std::reverse

 

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Defined in header <algorithm>
template<class BidirIt > void reverse( BidirIt first, BidirIt last );	(1)	(constexpr since C++20)
template<class ExecutionPolicy, class BidirIt > void reverse( ExecutionPolicy&& policy, BidirIt first, BidirIt last );	(2)	(since C++17)

1) Reverses the order of the elements in the range [first, last).

Behaves as if applying std::iter_swap to every pair of iterators first + i and (last - i)-1 for each integer i in [​0​, std::distance(first, last)/2).

2) Same as (1), but executed according to policy.

This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.	(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.	(since C++20)

If *first is not Swappable(until C++20)BidirIt is not ValueSwappable(since C++20), the behavior is undefined.

Parameters

first, last	-	the pair of iterators defining the range of elements to reverse
policy	-	the execution policy to use
Type requirements
- BidirIt must meet the requirements of LegacyBidirectionalIterator.

Complexity

Exactly std::distance(first, last)/2 swaps.

Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

See also the implementations in libstdc++, libc++, and MSVC STL.

template<class BidirIt>constexpr// since C++20void reverse(BidirIt first, BidirIt last){using iter_cat =typenamestd::iterator_traits<BidirIt>::iterator_category;   // Tag dispatch, e.g. calling reverse_impl(first, last, iter_cat()),// can be used in C++14 and earlier modes.ifconstexpr(std::is_base_of_v<std::random_access_iterator_tag, iter_cat>){if(first == last)return;   for(--last; first < last;(void)++first, --last)std::iter_swap(first, last);}elsewhile(first != last && first !=--last)std::iter_swap(first++, last);}

Notes

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type satisfies LegacyContiguousIterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

Example

Run this code

#include <algorithm>#include <iostream>#include <iterator>#include <vector>   void println(auto rem, autoconst& v){for(std::cout<< rem;auto e : v)std::cout<< e <<' ';std::cout<<'\n';}   int main(){std::vector<int> v {1, 2, 3}; std::reverse(v.begin(), v.end()); println("after reverse, v = ", v);   int a[]={4, 5, 6, 7}; std::reverse(std::begin(a), std::end(a)); println("after reverse, a = ", a);}

Output:

after reverse, v = 3 2 1 after reverse, a = 7 6 5 4

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 223	C++98	std::swap was applied to each pair of iterators	applies std::iter_swap instead
LWG 2039	C++98	std::iter_swap was also applied when i equals std::distance(first, last)/2	not applied

See also

reverse_copy	creates a copy of a range that is reversed (function template)
ranges::reverse (C++20)	reverses the order of elements in a range (algorithm function object)