std:: slice
|
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
size
stride
|
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) |