1 Revision History

2 Revision 1:

• LWG wants the default constructor to behave the same regardless of whether all the extents are static: that is, it should fill in the strides in the same way as layout_right::mapping would.
• add precondtion to ensure representability in index type of the required span size.

2.1 Initial Version 2023-01 Mailing

3 Rationale

During work on the padded layouts project an oversight in the layout_stride definition was discovered for a corner case. Specifically, the default constructor of a layout_stride with fully static extents will produce an invalid mapping.

The default constructor is defaulted, which means that the array for the strides member will be all zeros. As such the mappings operator() will return always zero for a defaulted layout_stride::mapping. In the case of at least one dynamic extent this is not an issue, because the valid index space for calling operator() is empty. Thus the statement that layout_stride::mapping is always unique is not violated. However for fully static (non zero) extents, the valid index space is not empty, and thus no precondition is violated if a user calls operator() with a valid multidimensional index.

layout_stride::mapping<extents<int, 4>> map;
// map.is_unique() == true;
// map(0) == 0; 
// map(1) == 0;
// map(2) == 0;
// map(3) == 0; 

Note: this issue does NOT affect mappings with extents of rank zero. It also does not affect layout_left and layout_right mappings. Their default constructed mappings with static extents are fully useable.

layout_left::mapping<extents<int, 4>> map;
// map.is_unique() == true;
// map(0) == 0; 
// map(1) == 1;
// map(2) == 2;
// map(3) == 3; 

We believe that this issue is a defect in the C++23 draft and should be considered for defect resolution process.

4 Possible Fixes

4.1 Delete default constructor for fully static extents

One option is to simply make the default constructor unavailable for layout_stride mappings of fully static extents.

The wording change would simply add a requires clause to the class synopsis

In subsection 24.7.3.4.7.1 [mdspan.layout.stride.overview] replace:

    // [mdspan.layout.stride.cons], constructors
    constexpr mapping() noexcept = default;
    constexpr mapping(const mapping&) noexcept = default;

With:

    // [mdspan.layout.stride.cons], constructors
    constexpr mapping()
      requires(extents_type::rank_dynamic()>0 || extents_type::rank()==0) noexcept = default;
    constexpr mapping(const mapping&) noexcept = default;

Note that this approach mirrors the approach of mdspan itself, where the default constructor requires rank_dynamic()>0. For mdspan we can’t avoid that, since there is no way to generate a valid data_handle.

4.2 Construct a layout_right equivalent mapping for fully static extents

If one would want to preserve the default constructibility of every layout_stride mapping, one could create some valid strides in the case where all extents are static.

Specifically, one could constructs strides such that the resulting mapping is the standard C-Layout. Conveniently, we can obtain such strides from a default constructed layout_right mapping with the same extents.

It would be desirable to preserve the trivial constructibility of layout_stride mappings for all the cases where that is still possible.

In subsection 24.7.3.4.7.1 [mdspan.layout.stride.overview] replace:

    // [mdspan.layout.stride.cons], constructors
    constexpr mapping() noexcept = default;
    constexpr mapping(const mapping&) noexcept = default;

With:

    // [mdspan.layout.stride.cons], constructors
    constexpr mapping() 
      requires(extents_type::rank_dynamic()>0 || extents_type::rank()==0) noexcept = default;
    constexpr mapping() 
      requires(extents_type::rank_dynamic()==0 && extents_type::rank()>0) noexcept;
    constexpr mapping(const mapping&) noexcept = default;

Add at the beginning of subsection 24.7.3.4.7.3 [mdspan.layout.stride.cons] insert:

    constexpr mapping() 
      requires(extents_type::rank_dynamic()==0 && extents_type::rank()>0) noexcept;

Effects: Direct-non-list-initializes extents_ with extents_type() , and for all d in the range [0,rank_), direct-non-list-initializes strides_[d] with layout_right::mapping<extents_type>().stride(d).

4.3 Preference for Resolution

We believe that it is preferable to preserve default constructibility of layout_stride for all specializations of layout_stride to simplify a number of generic programming cases – specifically, if layout policies are used directly in higher-level data structures for which the user wants to enable default constructibility.

Based on feedback in LWG we leave out the defaulted constructor for cases with dynamic extents and just always default construct layout_stride::mapping as if it were a layout_right::mapping with the same default constructed extents.

4.4 Proposed Wording

In subsection 24.7.3.4.7.1 [mdspan.layout.stride.overview] replace:

    // [mdspan.layout.stride.cons], constructors
    constexpr mapping() noexcept = default;
    constexpr mapping(const mapping&) noexcept = default;

With:

    // [mdspan.layout.stride.cons], constructors
    constexpr mapping() noexcept; 
    constexpr mapping(const mapping&) noexcept = default;

Add at the beginning of subsection 24.7.3.4.7.3 [mdspan.layout.stride.cons] insert:

    constexpr mapping() noexcept;

Preconditions: layout_right::mapping<extents_type>().required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with extents_type() , and for all d in the range [0,rank_), direct-non-list-initializes strides_[d] with layout_right::mapping<extents_type>().stride(d).