std::strided_slice
| Defined in header <mdspan>
|
||
| template< class OffsetType, class ExtentType, class StrideType > struct strided_slice; |
(since C++26) | |
An instance of every specialization of strided_slice is a slice specifier used in std::submdspan to select a subset of elements using a set of regularly spaced indices at a specified dimension in std::mdspan.
Each strided_slice object s is characterized by three data members: the offset index s.offset, the extent s.extent, and the stride s.stride.
Given that s.stride is greater than zero, the number of selected indices denoted by N is determined by 1 + (s.extent - 1) / s.stride if s.extent is nonzero, otherwise 0. The half-open interval from which indices will be selected is given by [s.offset, s.offset + s.extent). The sequence of selected indices is produced as follows: s.offset, ..., s.offset + (N - 1) * s.stride.
This class template has no base classes or declared members other than those shown below.
Template parameters
| OffsetType | - | the type of offset |
| ExtentType | - | the type of extent |
| StrideType | - | the type of stride |
| Type requirements | ||
| -All template parameters must be unsigned or signed integer types or must satisfy integral-constant-like | ||
The program is ill-formed if the type requirement is not satisfied.
Member types
| Member type | Definition |
offset_type
|
OffsetType
|
extent_type
|
ExtentType
|
stride_type
|
StrideType
|
Data members
| Member name | Definition |
| offset |
a starting index of type offset_type (public member object) |
| extent |
a value of type extent_type added to offset used to define the upper bound of indices (public member object) |
| stride |
an increment value of type stride_type which is equivalent to the distance between two indices (public member object) |
All these members are declared with [[no_unique_address]] attribute and have default member initializers where each data member is value initialized.
Notes
Every specialization of strided_slice is an aggregate class which allows aggregate initialization (including designated initialization) of data members (for instance, std::strided_slice{.offset = 2, .extent = 10, .stride = 3}).
The slice specification of strided_slice takes advantage of the data member extent, as opposed to other slice specifications that use end to indicate the upper bound value. This is because it can directly generate a static extent for subview of std::mdspan if both extent and stride are of types that satisfy integral-constant-like. This enables efficient extraction of subviews with static extents by mixing compile time values with a runtime value of offset.
Example
#include <mdspan> #include <print> template <typename View, typename O = int, typename E = int, typename S = int> requires (View::extents_type::rank() == 1) void print_sliced_view(View v, std::strided_slice<O, E, S> s) { using index_type = View::index_type; auto subview = std::submdspan(v, s); const auto& submap = subview.mapping(); std::print("["); bool uses_comma = false; for (index_type i = 0; i != subview.extent(0); ++i) { if (uses_comma) std::print(", "); std::print("{}", subview[i]); uses_comma = true; } uses_comma = false; std::print("] extracted from indices ["); for (index_type i = 0; i != subview.extent(0); ++i) { if (uses_comma) std::print(", "); std::print("{}", submap(i) + s.offset); uses_comma = true; } std::println("]"); } int main() { static constexpr char letters[] { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z' }; constexpr std::mdspan md(letters, 26); print_sliced_view(md, {.offset = 0, .extent = 10, .stride = 1}); print_sliced_view(md, {.offset = 2, .extent = 10, .stride = 1}); print_sliced_view(md, {.offset = 0, .extent = 5, .stride = 1}); print_sliced_view(md, {.offset = 2, .extent = 5, .stride = 1}); print_sliced_view(md, {.offset = 0, .extent = 10, .stride = 2}); print_sliced_view(md, {.offset = 2, .extent = 10, .stride = 3}); print_sliced_view(md, {.offset = 0, .extent = 15, .stride = 5}); print_sliced_view(md, {.offset = 6, .extent = 15, .stride = 5}); }
Output:
[A, B, C, D, E, F, G, H, I, J] extracted from indices [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [C, D, E, F, G, H, I, J, K, L] extracted from indices [2, 3, 4, 5, 6, 7, 8, 9, 10, 11] [A, B, C, D, E] extracted from indices [0, 1, 2, 3, 4] [C, D, E, F, G] extracted from indices [2, 3, 4, 5, 6] [A, C, E, G, I] extracted from indices [0, 2, 4, 6, 8] [C, F, I, L] extracted from indices [2, 5, 8, 11] [A, F, K] extracted from indices [0, 5, 10] [G, L, Q] extracted from indices [6, 11, 16]
See also
| BLAS-like slice of a valarray: starting index, length, stride (class) | |
| (C++26) |
returns a view of a subset of an existing mdspan (function template) |