23 Containers library [containers]

23.7 Views [views]

23.7.3 Multidimensional access [views.multidim]

23.7.3.7 submdspan [mdspan.sub]

23.7.3.7.8 submdspan function template [mdspan.sub.sub]

template<class ElementType, class Extents, class LayoutPolicy,
         class AccessorPolicy, class... SliceSpecifiers>
  constexpr auto submdspan(
    const mdspan<ElementType, Extents, LayoutPolicy, AccessorPolicy>& src,
    SliceSpecifiers... slices) -> see below;

Let index_type be typename Extents::index_type.

Let slices be the pack introduced by the following declaration: auto [...slices] = submdspan_canonicalize_slices(src, raw_slices...);

Let sub_map_offset be the result of submdspan_mapping(src.mapping(), slices...).

[Note 1:

This invocation of submdspan_mapping selects a function call via overload resolution on a candidate set that includes the lookup set found by argument-dependent lookup ([basic.lookup.argdep]).

— end note]

Constraints:

(4.1)
sizeof...(slices) equals Extents::rank(), and
(4.2)
LayoutPolicy::mapping<Extents> models sliceable-mapping.

Mandates: For each rank index k of src:

(5.1)
SliceSpecifiers...[k] is a submdspan slice type for index_type, and
(5.2)
decltype(slices...[k]) is a valid submdspan slice type for the $k^{th}$ extent of Extents.

Preconditions: For each rank index k of src.extents(), slices...[k] is a valid submdspan slice for the

k^{th}

extent of src.extents().

Effects: Equivalent to: auto sub_map_result = submdspan_mapping(src.mapping(), slices...); return mdspan(src.accessor().offset(src.data_handle(), sub_map_result.offset), sub_map_result.mapping, typename AccessorPolicy::offset_policy(src.accessor()));

[Example 1:

Given a rank-3 mdspan grid3d representing a three-dimensional grid of regularly spaced points in a rectangular prism, the function zero_surface sets all elements on the surface of the 3-dimensional shape to zero.

It does so by reusing a function zero_2d that takes a rank-2 mdspan.

// zero out all elements in an mdspan template<class T, class E, class L, class A> void zero_2d(mdspan<T, E, L, A> a) { static_assert(a.rank() == 2); for (int i = 0; i < a.extent(0); i++) for (int j = 0; j < a.extent(1); j++) a[i, j] = 0; } // zero out just the surface template<class T, class E, class L, class A> void zero_surface(mdspan<T, E, L, A> grid3d) { static_assert(grid3d.rank() == 3); zero_2d(submdspan(grid3d, 0, full_extent, full_extent)); zero_2d(submdspan(grid3d, full_extent, 0, full_extent)); zero_2d(submdspan(grid3d, full_extent, full_extent, 0)); zero_2d(submdspan(grid3d, grid3d.extent(0) - 1, full_extent, full_extent)); zero_2d(submdspan(grid3d, full_extent, grid3d.extent(1) - 1, full_extent)); zero_2d(submdspan(grid3d, full_extent, full_extent, grid3d.extent(2) - 1)); } — end example]