P0009 subspan wording

[crtrott]

Not quite done yet:

  // [mdspan.subspan], subspan creation
  template<class ElementType, class Extents, class LayoutPolicy,
           class Accessor, class... SliceSpecifiers>
    basic_mdspan<ElementType, E /* see-below */, layout_stride, Accessor>
      subspan(const basic_mdspan<ElementType, Extents, LayoutPolicy, Accessor>& src, SliceSpecifiers ... slices) noexcept;

• subspan creates a basic_mdspan that is a view on a (potentially trivial) subset of another basic_mdspan.
• The SliceSpecifier parameters indicate the subset that the return value references.
• Let slices[r] denote the rth value in the parameter pack slices.
• The second template parameter of the return type is E, a specialization of extents.
• Let C be the number of parameters in SliceSpecifiers which are convertible to ptrdiff_t
• Let ranges[r] denote the rth value in the parameter pack slices, which is not convertible to ptrdiff_t
• Let sub be the return value of subspan
• Let first[r] denote the rth lower bound of slices[r].
  • If slices[r] is convertible to ptrdiff_t first[r]==slices[r]
  • If slices[r] is convertible to pair<ptrdiff_t,ptrdiff_t> first[r]==pair<ptrdiff_t,ptrdiff_t>(slices[r]).first()
  • If slices[r] is convertible to alltype_t first[r]==0
• Let last[r] denote the rth upper bound of slices[r].
  • If slices[r] is convertible to ptrdiff_t last[r]==slices[r]+1
  • If slices[r] is convertible to pair<ptrdiff_t,ptrdiff_t> last[r]==pair<ptrdiff_t,ptrdiff_t>(slices[r]).second()
  • If slices[r] is convertible to alltype_t last[r]==src.extent(r)
• Requires:
  • sizeof(slices...)==Extents::rank()
  • slices[r] is convertible to ptrdiff_t, pair<ptrdiff_t,ptrdiff_t> or alltype_t
  • 0<=first[r]<src.extent(r)
  • 0<last[r]<=src.extent(r)
  • LayoutPolicy is layout_right, layout_left, or layout_stride
• Postcondition:
  • E::rank() == Extents::rank()-C
  • If ranges[r] is the nth value of ranges convertible to alltype_t or pair<ptrdiff_t,ptrdiff_t> and slices[k] is the nth value of slices convertible to all type_t or pair<ptrdiff_t,ptrdiff_t> then sub.extent(r)==last[k]-first[k] and sub.stride(r)==src.stride(k)
  • sub.ptr_==src.ptr_ + first[0]*src.stride(0) + ... + first[Extents::rank()-1]*src.stride(Extents::rank()-1)
• Note: it is quality of implementation whether static extents are preserved if possible.
• Examples:

  typedef extents<100,dynamic_extent,3> Extents3D;
  layout_right::mapping<Extents3D> map_right(
  basic_mdspan<int,Extents3D,layout_right> a()

[mhoemmen]

@crtrott It looks like this isn't in the paper yet -- should I add this, or wait for a PR?

[crtrott]

No this isn't done yet. We need to finish this definition first. Its kind of complicated ;-)

[crtrott]

Check whether this is postcondition or return clause.

[crtrott]

Closing this for now. We got a working definition committed.