Breaking changes due to C++ committee review

Hi all,

there are a number of breaking changes upcoming which stem from the intense review mdspan got now that its is nearing inclusion into the C++23 standard. These are described in P2553, P2554, P2604 and P2599.

All of these changes are getting merged into P0009 (the main mdspan paper) as part of the final wording review in LWG.

We created an mdspan-0.3.0 tag before any of this hits the repo.

Extents gets a controllable integral type

https://wg21.link/p2553

This allows (and makes it mandatory) for a user to choose what the underlying integral type is for extents,
a type which can be unsigned or signed.

Before:

template<size_t ... Extents>
class extents;

After

template<class IndexType, size_t ... Extents>
class extents;

Fix up deduction guide from 1D c-arrays.

https://wg21.link/p2554

This changes the behavior of the deduction guide then, constructing from a carray which is convertable to pointer.

Before:

int data[6];
mdspan a(data);
static_assert(a.rank()==0);

mdspan b(data,2,3);
static_assert(b.rank()==2);

After:

int data[6];
mdspan a(data);
static_assert(a.rank()==1);
static_assert(a.static_extents(1)==6);

mdspan b(data,2,3);
static_assert(b.rank()==2);

Renamed members

https://isocpp.org/files/papers/P2604R0.html and https://isocpp.org/files/papers/P2599R1.pdf

• extents
  • size_type -> index_type
• mappings:
  • size_type -> index_type
  • is_contiguous -> is_exhaustive
  • is_always_contiguous -> is_always_exhaustive
• accessors:
  • pointer -> data_handle_type
• mdspan
  • size_type -> index_type
  • pointer -> data_handle_type
  • data() -> data_handle
  • is_contiguous -> is_exhaustive
  • is_always_contiguous -> is_always_exhaustive

Read P2553, P2554, P2604, and P2599, then inspect the extents, mappings, accessors, and mdspan APIs described in the issue. Done means the implementation reflects the listed template, deduction-guide, and member renames and matches the reviewed C++23 wording.