std::ranges::reverse
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< std::bidirectional_iterator I, std::sentinel_for<I> S > requires std::permutable<I> constexpr I reverse( I first, S last ); |
(1) | (since C++20) |
template< ranges::bidirectional_range R > requires std::permutable<ranges::iterator_t<R>> constexpr ranges::borrowed_iterator_t<R> reverse( R&& r ); |
(2) | (since C++20) |
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 algorithm function objects (informally known as 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.
Parameters
| first, last | - | the iterator-sentinel pair defining 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 {};
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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
(C++20) |
creates a copy of a range that is reversed (algorithm function object) |
a view that iterates over the elements of another bidirectional view in reverse order(class template) (range adaptor object) | |
| reverses the order of elements in a range (function template) |