[alg.reverse]

# 26 Algorithms library [[algorithms]](./#algorithms)

## 26.7 Mutating sequence operations [[alg.modifying.operations]](alg.modifying.operations#alg.reverse)

### 26.7.10 Reverse [alg.reverse]

[🔗](#lib:reverse)

`template<class BidirectionalIterator>
  constexpr void reverse(BidirectionalIterator first, BidirectionalIterator last);
template<class ExecutionPolicy, class BidirectionalIterator>
  void reverse(ExecutionPolicy&& exec,
               BidirectionalIterator first, BidirectionalIterator last);

template<[bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]") I, [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<I> S>
  requires [permutable](alg.req.permutable#concept:permutable "24.3.7.6 Concept permutable [alg.req.permutable]")<I>
  constexpr I ranges::reverse(I first, S last);
template<[bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]") R>
  requires [permutable](alg.req.permutable#concept:permutable "24.3.7.6 Concept permutable [alg.req.permutable]")<iterator_t<R>>
  constexpr borrowed_iterator_t<R> ranges::reverse(R&& r);

template<[execution-policy](algorithms.parallel.defns#concept:execution-policy "26.3.1 Preamble [algorithms.parallel.defns]") Ep, [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]") I, [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<I> S>
  requires [permutable](alg.req.permutable#concept:permutable "24.3.7.6 Concept permutable [alg.req.permutable]")<I>
  I ranges::reverse(Ep&& exec, I first, S last);
template<[execution-policy](algorithms.parallel.defns#concept:execution-policy "26.3.1 Preamble [algorithms.parallel.defns]") Ep, [sized-random-access-range](range.refinements#concept:sized-random-access-range "25.4.6 Other range refinements [range.refinements]") R>
  requires [permutable](alg.req.permutable#concept:permutable "24.3.7.6 Concept permutable [alg.req.permutable]")<iterator_t<R>>
  borrowed_iterator_t<R> ranges::reverse(Ep&& exec, R&& r);
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8064)

*Preconditions*: For the overloads in namespace std,BidirectionalIterator meets
the [*Cpp17ValueSwappable*](swappable.requirements#:Cpp17ValueSwappable "16.4.4.3 Swappable requirements [swappable.requirements]") requirements ([[swappable.requirements]](swappable.requirements "16.4.4.3 Swappable requirements"))[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8070)

*Effects*: For each non-negative integer i < (last - first) / 2,
applies std​::​iter_swap, orranges​::​​iter_swap for the overloads in namespace ranges,
to all pairs of iterators first + i, (last - i) - 1[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8077)

*Returns*: last for the overloads in namespace ranges[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8081)

*Complexity*: Exactly (last - first)/2 swaps[.](#4.sentence-1)

[🔗](#lib:reverse_copy)

`template<class BidirectionalIterator, class OutputIterator>
  constexpr OutputIterator
    reverse_copy(BidirectionalIterator first, BidirectionalIterator last,
                 OutputIterator result);
template<class ExecutionPolicy, class BidirectionalIterator, class ForwardIterator>
  ForwardIterator
    reverse_copy(ExecutionPolicy&& exec,
                 BidirectionalIterator first, BidirectionalIterator last,
                 ForwardIterator result);

template<[bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]") I, [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<I> S, [weakly_incrementable](iterator.concept.winc#concept:weakly_incrementable "24.3.4.4 Concept weakly_­incrementable [iterator.concept.winc]") O>
  requires [indirectly_copyable](alg.req.ind.copy#concept:indirectly_copyable "24.3.7.3 Concept indirectly_­copyable [alg.req.ind.copy]")<I, O>
  constexpr ranges::reverse_copy_result<I, O>
    ranges::reverse_copy(I first, S last, O result);
template<[bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]") R, [weakly_incrementable](iterator.concept.winc#concept:weakly_incrementable "24.3.4.4 Concept weakly_­incrementable [iterator.concept.winc]") O>
  requires [indirectly_copyable](alg.req.ind.copy#concept:indirectly_copyable "24.3.7.3 Concept indirectly_­copyable [alg.req.ind.copy]")<iterator_t<R>, O>
  constexpr ranges::reverse_copy_result<borrowed_iterator_t<R>, O>
    ranges::reverse_copy(R&& r, O result);
`

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8109)

Let N be last - first[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8112)

*Preconditions*: The ranges [first, last) and [result, result + N)
do not overlap[.](#6.sentence-1)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8117)

*Effects*: Copies the range [first, last) to the range [result, result + N)
such that for every non-negative integer i < N the following assignment takes place:*(result + N - 1 - i) = *(first + i)[.](#7.sentence-1)

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8124)

*Returns*: 

- [(8.1)](#8.1)

  result + N for the overloads in namespace std[.](#8.1.sentence-1)

- [(8.2)](#8.2)

  {last, result + N} for the overloads in namespace ranges[.](#8.2.sentence-1)

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8133)

*Complexity*: Exactly N assignments[.](#9.sentence-1)

[🔗](#itemdecl:3)

`template<[execution-policy](algorithms.parallel.defns#concept:execution-policy "26.3.1 Preamble [algorithms.parallel.defns]") Ep, [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]") I, [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<I> S,
         [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]") O, [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<O> OutS>
  requires [indirectly_copyable](alg.req.ind.copy#concept:indirectly_copyable "24.3.7.3 Concept indirectly_­copyable [alg.req.ind.copy]")<I, O>
  ranges::reverse_copy_truncated_result<I, O>
    ranges::reverse_copy(Ep&& exec, I first, S last, O result,
                         OutS result_last);
template<[execution-policy](algorithms.parallel.defns#concept:execution-policy "26.3.1 Preamble [algorithms.parallel.defns]") Ep, [sized-random-access-range](range.refinements#concept:sized-random-access-range "25.4.6 Other range refinements [range.refinements]") R, [sized-random-access-range](range.refinements#concept:sized-random-access-range "25.4.6 Other range refinements [range.refinements]") OutR>
  requires [indirectly_copyable](alg.req.ind.copy#concept:indirectly_copyable "24.3.7.3 Concept indirectly_­copyable [alg.req.ind.copy]")<iterator_t<R>, iterator_t<OutR>>
  ranges::reverse_copy_truncated_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
    ranges::reverse_copy(Ep&& exec, R&& r, OutR&& result_r);
`

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8152)

Let N be min(last - first, result_last - result),
and let *NEW_FIRST* be first + (last - first) - N[.](#10.sentence-1)

[11](#11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8156)

*Preconditions*: The ranges [first, last) and [result, result + N)
do not overlap[.](#11.sentence-1)

[12](#12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8161)

*Effects*: Copies the range [*NEW_FIRST*, last)
to the range [result, result + N)
such that for every non-negative integer i<N the following assignment takes place:*(result + N - 1 - i) = *(*NEW_FIRST* + i)[.](#12.sentence-1)

[13](#13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8169)

*Returns*: {last, *NEW_FIRST*, result + N}[.](#13.sentence-1)

[14](#14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8173)

*Complexity*: Exactly N assignments[.](#14.sentence-1)