std::slice
| Defined in header <valarray>
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| class slice; |
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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) |
| startsizestride |
returns the parameters of the slice (public member function) |
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.
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) (C++20) |
checks if two slices are equal (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.
#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 constructed slice is usable | it is usable (as an empty subset) |
See also
| 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) | |
| (C++23) |
a multi-dimensional non-owning array view (class template) |