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std::reverse

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Defined in header <algorithm>
template< class BidirIt >
void reverse( BidirIt first, BidirIt last );
(1)
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, (last-i) - 1 for each non-negative i < (last-first)/2
2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Contents

[edit] Parameters

first, last - the range of elements to reverse
policy - the execution policy to use. See execution policy for details.
Type requirements
-
BidirIt must meet the requirements of BidirectionalIterator.
-
The type of dereferenced BidirIt must meet the requirements of Swappable.

[edit] Return value

(none)

[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,
  • if policy is std::parallel_vector_execution_policy, std::terminate is called
  • if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
  • if policy is some other type, the behavior is implementation-defined
  • If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

[edit] Possible implementation

template<class BidirIt>
void reverse(BidirIt first, BidirIt last)
{
    while ((first != last) && (first != --last)) {
        std::iter_swap(first++, last);
    }
}

[edit] Example

#include <vector>
#include <iostream>
#include <iterator>
#include <algorithm>
 
int main()
{
    std::vector<int> v({1,2,3});
    std::reverse(std::begin(v), std::end(v));
    std::cout << v[0] << v[1] << v[2] << '\n';
 
    int a[] = {4, 5, 6, 7};
    std::reverse(std::begin(a), std::end(a));
    std::cout << a[0] << a[1] << a[2] << a[3] << '\n';
}

Output:

321
7654

[edit] Complexity

linear in the distance between first and last

[edit] See also

creates a copy of a range that is reversed
(function template) [edit]
parallelized version of std::reverse
(function template) [edit]