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Standard library header <mdspan> (C++23)

This header is part of the containers library.

Classes
(C++23)
 a multi-dimensional non-owning array view
(class template)
(C++23)
 a descriptor of a multidimensional index space of some rank
(class template)
(C++23)
 column-major multidimensional array layout mapping; leftmost extent has stride 1
(class)
(C++23)
 row-major multidimensional array layout mapping; rightmost extent has stride 1
(class)
(C++23)
 a layout mapping with user-defined strides
(class)
(C++23)
 a type for indexed access to elements of mdspan
(class template)

Synopsis

namespace std {
  // class template extents
  template<class IndexType, size_t... Extents>
    class extents;
 
  // alias template dextents
  template<class IndexType, size_t Rank>
    using dextents = /* see description */;
 
  // layout mapping
  struct layout_left;
  struct layout_right;
  struct layout_stride;
 
  // class template default_accessor
  template<class ElementType>
    class default_accessor;
 
  // class template mdspan
  template<class ElementType, class Extents, class LayoutPolicy = layout_right,
           class AccessorPolicy = default_accessor<ElementType>>
    class mdspan;
}

Class template std::mdspan 

namespace std {
  template<class ElementType, class Extents, class LayoutPolicy, class AccessorPolicy>
  class mdspan {
  public:
    using extents_type = Extents;
    using layout_type = LayoutPolicy;
    using accessor_type = AccessorPolicy;
    using mapping_type = typename layout_type::template mapping<extents_type>;
    using element_type = ElementType;
    using value_type = remove_cv_t<element_type>;
    using index_type = typename extents_type::index_type;
    using size_type = typename extents_type::size_type;
    using rank_type = typename extents_type::rank_type;
    using data_handle_type = typename accessor_type::data_handle_type;
    using reference = typename accessor_type::reference;
 
    static constexpr rank_type rank() noexcept { return extents_type::rank(); }
    static constexpr rank_type rank_dynamic() noexcept {
        return extents_type::rank_dynamic(); }
    static constexpr size_t static_extent(rank_type r) noexcept
      { return extents_type::static_extent(r); }
    constexpr index_type extent(rank_type r) const noexcept { 
        return extents().extent(r); }
 
    // constructors
    constexpr mdspan();
    constexpr mdspan(const mdspan& rhs) = default;
    constexpr mdspan(mdspan&& rhs) = default;
 
    template<class... OtherIndexTypes>
      constexpr explicit mdspan(data_handle_type ptr, OtherIndexTypes... exts);
    template<class OtherIndexType, size_t N>
      constexpr explicit(N != rank_dynamic())
        mdspan(data_handle_type p, span<OtherIndexType, N> exts);
    template<class OtherIndexType, size_t N>
      constexpr explicit(N != rank_dynamic())
        mdspan(data_handle_type p, const array<OtherIndexType, N>& exts);
    constexpr mdspan(data_handle_type p, const extents_type& ext);
    constexpr mdspan(data_handle_type p, const mapping_type& m);
    constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a);
 
    template<class OtherElementType, class OtherExtents,
             class OtherLayoutPolicy, class OtherAccessorPolicy>
      constexpr explicit(/* see description */)
        mdspan(const mdspan<OtherElementType, OtherExtents,
                            OtherLayoutPolicy, OtherAccessorPolicy>& other);
 
    constexpr mdspan& operator=(const mdspan& rhs) = default;
    constexpr mdspan& operator=(mdspan&& rhs) = default;
 
    // members
    template<class... OtherIndexTypes>
      constexpr reference operator[](OtherIndexTypes... indices) const;
    template<class OtherIndexType>
      constexpr reference operator[](span<OtherIndexType, rank()> indices) const;
    template<class OtherIndexType>
      constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;
 
    constexpr size_type size() const noexcept;
    [[nodiscard]] constexpr bool empty() const noexcept;
 
    friend constexpr void swap(mdspan& x, mdspan& y) noexcept;
 
    constexpr const extents_type& extents() const noexcept { return map_.extents(); }
    constexpr const data_handle_type& data_handle() const noexcept { return ptr_; }
    constexpr const mapping_type& mapping() const noexcept { return map_; }
    constexpr const accessor_type& accessor() const noexcept { return acc_; }
 
    static constexpr bool is_always_unique()
      { return mapping_type::is_always_unique(); }
    static constexpr bool is_always_exhaustive()
      { return mapping_type::is_always_exhaustive(); }
    static constexpr bool is_always_strided()
      { return mapping_type::is_always_strided(); }
 
    constexpr bool is_unique() const
      { return map_.is_unique(); }
    constexpr bool is_exhaustive() const
      { return map_.is_exhaustive(); }
    constexpr bool is_strided() const
      { return map_.is_strided(); }
    constexpr index_type stride(rank_type r) const
      { return map_.stride(r); }
 
  private:
    accessor_type acc_;         // exposition only
    mapping_type map_;          // exposition only
    data_handle_type ptr_;      // exposition only
  };
 
  template<class CArray>
    requires(is_array_v<CArray> && rank_v<CArray> == 1)
    mdspan(CArray&)
      -> mdspan<remove_all_extents_t<CArray>, extents<size_t, extent_v<CArray, 0>>>;
 
  template<class Pointer>
    requires(is_pointer_v<remove_reference_t<Pointer>>)
    mdspan(Pointer&&)
      -> mdspan<remove_pointer_t<remove_reference_t<Pointer>>, extents<size_t>>;
 
  template<class ElementType, class... Integrals>
    requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
    explicit mdspan(ElementType*, Integrals...)
      -> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>;
 
  template<class ElementType, class OtherIndexType, size_t N>
    mdspan(ElementType*, span<OtherIndexType, N>)
      -> mdspan<ElementType, dextents<size_t, N>>;
 
  template<class ElementType, class OtherIndexType, size_t N>
    mdspan(ElementType*, const array<OtherIndexType, N>&)
      -> mdspan<ElementType, dextents<size_t, N>>;
 
  template<class ElementType, class IndexType, size_t... ExtentsPack>
    mdspan(ElementType*, const extents<IndexType, ExtentsPack...>&)
      -> mdspan<ElementType, extents<IndexType, ExtentsPack...>>;
 
  template<class ElementType, class MappingType>
    mdspan(ElementType*, const MappingType&)
      -> mdspan<ElementType, typename MappingType::extents_type,
                typename MappingType::layout_type>;
 
  template<class MappingType, class AccessorType>
    mdspan(const typename AccessorType::data_handle_type&, const MappingType&,
           const AccessorType&)
      -> mdspan<typename AccessorType::element_type, typename MappingType::extents_type,
                typename MappingType::layout_type, AccessorType>;
}

Class template std::extents 

namespace std {
  template<class IndexType, size_t... Extents>
  class extents {
  public:
    using index_type = IndexType;
    using size_type = make_unsigned_t<index_type>;
    using rank_type = size_t;
 
    // observers of the multidimensional index space
    static constexpr rank_type rank() noexcept { return sizeof...(Extents); }
    static constexpr rank_type rank_dynamic() noexcept {
        return /*dynamic-index*/(rank()); }
    static constexpr size_t static_extent(rank_type) noexcept;
    constexpr index_type extent(rank_type) const noexcept;
 
    // constructors
    constexpr extents() noexcept = default;
 
    template<class OtherIndexType, size_t... OtherExtents>
      constexpr explicit(/* see description */)
        extents(const extents<OtherIndexType, OtherExtents...>&) noexcept;
    template<class... OtherIndexTypes>
      constexpr explicit extents(OtherIndexTypes...) noexcept;
    template<class OtherIndexType, size_t N>
      constexpr explicit(N != rank_dynamic())
        extents(span<OtherIndexType, N>) noexcept;
    template<class OtherIndexType, size_t N>
      constexpr explicit(N != rank_dynamic())
        extents(const array<OtherIndexType, N>&) noexcept;
 
    // comparison operators
    template<class OtherIndexType, size_t... OtherExtents>
      friend constexpr bool operator==(const extents&,
                                       const extents<OtherIndexType,
                                       OtherExtents...>&) noexcept;
 
    // exposition-only helpers
    constexpr size_t /*fwd-prod-of-extents*/(rank_type) const noexcept; // exposition only
    constexpr size_t /*rev-prod-of-extents*/(rank_type) const noexcept; // exposition only
    template<class OtherIndexType>
      static constexpr auto /*index-cast*/(OtherIndexType&&) noexcept;  // exposition only
 
  private:
    static constexpr rank_type /*dynamic-index*/(rank_type) noexcept;   // exposition only
    static constexpr rank_type
        /*dynamic-index-inv*/(rank_type) noexcept;                      // exposition only
    array<index_type, rank_dynamic()>/*dynamic-extents*/{};             // exposition only
  };
 
  template<class... Integrals>
    explicit extents(Integrals...)
      -> /* see description */;
}

Layout mapping policies

namespace std {
  struct layout_left {
    template<class Extents>
      class mapping;
  };
  struct layout_right {
    template<class Extents>
      class mapping;
  };
  struct layout_stride {
    template<class Extents>
      class mapping;
  };
}

Class template std::layout_left::mapping 

namespace std {
  template<class Extents>
  class layout_left::mapping {
  public:
    using extents_type = Extents;
    using index_type = typename extents_type::index_type;
    using size_type = typename extents_type::size_type;
    using rank_type = typename extents_type::rank_type;
    using layout_type = layout_left;
 
    // constructors
    constexpr mapping() noexcept = default;
    constexpr mapping(const mapping&) noexcept = default;
    constexpr mapping(const extents_type&) noexcept;
    template<class OtherExtents>
      constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
        mapping(const mapping<OtherExtents>&) noexcept;
    template<class OtherExtents>
      constexpr explicit(/* see description */)
        mapping(const layout_right::mapping<OtherExtents>&) noexcept;
    template<class OtherExtents>
      explicit(extents_type::rank() > 0)
      constexpr mapping(const layout_stride::mapping<OtherExtents>&);
 
    constexpr mapping& operator=(const mapping&) noexcept = default;
 
    // observers
    constexpr const extents_type& extents() const noexcept { return extents_; }
 
    constexpr index_type required_span_size() const noexcept;
 
    template<class... Indices>
      constexpr index_type operator()(Indices...) const noexcept;
 
    static constexpr bool is_always_unique() noexcept { return true; }
    static constexpr bool is_always_exhaustive() noexcept { return true; }
    static constexpr bool is_always_strided() noexcept { return true; }
 
    static constexpr bool is_unique() noexcept { return true; }
    static constexpr bool is_exhaustive() noexcept { return true; }
    static constexpr bool is_strided() noexcept { return true; }
 
    constexpr index_type stride(rank_type) const noexcept;
 
    template<class OtherExtents>
      friend constexpr bool operator==(const mapping&,
                                       const mapping<OtherExtents>&) noexcept;
 
  private:
    extents_type extents_{};    // exposition only
  };
}

Class template std::layout_right::mapping 

namespace std {
  template<class Extents>
  class layout_right::mapping {
  public:
    using extents_type = Extents;
    using index_type = typename extents_type::index_type;
    using size_type = typename extents_type::size_type;
    using rank_type = typename extents_type::rank_type;
    using layout_type = layout_right;
 
    // constructors
    constexpr mapping() noexcept = default;
    constexpr mapping(const mapping&) noexcept = default;
    constexpr mapping(const extents_type&) noexcept;
    template<class OtherExtents>
      constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
        mapping(const mapping<OtherExtents>&) noexcept;
    template<class OtherExtents>
      constexpr explicit(/* see description */)
        mapping(const layout_left::mapping<OtherExtents>&) noexcept;
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0)
        mapping(const layout_stride::mapping<OtherExtents>&) noexcept;
 
    constexpr mapping& operator=(const mapping&) noexcept = default;
 
    // observers
    constexpr const extents_type& extents() const noexcept { return extents_; }
 
    constexpr index_type required_span_size() const noexcept;
 
    template<class... Indices>
      constexpr index_type operator()(Indices...) const noexcept;
 
    static constexpr bool is_always_unique() noexcept { return true; }
    static constexpr bool is_always_exhaustive() noexcept { return true; }
    static constexpr bool is_always_strided() noexcept { return true; }
 
    static constexpr bool is_unique() noexcept { return true; }
    static constexpr bool is_exhaustive() noexcept { return true; }
    static constexpr bool is_strided() noexcept { return true; }
 
    constexpr index_type stride(rank_type) const noexcept;
 
    template<class OtherExtents>
      friend constexpr bool operator==(const mapping&, 
                                       const mapping<OtherExtents>&) noexcept;
 
  private:
    extents_type extents_{};    // exposition only
  };
}

Class template std::layout_stride::mapping 

namespace std {
  template<class Extents>
  class layout_stride::mapping {
  public:
    using extents_type = Extents;
    using index_type = typename extents_type::index_type;
    using size_type = typename extents_type::size_type;
    using rank_type = typename extents_type::rank_type;
    using layout_type = layout_stride;
 
  private:
    static constexpr rank_type rank_ = extents_type::rank();    // exposition only
 
  public:
    // constructors
    constexpr mapping() noexcept = default;
    constexpr mapping(const mapping&) noexcept = default;
    template<class OtherIndexType>
      constexpr mapping(const extents_type&, span<OtherIndexType, rank_>) noexcept;
    template<class OtherIndexType>
      constexpr mapping(const extents_type&, const array<OtherIndexType, 
                        rank_>&) noexcept;
 
    template<class StridedLayoutMapping>
      constexpr explicit(/* see description */) mapping(
          const StridedLayoutMapping&) noexcept;
 
    constexpr mapping& operator=(const mapping&) noexcept = default;
 
    // observers
    constexpr const extents_type& extents() const noexcept { return extents_; }
    constexpr array<index_type, rank_> strides() const noexcept { return strides_; }
 
    constexpr index_type required_span_size() const noexcept;
 
    template<class... Indices>
      constexpr index_type operator()(Indices...) const noexcept;
 
    static constexpr bool is_always_unique() noexcept { return true; }
    static constexpr bool is_always_exhaustive() noexcept { return false; }
    static constexpr bool is_always_strided() noexcept { return true; }
 
    static constexpr bool is_unique() noexcept { return true; }
    constexpr bool is_exhaustive() const noexcept;
    static constexpr bool is_strided() noexcept { return true; }
 
    constexpr index_type stride(rank_type i) const noexcept { return strides_[i]; }
 
    template<class OtherMapping>
      friend constexpr bool operator==(const mapping&, const OtherMapping&) noexcept;
 
  private:
    extents_type extents_{};                     // exposition only
    array<index_type, rank_> strides_{};         // exposition only
  };
}

Exposition-only helpers

template<class T>
  constexpr bool /*is-extents*/ = false;                              // exposition only
template<class SizeType, size_t... Args>
  constexpr bool /*is-extents*/<extents<IndexType, Args...>> = true;  // exposition only
 
template<class M>
concept /*layout-mapping-alike*/ = requires {                         // exposition only
  requires /*is-extents*/<typename M::extents_type>;
  { M::is_always_strided() } -> same_as<bool>;
  { M::is_always_exhaustive() } -> same_as<bool>;
  { M::is_always_unique() } -> same_as<bool>;
  bool_constant<M::is_always_strided()>::value;
  bool_constant<M::is_always_exhaustive()>::value;
  bool_constant<M::is_always_unique()>::value;
};

Class template std::default_accessor 

namespace std {
  template<class ElementType>
  struct default_accessor {
    using offset_policy = default_accessor;
    using element_type = ElementType;
    using reference = ElementType&;
    using data_handle_type = ElementType*;
 
    constexpr default_accessor() noexcept = default;
    template<class OtherElementType>
      constexpr default_accessor(default_accessor<OtherElementType>) noexcept;
    constexpr reference access(data_handle_type p, size_t i) const noexcept;
    constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;
  };
}

References

  • C++23 standard (ISO/IEC 14882:2023):
    • 24.7.4 Header <mdspan> synopsis [mdspan.syn]

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