1 Revision History

1.1 Initial Version 2023-02 Mailing

2 Rationale

When default constructing a layout_left, layout_right or layout_stride mapping with all static extents, there is currently no precondition check guarding against overflow of the index_type.

A simple example is constructing the following layout_left mapping:

constexpr size_t N = 4'000'000;
layout_left::mapping<extents<int, N, N>> map;

Each individual extent fits into int but the theoretical required span size does not.

The non-default constructors already deal with this issue via a preconditions such as:

Preconditions: The size of the multidimensional index space e is representable as a value of type index_type ([basic.fundamental]).

For cases where at least one of the extents is dynamic, the default constructor is also fine, since it will result in an empty index space. That leaves only the case for all static extents to be fixed. We propose here to add a mandate to the mapping classes of layout_left, layout_right, and layout_stride that the forward product of extents is representable as index_type.

2.1 Proposed Wording

In subsection 24.7.3.4.5.1 [mdspan.layout.left.overview] add after paragraph 3:

Mandates: If Extents::rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents() is representable as a value of type typename Extents::index_type.

In subsection 24.7.3.4.6.1 [mdspan.layout.right.overview] add after paragraph 3:

Mandates: If Extents::rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents() is representable as a value of type typename Extents::index_type.

In subsection 24.7.3.4.7.1 [mdspan.layout.stride.overview] add after paragraph 3:

Mandates: If Extents::rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents() is representable as a value of type typename Extents::index_type.