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 indexes with valarray's operator[].
| (constructor) | constructs a slice (public member function) |
| startsizestride | returns the parameters of the slice (public member function) |
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.
slice(0, 0, 0). This constructor exists only to allow construction of arrays of slices.start, size, stride. This slice will refer to size number of elements, each with the position: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::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.
(none).
The parameters of the slice -- start, size and stride respectively.
Constant.
| operator==(std::slice)
(C++20) | checks if two slices are equal (function) |
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 |
lhs.start() == rhs.start() && lhs.size() == rhs.size() && lhs.stride() == rhs.stride().
Barebones valarray-backed Matrix class with a trace calculating function.
#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
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) |
| get/set valarray element, slice, or mask (public member function) |
|
| generalized slice of a valarray: starting index, set of lengths, set of strides (class) |
|
| proxy to a subset of a valarray after applying a slice (class template) |
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