std::move_backward

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

Moves the elements from the source range [first, last) to the destination range ending at d_last, starting from last and proceeding to first (reverse order).

If d_last is in (first, last], the behavior is undefined.

Parameters

first, last	-	the pair of iterators defining the source range
d_last	-	the past-the-end iterator of the destination range
Type requirements
-`BidirIt1, BidirIt2` must meet the requirements of LegacyBidirectionalIterator.

Return value

An iterator to the last move-assigned element in the destination range, or d_last if no element is moved.

Complexity

Exactly std::distance(first, last) assignments.

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).

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;
}

Example

#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

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