std::reverse
From cppreference.com
| Defined in header <algorithm>
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template< class BidirIt >
void reverse( BidirIt first, BidirIt last );
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(1) | (constexpr since C++20) |
template< class ExecutionPolicy, class BidirIt >
void reverse( ExecutionPolicy&& policy, BidirIt first, BidirIt last );
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(2) | (since C++17) |
1) Reverses the order of the elements in the target range
[first, last) by swapping each pair of corresponding elements using std::iter_swap.2) Same as (1), but executed according to
policy. This overload participates in overload resolution only if the value of the following expression is
true:
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(until C++20) |
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(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 target range |
| policy | - | the execution policy to use |
| Type requirements | ||
-BidirIt must meet the requirements of LegacyBidirectionalIterator.
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Complexity
Exactly std::distance(first, last) / 2 swaps.
Exceptions
2) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
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.
Possible implementation
See also the implementations in libstdc++, libc++, and MSVC STL.
template<class BidirIt>
constexpr // since C++20
void reverse(BidirIt first, BidirIt last)
{
using iter_cat = typename std::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.
if constexpr (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);
}
else
while (first != last && first != --last)
std::iter_swap(first++, last);
}
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Example
Run this code
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
void println(auto rem, const auto& 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 appliedstd::distance(first, last) / 2 + 1 times
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only appliedstd::distance(first, last) / 2 times
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See also
(C++20) |
reverses the order of elements in a range (algorithm function object) |
| creates a copy of a range that is reversed (function template & algorithm function object) | |
(C++20) |