P0009 LayoutStride Wording

[crtrott]

struct layout_stride {
  template <typename Extents>
  class mapping {
  public:
    constexpr mapping() noexcept;
    constexpr mapping(mapping const& other) noexcept;
    constexpr mapping(mapping&& other) noexcept;
    constexpr mapping(Extents e, array<ptrdiff_t, Extents::rank()> s) noexcept;
    template<class OtherExtents>
      constexpr mapping(const mapping<OtherExtents>& other);

    mapping& operator=() noexcept = default;
    mapping& operator=(mapping const& other) noexcept = default;
    template<class OtherExtents>
      constexpr mapping& operator=(const mapping<OtherExtents>& other);

    Extents get_extents() const noexcept;
    array<ptrdiff_t, Extents::rank()> get_strides() const noexcept;

    constexpr typename Extents::index_type required_span_size() const noexcept;

    template <class... Indices>
      typename Extents::index_type operator()(Indices... is) const;

    static constexpr bool is_always_unique() noexcept;
    static constexpr bool is_always_contiguous() noexcept;
    static constexpr bool is_always_strided() noexcept;

    constexpr bool is_unique() const noexcept;
    constexpr bool is_contiguous() const noexcept;
    constexpr bool is_strided() const noexcept;

    ptrdiff_t stride(size_t rank) const noexcept;

  private:
    Extents extents_; // exposition only
    //TODO: should this be extents<dynamic_extent,...,dynamic_extent> ?
    array<ptrdiff_t, Extents::rank()> strides_; // exposition only
  };
};

26.7.�.3.1 layout_right::mapping constructors [mdspan.layout.right.cons]

constexpr mapping() noexcept;

• Effects: Default-initializes extents_.
• Postconditions: get_extents() == Extents() and get_strides() == array<ptrdiff_t, Extents::rank()>()

constexpr mapping(mapping const& other) noexcept;

• Effects: Initializes extents with other.extents.
• Postconditions: get_extents() == other.get_extents() and get_strides() == other.get_strides()

constexpr mapping(mapping&& other) noexcept;

• Effects: Initializes extents with `move(other.extents)`.
• Postconditions: get_extents() returns a copy of an Extents that is equal to the copy returned by other.get_extents() before the invocation of the move and get_strides() returns a copy of an array<ptrdiff_t,Extents::rank()> that is equal to the copy returned by other.get_strides() before the invocation of the move

constexpr mapping(Extents e, array<ptrdiff_t, Extents::rank()> s) noexcept;

• Requires: there is a permutation of the numbers 0,...,Extents::rank()-1 o(i) with 0 <= i < Extents::rank() such that stride(o(i)) >= stride(o(i-1))*get_extent(o(i-1)) for 1 <= i < Extents::rank()
• Effects: Initializes extents with e and strides with s
• Postconditions: get_extents() == e and get_strides() == s.
• Throws: nothing

template<class OtherExtents>
  constexpr mapping(const mapping<OtherExtents>& other);

• Requires: other.getextents() meets the requirements for use in the initialization of extents.
• Effects: Initializes extents_ with other.getextents() and initializes strides with other.get_strides().
• Postconditions: get_extents() == other.get_extents() and get_strides() == other.get_strides().
• Remarks: This constructor does not participate in overload resolution unless OtherExtents is convertible to Extents.
• Throws: nothing.

26.7.�.3.2 layout_right::mapping operations [mdspan.layout.right.ops]

Extents get_extents() const noexcept;

• Returns: extents_.

Extents get_strides() const noexcept;

• Returns: strides_.

typename Extents::index_type required_span_size() const noexcept;

• Returns: The maximum of of get_extents().extent(r) * stride(r) for all r where 0 <= r < get_extents().rank()

template <class... Indices>
  typename Extents::index_type operator()(Indices... i) const noexcept;

• Returns: If i... is i0, i1, i2,..., ik (where k == Extents::rank() - 1) and s = get_strides(), returns i0*s[1] + i1*s[2] +...+ ik*s[k]
• Remarks: This operator shall not participate in overload resolution unless
  • sizeof...(Indices) == Extents::rank(),
  • and is_convertible_v<Indices, typename Extents::index_type> && ...

static constexpr bool is_always_unique() noexcept;
static constexpr bool is_always_strided() noexcept;
constexpr bool is_unique() const noexcept;
constexpr bool is_strided() const noexcept;

TODO: should this be always unique?

• Returns: true

static constexpr bool is_always_contiguous() noexcept;

• Returns: false

constexpr bool is_contiguous() const noexcept;

• Returns: true if there is a permutation of the numbers 0,...,Extents::rank()-1 o(i) with 0 <= i < Extents::rank() such that min(stride(o(i)) == 1 and stride(o(i)) == stride(o(i-1))*get_extent(o(i-1)) with 1 <= i < Extents::rank() otherwise returns false

index_type stride(int r) const noexcept;

• Returns: strides_(r)

constexpr bool operator==(const mapping& other) const noexcept;

• Returns: (get_extents() == other.get_extents()) && (get_strides() == other.get_strides())

constexpr bool operator!=(const mapping& other) const noexcept;

• Returns: (get_extents() != other.get_extents()) || (get_strides() != other.get_strides())

[dhollman]

• Postconditions: get_extents() == Extents() and get_strides() == array<ptrdiff_t, Extents::rank()>()

I'm pretty sure this would make get_strides() into uninitialized memory. I think what you'd actually want is get_strides() == array<ptrdiff_t, Extents::rank()>{}, though I'm not positive about this.

26.7.�.3.1 layout_right::mapping constructors

This should be layout_stride (also a few other places)

• Effects: Initializes extents with move(other.extents).

Two things:

• Also needs "Initializes strides_ with std::move(other.strides_)
• I think you're supposed to namespace-qualify move here so that it doesn't participate in ADL, though I'm not sure if the rest of the standard does this or not.

(Also need to make the analogous changes to the copy constructor.)

• constexpr mapping(Extents e, array<ptrdiff_t, Extents::rank()> s) noexcept;

Remove noexcept here; it has a requires clause so it can't be noexcept.

more to come...

[crtrott]

I think this is resolved