NAME¶

std::slice - std::slice

Synopsis¶

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[].

Member functions¶

constructor constructs a slice
(public member function)
start returns the parameters of the slice
size (public member function)
stride

std::slice::slice

slice() (1)
slice( std::size_t start, std::size_t size, std::size_t stride ); (2)
slice( const slice& other ); (3)

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.

Parameters¶

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; (1)
std::size_t size() const; (2)
std::size_t stride() const; (3)

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

Parameters¶

(none)

Return value¶

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

Complexity¶

Constant.

Non-member functions¶

operator==(std::slice) checks if two slices are equal
(C++20) (function)

operator==(std::slice)

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==.

Parameters¶

lhs, rhs - slices to compare

Return value¶

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

Example¶

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

// Run this code

#include <iostream>
#include <valarray>

class Matrix
{
std::valarray<int> data;
int dim;
public:
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';
}

Output:¶

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

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 it is usable (as an empty
constructed slice is usable subset)

See also¶

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