std::move_backward

 

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Defined in header <algorithm>
template< class BidirIt1, class BidirIt2 > BidirIt2 move_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );		(since C++11) (until C++20)
template< class BidirIt1, class BidirIt2 > constexpr BidirIt2 move_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );		(since C++20)

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

The behavior is undefined if d_last is within (first, last]. std::move must be used instead of std::move_backward in that case.

Parameters

first, last	-	the range of the elements to move
d_last	-	end of the destination range
Type requirements
- BidirIt1, BidirIt2 must meet the requirements of LegacyBidirectionalIterator.

Return value

Iterator in the destination range, pointing at the last element moved.

Complexity

Exactly last - first move assignments.

Possible implementation

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

Notes

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

Example

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
#include <string_view>
#include <vector>
 
using container = std::vector<std::string>;
 
void print(std::string_view comment, const container& src, const container& dst = {})
{
    auto prn = [](std::string_view name, const container& cont) {
        std::cout << name;
        for (const auto &s: cont) { std::cout << (s.empty() ? "∙" : s.data()) << ' '; }
        std::cout << '\n';
    };
    std::cout << comment << '\n';
    prn("src: ", src);
    if (dst.empty()) return;
    prn("dst: ", dst);
}
 
int main()
{
    container src{"foo", "bar", "baz"};
    container dst{"qux", "quux", "quuz", "corge"};
    print("Non-overlapping case; before move_backward:", src, dst);
    std::move_backward(src.begin(), src.end(), dst.end());
    print("After:", src, dst);
 
    src = {"snap", "crackle", "pop", "lock", "drop"};
    print("Overlapping case; before move_backward:", src);
    std::move_backward(src.begin(), std::next(src.begin(), 3), src.end());
    print("After:", src);
}

Output:

Non-overlapping case; before move_backward:
src: foo bar baz 
dst: qux quux quuz corge 
After:
src: ∙ ∙ ∙ 
dst: qux foo bar baz 
Overlapping case; before move_backward:
src: snap crackle pop lock drop 
After:
src: ∙ ∙ snap crackle pop

See also

move (C++11)	moves a range of elements to a new location (function template)
ranges::move_backward (C++20)	moves a range of elements to a new location in backwards order (niebloid)