ranges::iota, ranges::shift_left, and ranges::shift_right

Document #: P2440R1
Date: 2021-12-05
Project: Programming Language C++
Audience: LWG
Reply-to: Tim Song
<>

1 Abstract

This paper proposes adding the algorithms ranges::iota, ranges::shift_left, and ranges::shift_right, to match their std counterparts.

2 Revision history

3 Discussion

3.1 ranges::iota

As [P2214R0] explains, while conceptification of other numeric algorithms is a far more complex endeavor (see, e.g., [P1813R0]), iota is straightforward:

We already have views::iota in C++20, which importantly means that we already have all the correct constraints in place. In that sense, it almost takes less time to adopt ranges::iota than it would take to discuss whether or not it’s worth spending time adopting it.

The ranges::iota algorithm isn’t redundant to views::iota either: the algorithm determines the number of values to write based on the output range, while using ranges::copy or similar algorithms with views::iota requires that information to be computed upfront. When you already have a range, ranges::iota can be more convenient and efficient.

Following the law of useful returns, this paper proposes returning both the end of the range and the final value.

3.2 ranges::shift_left and ranges::shift_right

These were proposed in [P1243R3] but removed in Prague due to concerns about the return type of shift_left losing information about the original end of the range. There were particularly concerns about difficulty in recovering the original end if sentinels are involved, since the elements between the end of the new range and the previous end could have been moved from.

However, this argument overlooks the fact that:

Moreover, the standard library already provides a view factory for users who want to iterate [new_last, last): views::counted. In some cases, using counted_iterator might even be more efficient by reducing iterator comparisons to integer comparisons (comparing counted_iterators only need to compare the counter).

Meanwhile, just returning a subrange that represents the new range of elements instead of a more complex, bespoke type that users need to decompose themselves is substantially more convenient, and it is hardly unprecedented for some algorithms to not return the end of the range despite necessarily computing it (e.g., ranges::count, ranges::min/max/minmax_element), and here we don’t even necessarily compute it. It also allows shift_left and shift_right to have the same return type, without having to complicate the latter.

This paper therefore proposes maintaining the [P1243R3] design: ranges::shift_left and ranges::shift_right should return a subrange that represents the new range after the shift.

4 Wording

4.1 ranges::iota

  1. Edit 25.9 [numeric.ops.overview], header <numeric> synopsis, as indicated:

 namespace std {
+  namespace ranges {
+    // 25.5 [algorithms.results], algorithm result types
+
+    template<class O, class T>
+      struct out_value_result;
+  }

   // [...]

   // 25.10.13 [numeric.iota], iota
   template<class ForwardIterator, class T>
     constexpr void iota(ForwardIterator first, ForwardIterator last, T value);

+  namespace ranges {
+    template<class O, class T>
+      using iota_result = out_value_result<O, T>;
+
+    template<input_or_output_iterator O, sentinel_for<O> S, weakly_incrementable T>
+      requires indirectly_writable<O, const T&>
+      constexpr iota_result<O, T> iota(O first, S last, T value);
+
+    template<weakly_incrementable T, output_range<const T&> R>
+      constexpr iota_result<borrowed_iterator_t<R>, T> iota(R&& r, T value);
+  }

   // [...]
 }
  1. Add the following to 25.5 [algorithms.results]:
namespace std::ranges {
  // ...

  template<class O, class T>
  struct out_value_result {
    [[no_unique_address]] O out;
    [[no_unique_address]] T value;

    template<class O2, class T2>
      requires convertible_to<const O&, O2> && convertible_to<const T&, T2>
    constexpr operator out_value_result<O2, T2>() const & {
      return {out, value};
    }

    template<class O2, class T2>
      requires convertible_to<O, O2> && convertible_to<T, T2>
    constexpr operator out_value_result<O2, T2>() && {
      return {std::move(out), std::move(value)};
    }
  };
  1. Add the following to 25.10.13 [numeric.iota]:
template<input_or_output_iterator O, sentinel_for<O> S, weakly_incrementable T>
  requires indirectly_writable<O, const T&>
  constexpr ranges::iota_result<O, T> ranges::iota(O first, S last, T value);

template<weakly_incrementable T, output_range<const T&> R>
  constexpr ranges::iota_result<borrowed_iterator_t<R>, T> ranges::iota(R&& r, T value);

? Effects: Equivalent to: [ Drafting note: Matching range-v3, we write as_const(value) to the output to enforce that value should not be modified by the writing operation. ]

while (first != last) {
  *first = as_const(value);
  ++first;
  ++value;
}

return {std::move(first), std::move(value)};
  1. Add the following macro definition to 17.3.2 [version.syn], header <version> synopsis, with the value selected by the editor to reflect the date of adoption of this proposal:
#define __cpp_lib_ranges_iota  20XXXXL // also in <numeric>

4.2 ranges::shift_left and ranges::shift_right

  1. Edit 25.4 [algorithm.syn], header <algorithm> synopsis, as indicated:

 namespace std {

   // [...]

   // 25.7.14 [alg.shift], shift
   template<class ForwardIterator>
     constexpr ForwardIterator
       shift_left(ForwardIterator first, ForwardIterator last,
                  typename iterator_traits<ForwardIterator>::difference_type n);
   template<class ExecutionPolicy, class ForwardIterator>
     ForwardIterator
       shift_left(ExecutionPolicy&& exec,                        // see [algorithms.parallel.overloads]
                  ForwardIterator first, ForwardIterator last,
                  typename iterator_traits<ForwardIterator>::difference_type n);

+  namespace ranges {
+    template<permutable I, sentinel_for<I> S>
+      constexpr subrange<I> shift_left(I first, S last, iter_difference_t<I> n);
+    template<forward_range R>
+      requires permutable<iterator_t<R>>
+      constexpr borrowed_subrange_t<R> shift_left(R&& r, range_difference_t<R> n);
+  }

   template<class ForwardIterator>
     constexpr ForwardIterator
       shift_right(ForwardIterator first, ForwardIterator last,
                   typename iterator_traits<ForwardIterator>::difference_type n);
   template<class ExecutionPolicy, class ForwardIterator>
     ForwardIterator
       shift_right(ExecutionPolicy&& exec,                       // see [algorithms.parallel.overloads]
                   ForwardIterator first, ForwardIterator last,
                   typename iterator_traits<ForwardIterator>::difference_type n);

+  namespace ranges {
+    template<permutable I, sentinel_for<I> S>
+      constexpr subrange<I> shift_right(I first, S last, iter_difference_t<I> n);
+    template<forward_range R>
+      requires permutable<iterator_t<R>>
+      constexpr borrowed_subrange_t<R> shift_right(R&& r, range_difference_t<R> n);
+  }

   // [...]
 }
  1. Edit 25.7.14 [alg.shift] as indicated:
 template<class ForwardIterator>
   constexpr ForwardIterator
     shift_left(ForwardIterator first, ForwardIterator last,
                typename iterator_traits<ForwardIterator>::difference_type n);
 template<class ExecutionPolicy, class ForwardIterator>
   ForwardIterator
     shift_left(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last,
                typename iterator_traits<ForwardIterator>::difference_type n);
+
+template<permutable I, sentinel_for<I> S>
+  constexpr subrange<I> ranges::shift_left(I first, S last, iter_difference_t<I> n);
+template<forward_range R>
+  requires permutable<iterator_t<R>>
+  constexpr borrowed_subrange_t<R> ranges::shift_left(R&& r, range_difference_t<R> n);

1 Preconditions: n >= 0 is true. For the overloads in namespace std, tThe type of *first meets the Cpp17MoveAssignable requirements.

2 Effects: If n == 0 or n >= last - first, does nothing. Otherwise, moves the element from position first + n + i into position first + i for each non-negative integer i < (last - first) - n. For the overloads without an ExecutionPolicy template parameterIn the first overload case, does so in order starting from i = 0 and proceeding to i = (last - first) - n - 1.

3 Returns: Let NEW_LAST be first + (last - first - n) if n < last - first, otherwise first.

  • (3.1) NEW_LAST for the overloads in namespace std.
  • (3.2) {first, NEW_LAST} for the overloads in namespace ranges.

4 Complexity: At most (last - first) - n assignments.

 template<class ForwardIterator>
   constexpr ForwardIterator
     shift_right(ForwardIterator first, ForwardIterator last,
                 typename iterator_traits<ForwardIterator>::difference_type n);
 template<class ExecutionPolicy, class ForwardIterator>
   ForwardIterator
     shift_right(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last,
                 typename iterator_traits<ForwardIterator>::difference_type n);
+
+template<permutable I, sentinel_for<I> S>
+  constexpr subrange<I> ranges::shift_right(I first, S last, iter_difference_t<I> n);
+template<forward_range R>
+  requires permutable<iterator_t<R>>
+  constexpr borrowed_subrange_t<R> ranges::shift_right(R&& r, range_difference_t<R> n);

5 Preconditions: n >= 0 is true. For the overloads in namespace std, tThe type of *first meets the Cpp17MoveAssignable requirements, and. ForwardIterator meets the Cpp17BidirectionalIterator requirements (23.3.5.6 [bidirectional.iterators]) or the Cpp17ValueSwappable requirements.

6 Effects: If n == 0 or n >= last - first, does nothing. Otherwise, moves the element from position first + i into position first + n + i for each non-negative integer i < (last - first) - n. In the first overload case, if ForwardIterator meets the Cpp17BidirectionalIterator requirements, dDoes so in order starting from i = (last - first) - n - 1 and proceeding to i = 0 if:.

  • (6.1) for the overload in namespace std without an ExecutionPolicy template parameter, ForwardIterator meets the Cpp17BidirectionalIterator requirements.
  • (6.2) for the overloads in namespace ranges, I models bidirectional_iterator.

7 Returns: Let NEW_FIRST be first + n if n < last - first, otherwise last.

  • (7.1) NEW_FIRST for the overloads in namespace std.
  • (7.2) {NEW_FIRST, last} for the overloads in namespace ranges.

8 Complexity: At most (last - first) - n assignments or swaps.

  1. Update the value of __cpp_lib_shift in 17.3.2 [version.syn] to reflect the date of adoption of this proposal.

5 References

[P1243R3] Dan Raviv. 2020-01-09. Rangify New Algorithms.
https://wg21.link/p1243r3

[P1813R0] Christopher Di Bella. 2019-08-02. A Concept Design for the Numeric Algorithms.
https://wg21.link/p1813r0

[P2214R0] Barry Revzin, Conor Hoekstra, Tim Song. 2020-10-15. A Plan for C++23 Ranges.
https://wg21.link/p2214r0