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:
If *first is not Swappable (until C++20)BidirIt is not ValueSwappable (since C++20), the behavior is undefined.
[edit] Parameters
first, last
-
the pair of iterators defining the
range of elements to reverse
Type requirements
[edit] Complexity
Exactly std::distance (first, last) / 2 swaps.
[edit] 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.
[edit] 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);
}
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.
[edit] Example
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
void println(auto rem, auto const& 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
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
[edit] See also
creates a copy of a range that is reversed
(function template) [edit]
reverses the order of elements in a range
(algorithm function object)[edit]