std::copy_backward

 

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Defined in header <algorithm>
template<class BidirIt1, class BidirIt2 > BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );		(constexpr 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 pair of iterators defining the source range of 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 std::distance(first, last) 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 (algorithm function object)