std::copy_backward

 

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is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
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uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Defined in header <algorithm>
template< class BidirIt1, class BidirIt2 > BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );		(until C++20)
template< class BidirIt1, class BidirIt2 > constexpr BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );		(since C++20)

Copies the elements from the range [first, last) to another range ending at d_last. The elements are copied in reverse order (the last element is copied first), but their relative order is preserved.

The behavior is undefined if d_last is within (first, last). std::copy must be used instead of std::copy_backward in that case.

Parameters

first, last	-	the range of the elements to copy from
d_last	-	the end of the destination range
Type requirements
- BidirIt must meet the requirements of LegacyBidirectionalIterator.

Return value

Iterator to the last element copied.

Complexity

Exactly last - first assignments.

Notes

When copying overlapping ranges, std::copy is appropriate when copying to the left (beginning of the destination range is outside the source range) while std::copy_backward is appropriate when copying to the right (end of the destination range is outside the source range).

Possible implementation

template<class BidirIt1, class BidirIt2> BidirIt2 copy_backward(BidirIt1 first, BidirIt1 last, BidirIt2 d_last) { while (first != last) *(--d_last) = *(--last); return d_last; }

Example

Run this code

#include <algorithm>
#include <iostream>
#include <numeric>
#include <vector>
 
int main()
{
    std::vector<int> source(4);
    std::iota(source.begin(), source.end(), 1); // fills with 1, 2, 3, 4
 
    std::vector<int> destination(6);
 
    std::copy_backward(source.begin(), source.end(), destination.end());
 
    std::cout << "destination contains: ";
    for (auto i: destination)
        std::cout << i << ' ';
    std::cout << '\n';
}

Output:

destination contains: 0 0 1 2 3 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 1206	C++98	1. the behavior was well-defined if d_last == last 2. the behavior was undefined if d_last == first	1. made undefined 2. made well-defined

See also

copycopy_if (C++11)	copies a range of elements to a new location (function template)
ranges::copy_backward (C++20)	copies a range of elements in backwards order (niebloid)