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

      From cppreference.com
      <cpp‎ |named req
       
       
      C++ named requirements
       

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

      Contents

      [edit]Requirements

      A typeM satisfiesLayoutMapping if it modelscopyable andequality_comparable, and the following aretrue:

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

      [edit]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 inm.extents()
      r a (possibly const-qualified) rank index oftypename M​::​extents_type
      d_r a pack of (possibly const-qualified) integers for whichsizeof...(d_r)== M​::​extents_type​::​rank() istrue, the element at rank indexr is equal to1, and all other elements are equal to​0​

      [edit]Member types

      NameTypeRequirements
      M::extents_typeSpecialization of class templatestd::extents
      M::index_typetypename M::extents_type::index_type
      M::rank_typetypename M::extents_type::rank_type
      M::layout_typeLayout mapping policyMP wheretypename MP::template mapping<E> isM
      for some extents typeE      		LayoutMappingPolicy for whichM is mapping type ofMP

      [edit]Member functions and operators

      ExpressionReturn typeSemantics
      m.extents()consttypename M​::​extents_type&Returns constant reference to associated multidimensional index space
      m(i...)typename M​::​index_type
      • Returns a nonnegative integero such thato<std::numeric_limits<typename M​::​index_type>​::​max() ando<=std::numeric_limits<std::size_t>​::​max() are bothtrue.
      • Such expression is equivalent tom(static_cast<typename M::index_type>(i)...).
      m.required_span_size()typename M​::​index_type
      • Returns1 plus the maximum value ofm(i...) for alli if the size of the multidimensional index spacem.extents() is not 0.
      • Otherwise, returns​0​.
      m.is_unique()boolReturnstrue only if for everyi andj where(i!= j|| ...) istrue,m(i...)!= m(j...) istrue.[note 1]
      m.is_exhaustive()boolReturnstrue only if for allk in the range[​0​, m.required_span_size()), there exists ani such thatm(i...) equalsk.[note 2]
      m.is_strided()boolReturnstrue only if for every rank indexr ofm.extents(), there exists an integers_r such that, for alli where(i+ d_r) is a multidimensional index inm.extents(),m((i+ d_r)...)- m(i...) equalss_r.[note 3]
      m.stride(r)typename M​::​index_type
      • The precondition is thatm.is_strided() istrue.
      • Returns a strides_r at rank indexr as defined inm.is_strided() above.
      M::is_always_unique()bool
      • Returnstrue only ifm.is_unique() istrue for all possible objectsm of typeM.[note 4]
      • The return value is always a constant expression.
      M::is_always_exhaustive()bool
      • Returnstrue only ifm.is_exhaustive() istrue for all possible objectsm of typeM.[note 5]
      • The return value is always a constant expression.
      M::is_always_strided()bool
      • Returnstrue only ifm.is_strided() istrue for all possible objectsm of typeM.[note 6]
      • The return value is always a constant expression.
      1. ↑A mapping can returnfalse even if the condition is met. For certain layouts, it is possibly not feasible to determine efficiently whether the layout is unique.
      2. ↑Same as above, but in the case of exhaustive layouts.
      3. ↑Same as above, but in the case of strided layouts.
      4. ↑A mapping can returnfalse even if the condition is met. For certain layout mappings, it is possibly not feasible to determine whether every instance is unique.
      5. ↑Same as above, but in the case of exhaustive instances.
      6. ↑Same as above, but in the case of strided instances.

      [edit]Concept

      For the constraints uses understd::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 theLayoutMapping requirement. This concept checks that the predicate mapping trait functions above exist, are constant expressions, and have a return type ofbool.

      /*is-extents*/<E> istrue if and only ifE is a specialization ofstd::extents.

      [edit]Standard library

      The following standard library types satisfyLayoutMapping requirements:

      a layout mapping oflayout_left
      (public member class template ofstd::layout_left)[edit]
      a layout mapping oflayout_right
      (public member class template ofstd::layout_right)[edit]
      a layout mapping oflayout_stride
      (public member class template ofstd::layout_stride)[edit]
      a layout mapping oflayout_left_padded
      (public member class template ofstd::layout_left_padded<PaddingValue>)[edit]
      a layout mapping oflayout_right_padded
      (public member class template ofstd::layout_right_padded<PaddingValue>)[edit]

      [edit]See also

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