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std::reverse_copy

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
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reverse_copy
(until C++17)(C++11)
(C++20)(C++20)
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(C++17)

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Defined in header <algorithm>
template< class BidirIt, class OutputIt >

OutputIt reverse_copy( BidirIt first, BidirIt last,

                       OutputIt d_first );
(1) (constexpr 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) Given N as std::distance(first, last). Copies the elements from the range [firstlast) (source range) to another range of N elements beginning at d_first (destination range) in such a way that the elements in the destination range are in reverse order.
Behaves as if by executing the assignment *(d_first + N - 1 - i) = *(first + i)[1] once for each integer i in [0N).
If source and destination ranges overlap, the behavior is undefined.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if

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)

Contents

[edit] 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.

[edit] Return value

Output iterator to the element past the last element copied.

[edit] Complexity

Exactly N assignments.

[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 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.

[edit] 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;
}

[edit] 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.

[edit] Example

#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    auto print = [](const std::vector<int>& v)
    {
        for (const auto& value : v)
            std::cout << value << ' ';
        std::cout << '\n';
    };
 
    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

[edit] 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 2074 C++98 for each i, the assignment was
*(d_first + N - i) = *(first + i)[1]
corrected to
*(d_first + N - 1 - i) = *(first + i)[1]
LWG 2150 C++98 only one element was required to be assigned corrected the requirement
  1. 1.0 1.1 1.2 LegacyOutputIterator is not required to support binary + and -. The usages of + and - here are exposition-only: the actual computation does not need to use them.

[edit] See also

reverses the order of elements in a range
(function template) [edit]
creates a copy of a range that is reversed
(niebloid)[edit]