1 Revision History
- 1.1 Initial Version 2022-02 Mailing
2 Description
3 Wording
- 3.1 In SubSection 22.7.X.1 [mdspan.mdspan.overview]
- 3.2 In SubSection 22.7.X.2 [mdspan.mdspan.cons]

1 Revision History

1.1 Initial Version 2022-02 Mailing

2 Description

2.1 The Issue

Currently one cannot construct an mdspan from a multidimensional c-array.

int a[3][2] = {1,2,3,4,5,6};
mdspan<int,dextents<2>> b(a,3,2); // fail
mdspan<int,extents<3,2>> c(a); // fail

It only works for 1D c-arrays, which decay to pointers. However the deduction guide when using just the 1D c-array with no length argument, would deduce a rank-0 mdspan, which may be surprising to users:

int a[6] = {1,2,3,4,5,6};
mdspan b(a);
static_assert(decltype(b)::rank()==0);

2.2 Proposal

We cannot currently fix the multidimensional c-array construction, since it is UB to alias a nested C-Array with a element type pointer - per discussion on the C++ committee reflector in January 2022. However, in practice it works (on the compilers we tested e.g. clang-based and gcc) - and it may be something the committee changes in the future - i.e. make it not-UB. We propotyped this capability, which requires an additional constructor from c-array and a few deduction guides.

What we can fix today is the deduction from 1D c-array, by adding a deduction guide from c-array constraint to rank-1 arrays.

This would enable the following:

{
  int a[6] = {1,2,3,4,5,6};
  mdspan b(a);
  static_assert(decltype(b)::rank()==1);
  static_assert(decltype(b)::static_extent(0)==6);
}

It is currently possible, and will continue to be allowed to do the following with 1D c-arrays (via the decay to pointer code path):

{
  int a[6] = {1,2,3,4,5,6};
  mdspan b(a,3);
  static_assert(decltype(b)::rank()==1);
  static_assert(decltype(b)::static_extent(0)==dynamic_extent;
  // b.extent(0)==3
}
{
  int a[6] = {1,2,3,4,5,6};
  mdspan<int,extents<3>> b(a);
  static_assert(decltype(b)::rank()==1);
  static_assert(decltype(b)::static_extent(0)==3;
}

Adding this deduction guide now, means that if we later fix the general issue of constructing an mdspan from a nested c-array (which would require a core language change), we do not break prior behavior.

2.3 Implementation Prototype

Based on the reference implementation of P0009 we implemented the changes in a godbolt example: godbolt.

3 Wording

3.1 In SubSection 22.7.X.1 [mdspan.mdspan.overview]

Add the following deduction guides after the class definition:

template<class CArray>
mdspan(CArray&)
  -> mdspan<see below>;

template<class Pointer>
mdspan(Pointer&)
  -> mdspan<see below>;

3.2 In SubSection 22.7.X.2 [mdspan.mdspan.cons]

Insert after paragraph 19:

template<class CArray>
mdspan(CArray&)
  -> mdspan<see below>;

Constraints:

is_array_v<CArray> is true.
rank_v<CArray>==1 is true.

Remarks:

The deduced type is: mdspan< std::remove_all_extents_t<CArray>, extents<extent_v<CArray,0>> >

template<class Pointer>
mdspan(Pointer&)
  -> mdspan<see below>;