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std::slice(3)		       C++ Standard Libary		   std::slice(3)

NAME
     std::slice - std::slice

Synopsis
	Defined in header <valarray>
	class slice;

	std::slice is the selector class that identifies a subset of std::valar-
     ray 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 associ-
     ated 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  func-
     tion.

     // 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	retroac-
     tively 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)

http://cppreference.com 	   2024.06.10			   std::slice(3)
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