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    std::reverse

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Execution policies (C++17)
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    (C++17)
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    (C++11)                (C++11)(C++11)

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    (C++11)
    (C++11)
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    Transformation operations
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    (until C++17)(C++11)
    (C++20)(C++20)
    Sampling operations
    (C++17)

    Sorting and related operations
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    (on partitioned ranges)
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    (C++11)
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    C library
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    Defined in header <algorithm>
    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:
    See More

    std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

    		(until C++20)

    std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

    		(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 range of elements to reverse
    policy - the execution policy to use
    Type requirements
    -
    BidirIt must meet the requirements of LegacyBidirectionalIterator.

    Complexity

    Exactly std::distance(first, last) / 2 swaps.

    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.

    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);
}

    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.

    Example

    Run this code
    #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

    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 was also applied when i
    equals std::distance(first, last) / 2     		not applied

    See also

    		 creates a copy of a range that is reversed
    (function template) [edit]
    (C++20)
    		 reverses the order of elements in a range
    (algorithm function object)[edit]