# std::reverse_copy

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class BidirIt, class OutputIt > OutputIt reverse_copy( BidirIt first, BidirIt last, OutputIt d_first ); |  | (until C++20)
template< class BidirIt, class OutputIt > constexpr OutputIt reverse_copy( BidirIt first, BidirIt last, OutputIt d_first ); |  | (since C++20)
template< class ExecutionPolicy, class BidirIt, class ForwardIt > ForwardIt reverse_copy( ExecutionPolicy&& policy, BidirIt first, BidirIt last, ForwardIt d_first ); | (2) | (since C++17)

1) Copies the elements from the range `[`first`,` last`)` to another range beginning at d_first in such a way that the elements in the new range are in reverse order.
Behaves as if by executing the assignment *(d_first + (last - first) - 1 - i) = *(first + i) once for each non-negative i < (last - first).
If the source and destination ranges (that is, `[`first`,` last`)` and `[`d_first`,` d_first + (last - first)`)`, respectively) overlap, the behavior is undefined.
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. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of elements to copy
d_first | - | the beginning of the destination range
Type requirements
- `BidirIt` must meet the requirements of LegacyBidirectionalIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.

### Return value

Output iterator to the element past the last element copied.

### Complexity

Exactly last - first assignments.

### 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.

### Notes

Implementations (e.g. MSVC STL) may enable vectorization when the both iterator types satisfy LegacyContiguousIterator and have the same value type, and the value type is TriviallyCopyable.

### Possible implementation

See also the implementations in libstdc++, libc++, and MSVC STL.

    template<class BidirIt, class OutputIt>
    constexpr // since C++20
    OutputIt reverse_copy(BidirIt first, BidirIt last, OutputIt d_first)
    {
        for (; first != last; ++d_first)
            *d_first = *(--last);
        return d_first;
    }

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <vector>

    int main()
    {
        auto print = [](std::vector<int> const& v)
        {
            for (const auto& value : v)
                std::cout << value << ' ';
            std::cout << '\t';
        };

        std::vector<int> v {1, 2, 3};
        print(v);

        std::vector<int> destination(3);
        std::reverse_copy(std::begin(v), std::end(v), std::begin(destination));
        print(destination);

        std::reverse_copy(std::rbegin(v), std::rend(v), std::begin(destination));
        print(destination);
    }

Output:

    1 2 3   3 2 1   1 2 3

### See also

reverse | reverses the order of elements in a range (function template)
ranges::reverse_copy (C++20) | creates a copy of a range that is reversed (niebloid)

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