:

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

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

Legend

Type	Definition
M	a layout mapping class
Value	Definition
m	a value of type (possibly const-qualified) M
i, j	packs of (possibly const-qualified) integers that are multidimensional indices in m.extents()
r	a (possibly const-qualified) rank index of typename M​::​extents_type
d_r	a pack of (possibly const-qualified) integers for which sizeof...(d_r) == M​::​extents_type​::​rank() is true, the element at rank index r is equal to 1, and all other elements are equal to ​0​

Member types

Name	Type	Requirements
M::extents_type	Specialization of class template std::extents
M::index_type	typename M::extents_type::index_type
M::rank_type	typename M::extents_type::rank_type
M::layout_type	Layout mapping policy MP where typename MP::template mapping<E> is M for some extents type E	LayoutMappingPolicy for which M is mapping type of MP

Member functions and operators

Expression	Return type	Semantics
m.extents()	const typename M​::​extents_type&	Returns constant reference to associated multidimensional index space
m(i...)	typename M​::​index_type	Returns a nonnegative integer o such that o < std::numeric_limits<typename M​::​index_type>​::​max() and o <= std::numeric_limits<std::size_t>​::​max() are both true. Such expression is equivalent to m(static_cast<typename M::index_type>(i)...).
m.required_span_size()	typename M​::​index_type	Returns 1 plus the maximum value of m(i...) for all i if the size of the multidimensional index space m.extents() is not 0. Otherwise, returns ​0​.
m.is_unique()	bool	Returns true only if for every i and j where (i != j || ...) is true, m(i...) != m(j...) is true.
m.is_exhaustive()	bool	Returns true only if for all k in the range [​0​, m.required_span_size()), there exists an i such that m(i...) equals k.
m.is_strided()	bool	Returns true only if for every rank index r of m.extents(), there exists an integer s_r such that, for all i where (i + d_r) is a multidimensional index in m.extents(), m((i + d_r)...) - m(i...) equals s_r.
m.stride(r)	typename M​::​index_type	The precondition is that m.is_strided() is true. Returns a stride s_r at rank index r as defined in m.is_strided() above.
M::is_always_unique()	bool	Returns true only if m.is_unique() is true for all possible objects m of type M. The return value is always a constant expression.
M::is_always_exhaustive()	bool	Returns true only if m.is_exhaustive() is true for all possible objects m of type M. The return value is always a constant expression.
M::is_always_strided()	bool	Returns true only if m.is_strided() is true for all possible objects m of type M. The return value is always a constant expression.

Concept

For the constraints uses under std::layout_stride::mapping, the following exposition-only concept is defined.

template< class M > concept /*layout-mapping-alike*/ = requires {     requires /*is-extents*/<typename M::extents_type>;     { M::is_always_strided() }    -> std::same_as<bool>;     { M::is_always_exhaustive() } -> std::same_as<bool>;     { M::is_always_unique() }     -> std::same_as<bool>;     std::bool_constant<M::is_always_strided()>::value;     std::bool_constant<M::is_always_exhaustive()>::value;     std::bool_constant<M::is_always_unique()>::value; };		(exposition only*)

Defines the minimal usability constraints of the LayoutMapping requirement. This concept checks that the predicate mapping trait functions above exist, are constant expressions, and have a return type of bool.

/*is-extents*/<E> is true if and only if E is a specialization of std::extents.

Standard library

The following standard library types satisfy LayoutMapping requirements:

mapping	a layout mapping of layout_left (public member class template of std::layout_left)
mapping	a layout mapping of layout_right (public member class template of std::layout_right)
mapping	a layout mapping of layout_stride (public member class template of std::layout_stride)
mapping	a layout mapping of layout_left_padded (public member class template of std::layout_left_padded<PaddingValue>)
mapping	a layout mapping of layout_right_padded (public member class template of std::layout_right_padded<PaddingValue>)

See also

• LayoutMappingPolicy