Is there an equivalent of submdspan for mdarray?

Question

The repo of the exciting mdspan, a multi-dimensional analogue of std::span suggested for the C++ standard libraries, now also contains a reference implementation of the closely-related mdarray, which unlike mdspan owns its data.

But whereas the submdspan function can produce a subset of an mdspan, I can't find an analogue for mdarray. What I was expecting was a function that behaves exactly the same as submdspan and returns an mdspan, but which operates on an mdarray.

Is this planned but not implemented yet? If not, why not?

Edit:

I've temporarily solved it with a home-brew solution in the form of an overload of submdspan that takes an mdarray, then creates a temporary mdspan that maps to the entire mdarray, and calls submdspan on that.

It does the job for now! But I'm not confident this covers every conceivable mdarray, as there is almost no documentation at the moment. Would still love an answer to the original question.

template <class ElementType, class Extents, class LayoutPolicy, class... SliceSpecs>
auto submdspan(
    mdarray<ElementType, Extents, LayoutPolicy> &arr, 
    SliceSpecs... slices)
{
    return submdspan(
        mdspan<ElementType, Extents, LayoutPolicy>(arr.data(), arr.mapping()), 
        slices...);
}

@KamilCuk, because this seems like a mature proposal and likely to become part of the C++ standard AFAIK. Also, I don't know how to reach the author. — Simplex, Jul 26 '22 at 07:36

score 5 · Accepted Answer · answered Oct 10 '22 at 21:07

Just randomly ran across this: since I am the primary author/maintainer on all involved things (mdspan, mdarray, submdspan and the reference implementation) yeah we can add that. And the way I would do that is actually calling the "to_mdspan" function of the mdarray, and call submdspan on that:

auto sub = submdspan(mda.to_mdspan(), slice_specifiers...);

In order for the two proposals to be independent I will keep that apart for now (unless LEWG tells me otherwise) and write a follow up paper which simply adds an overload to submdspan which does the above to forward to the overload taking an mdspan.

But generally: you can just open an issue on the repo to ask about proposal issues.

Is there an equivalent of submdspan for mdarray?

1 Answers1