< cpp‎ | numeric‎ | valarray
Defined in header <valarray>
class slice;

std::slice is the selector class that identifies a subset of std::valarray similar to BLAS slice. An object of type std::slice holds three values: the starting index, the stride, and the total number of values in the subset. Objects of type std::slice can be used as indices with valarray's operator[].


[edit] Member functions

constructs a slice
(public member function)
returns the parameters of the slice
(public member function)


slice( std::size_t start, std::size_t size, std::size_t stride );
slice( const slice& other );

Constructs a new slice.

1) Default constructor. Equivalent to slice(0, 0, 0). This constructor exists only to allow construction of arrays of slices.
2) Constructs a new slice with parameters start, size, stride. This slice will refer to size number of elements, each with the position:
start + 0 * stride
start + 1 * stride
start + (size - 1) * stride
3) Constructs a copy of other.


start - the position of the first element
size - the number of elements in the slice
stride - the number of positions between successive elements in the slice
other - another slice to copy

std::slice::start, size, stride

std::size_t start() const;
std::size_t size() const;
std::size_t stride() const;

Returns the parameters passed to the slice on construction - start, size and stride respectively.



Return value

The parameters of the slice -- start, size and stride respectively.



[edit] Non-member functions

checks if two slices are equal


friend bool operator==( const slice& lhs, const slice& rhs );
(since C++20)

Checks if the parameters of lhs and rhs - start, size and stride are equal respectively.

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::slice is an associated class of the arguments.

The != operator is synthesized from operator==.


lhs, rhs - slices to compare

Return value

lhs.start() == rhs.start() && lhs.size() == rhs.size() && lhs.stride() == rhs.stride()

[edit] Example

Barebones valarray-backed Matrix class with a trace calculating function.

#include <iostream>
#include <valarray>
class Matrix
    std::valarray<int> data;
    int dim;
    Matrix(int r, int c) : data(r*c), dim(c) {}
    int& operator()(int r, int c) { return data[r * dim + c]; }
    int trace() const { return data[std::slice(0, dim, dim + 1)].sum(); }
int main()
    Matrix m(3, 3);
    int n = 0;
    for (int r = 0; r < 3; ++r)
       for (int c = 0; c < 3; ++c)
           m(r, c) = ++n;
    std::cout << "Trace of the matrix (1,2,3) (4,5,6) (7,8,9) is " << m.trace() << '\n';


Trace of the matrix (1,2,3) (4,5,6) (7,8,9) is 15

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 543 C++98 it was unclear whether a default constructed slice is usable it is usable (as an empty subset)

[edit] See also

get/set valarray element, slice, or mask
(public member function) [edit]
generalized slice of a valarray: starting index, set of lengths, set of strides
(class) [edit]
proxy to a subset of a valarray after applying a slice
(class template) [edit]