submdspan.hh File Reference

Slicing utilities for Dune::Std::mdspan. More...

#include <array>
#include <tuple>
#include <type_traits>
#include <dune/common/indices.hh>
#include <dune/common/std/extents.hh>
#include <dune/common/std/impl/submdspan.hh>
#include <dune/common/std/mdspan.hh>
#include <dune/common/std/submdspan_slices.hh>

Go to the source code of this file.

Namespaces
namespace	Dune
	Dune namespace
namespace	Dune::Std
	Namespace for features backported from new C++ standards.

Functions
template<class IndexType , std::size_t... exts, class... SliceSpecifiers, std::enable_if_t<(sizeof...(SliceSpecifiers)==sizeof...(exts)), int > = 0>
constexpr auto	Dune::Std::subextents (const extents< IndexType, exts... > &src, SliceSpecifiers... slices)
	Return the extents produced by slicing a source extents object.
template<class Mapping , class... SliceSpecifiers, std::enable_if_t<(sizeof...(SliceSpecifiers)==Mapping::extents_type::rank()), int > = 0>
constexpr auto	Dune::Std::submdspan_mapping (const Mapping &mapping, SliceSpecifiers... slices)
	Create the mapping and source-data offset produced by slicing a layout mapping.
template<class ElementType , class Extents , class LayoutPolicy , class AccessorPolicy , class... SliceSpecifiers, std::enable_if_t<(sizeof...(SliceSpecifiers)==Extents::rank()), int > = 0>
constexpr auto	Dune::Std::submdspan (const mdspan< ElementType, Extents, LayoutPolicy, AccessorPolicy > &src, SliceSpecifiers... slices)
	Create an mdspan view by slicing an existing mdspan.

Detailed Description

Slicing utilities for Dune::Std::mdspan.

submdspan creates a non-owning view into an existing Dune::Std::mdspan. It follows the interface of std::submdspan from P2630R4 and the C++ standard working draft [mdspan.sub].

A slice specifier is provided for every source dimension. Each slice either fixes a dimension and removes it from the result rank, or keeps a dimension with a full, contiguous, or strided index set.

Supported slice specifiers:

• An integral value or std::integral_constant fixes one source index and removes that dimension.
• Dune::Std::full_extent keeps the full source dimension.
• Dune::Std::range_slice{first,last} keeps the half-open range [first,last).
• Dune::Std::range_slice{first,last,stride} keeps the strided range first, first+stride, ....
• Dune::Std::extent_slice{offset,extent,stride} keeps extent entries starting at offset with the given stride.
• Pair-like objects such as std::pair{first,last} are accepted as half-open ranges.

The returned mdspan aliases the source data. The implementation preserves layout_right for fixed leading indices followed by full dimensions, e.g. row slices of a row-major tensor, and preserves layout_left for the symmetric fixed trailing-index case. Following P2642R0, contiguous submatrix-like slices of layout_left, layout_right, and the corresponding padded layouts return layout_left_padded or layout_right_padded when the resulting strides can be represented by those layouts. Other regular slices use layout_stride.

Examples:

std::array<double, 12> data{};
Dune::Std::mdspan a(data.data(), 3, 4);
 
// Fix the first index: a rank-1 row view.
auto row = Dune::Std::submdspan(a, 1, Dune::Std::full_extent);
 
// Keep a half-open range of columns.
auto cols = Dune::Std::submdspan(a, Dune::Std::full_extent,
                                 Dune::Std::range_slice{1, 4});
 
// Keep every second column.
auto evenCols = Dune::Std::submdspan(a, Dune::Std::full_extent,
                                     Dune::Std::extent_slice{0, 2, 2});
 
// Fix both indices: a rank-0 mdspan.
auto entry = Dune::Std::submdspan(a, 1, 2);