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std::strided_slice

From cppreference.com
< cpp‎ | container‎ | mdspan
 
 
 
 
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. It provides three values: the offset index, the extent, and the stride.

Given that stride is greater than zero, let N be the number of selected indices which is determined by 1 + (extent - 1) / stride if extent is nonzero, otherwise 0. The indices are selected from half-open interval [offset, offset + extent). Hence, the selected indices are offset, ..., offset + (N - 1) * stride.

This class template has no base classes or declared members other than those shown below.

Contents

[edit] Template parameters

OffsetType
ExtentType
StrideType
- the types of offset, extent, and stride, respectively.
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.

[edit] Member types

Member type Definition
offset_type OffsetType
extent_type ExtentType
stride_type StrideType

[edit] 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.

[edit] 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.

[edit] 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]

[edit] See also

BLAS-like slice of a valarray: starting index, length, stride
(class) [edit]
(C++26)
returns a view of a subset of an existing mdspan
(function template) [edit]