# std::slice

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< 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 indexes 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.

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) (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.

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

### See also

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

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