NAME¶

std::ranges::reverse - std::ranges::reverse

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::bidirectional_iterator I, std::sentinel_for<I> S >

requires std::permutable<I> (1) (since C++20)
constexpr I

reverse( I first, S last );
template< ranges::bidirectional_range R >

requires std::permutable<ranges::iterator_t<R>> (2) (since C++20)
constexpr ranges::borrowed_iterator_t<R>

reverse( R&& r );

1) Reverses the order of the elements in the range [first, last).
Behaves as if applying ranges::iter_swap to every pair of iterators first + i, last
- i - 1 for each integer i, where 0 ≤ i < (last - first) / 2.
2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first and
ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

* Explicit template argument lists cannot be specified when calling any of them.
* None of them are visible to argument-dependent lookup.
* When any of them are found by normal unqualified lookup as the name to the left
of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters¶

first, last - the range of elements to reverse
r - the range of elements to reverse

Return value¶

An iterator equal to last.

Complexity¶

Exactly (last - first) / 2 swaps.

Notes¶

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

Possible implementation¶

See also implementations in libstdc++ and MSVC STL.

struct reverse_fn
{
template<std::bidirectional_iterator I, std::sentinel_for<I> S>
requires std::permutable<I>
constexpr I operator()(I first, S last) const
{
auto last2 {ranges::next(first, last)};
for (auto tail {last2}; !(first == tail or first == --tail); ++first)
ranges::iter_swap(first, tail);
return last2;
}

template<ranges::bidirectional_range R>
requires std::permutable<ranges::iterator_t<R>>
constexpr ranges::borrowed_iterator_t<R>
operator()(R&& r) const
{
return (*this)(ranges::begin(r), ranges::end(r));
}
};

inline constexpr reverse_fn reverse {};

Example¶

// Run this code

#include <algorithm>
#include <array>
#include <iostream>
#include <string>

int main()
{
std::string s {"ABCDEF"};
std::cout << s << " → ";
std::ranges::reverse(s.begin(), s.end());
std::cout << s << " → ";
std::ranges::reverse(s);
std::cout << s << " │ ";

std::array a {1, 2, 3, 4, 5};
for (auto e : a)
std::cout << e << ' ';
std::cout << "→ ";
std::ranges::reverse(a);
for (auto e : a)
std::cout << e << ' ';
std::cout << '\n';
}

Output:¶

ABCDEF → FEDCBA → ABCDEF │ 1 2 3 4 5 → 5 4 3 2 1

See also¶

ranges::reverse_copy creates a copy of a range that is reversed
(C++20) (niebloid)
ranges::reverse_view a view that iterates over the elements of another bidirectional
views::reverse view in reverse order
(C++20) (class template) (range adaptor object)
reverse reverses the order of elements in a range
(function template)