Add element access via `at()` to `std::mdspan`

[stephanlachnit]

Closes #300

Possible implementation for ::at element access to mdspan with boundary checks. The boundary check is implemented with a for loop, as I did not see any other way to achieve this. The error message is inspired by the std::span::at error message from libstdc++ (https://github.com/gcc-mirror/gcc/commit/1fa85dcf656e2f2c7e483c9ed3c2680bf7db6858).

Note that this does not implement ::at element access to mdarray.

[stigrs]

Given that SizeTypes can be signed, it should also check for indices[r] < 0.

[stephanlachnit]

Given that SizeTypes can be signed, it should also check for indices[r] < 0.

Thanks for this comment! Is this true for std::span as well? At least in libstdc++, they do not check this:

https://github.com/gcc-mirror/gcc/blob/fd7dabc116b9abc40ee6aa25bcc5d240b8cc516a/libstdc%2B%2B-v3/include/std/span#L290-L300

[nmm0]

In span, I believe it's always just std::size_t, but mdspan has the size type as a template parameter for the extent and can be signed.

[stephanlachnit]

In span, I believe it's always just std::size_t, but mdspan has the size type as a template parameter for the extent and can be signed.

Ah right, thanks for pointing this out!

Edit: updated the pull request

[stephanlachnit]

if constexpr doesn't exist pre C++17, should I add a macro here? Or just go with if and assume the compiler optimizes for this anyway?

[stephanlachnit]

I don't quite get the build failure on MSVC, would need some help there...

[nmm0]

I don't quite get the build failure on MSVC, would need some help there...

What if you used the requires clause from operator()? I think that operator[] is not used in the MSVC build so we may not have updated it:

  MDSPAN_TEMPLATE_REQUIRES(
    class... SizeTypes,
    /* requires */ (
      extents_type::rank() == sizeof...(SizeTypes) &&
      (detail::are_valid_indices<index_type, SizeTypes...>())
    )
  )

[nmm0]

@stephanlachnit My apologies, we recently fixed a bunch of naming changes. Would you mind rebasing? If you don't have the time I can take this over just let me know. It would be very nice to have this merged in

[stephanlachnit]

@stephanlachnit My apologies, we recently fixed a bunch of naming changes. Would you mind rebasing? If you don't have the time I can take this over just let me know. It would be very nice to have this merged in

If you want to take over feel free to do so

[nmm0]

Closing as it is superseded by https://github.com/kokkos/mdspan/pull/400