# std::ranges::move_backward, std::ranges::move_backward_result

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::bidirectional_iterator I1, std::sentinel_for <I1> S1, std::bidirectional_iterator I2 > requires std::indirectly_movable <I1, I2> constexpr move_backward_result<I1, I2> move_backward( I1 first, S1 last, I2 result ); | (1) | (since C++20)
template< ranges::bidirectional_range R, std::bidirectional_iterator I > requires std::indirectly_movable < ranges::iterator_t <R>, I> constexpr move_backward_result< ranges::borrowed_iterator_t <R>, I> move_backward( R&& r, I result ); | (2) | (since C++20)
Helper types |  |
template< class I, class O > using move_backward_result = ranges::in_out_result <I, O>; | (3) | (since C++20)

1) Moves the elements in the range, defined by `[`first`,` last`)`, to another range `[`result - N`,` result`)`, where N = ranges::distance (first, 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 result is within `(first, last]`. In such a case, ranges::move may be used instead.
2) Same as (1), but uses r as the source range, as if using ranges::begin (r) as first, and ranges::end (r) as last.

The elements in the moved-from range will still contain valid values of the appropriate type, but not necessarily the same values as before the move, as if using *(result - n) = ranges::iter_move (last - n) for each integer `n`, where 0 ≤ n < N.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first | - | the beginning of the range of elements to move
last | - | the end of the range of elements to move
r | - | the range of the elements to move
result | - | the end of the destination range

### Return value

{last, result - N}.

### Complexity

1) Exactly N move assignments.
2) Exactly ranges::distance (r) move assignments.

### Notes

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

### Possible implementation

    struct move_backward_fn
    {
        template<std::bidirectional_iterator I1, std::sentinel_for<I1> S1,
                 std::bidirectional_iterator I2>
        requires std::indirectly_movable<I1, I2>
        constexpr ranges::move_backward_result<I1, I2>
            operator()(I1 first, S1 last, I2 result) const
        {
            auto i {last};
            for (; i != first; *--result = ranges::iter_move(--i))
            {}
            return {std::move(last), std::move(result)};
        }

        template<ranges::bidirectional_range R, std::bidirectional_iterator I>
        requires std::indirectly_movable<ranges::iterator_t<R>, I>
        constexpr ranges::move_backward_result<ranges::borrowed_iterator_t<R>, I>
            operator()(R&& r, I result) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::move(result));
        }
    };

    inline constexpr move_backward_fn move_backward {};

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <string>
    #include <string_view>
    #include <vector>

    using Vec = std::vector<std::string>;

    void print(std::string_view rem, Vec const& vec)
    {
        std::cout << rem << "[" << vec.size() << "]: ";
        for (const std::string& s : vec)
            std::cout << (s.size() ? s : std::string{"·"}) << ' ';
        std::cout << '\n';
    }

    int main()
    {
        Vec a{"▁", "▂", "▃", "▄", "▅", "▆", "▇", "█"};
        Vec b(a.size());

        print("Before move:\n" "a", a);
        print("b", b);

        std::ranges::move_backward(a, b.end());

        print("\n" "Move a >> b:\n" "a", a);
        print("b", b);

        std::ranges::move_backward(b.begin(), b.end(), a.end());
        print("\n" "Move b >> a:\n" "a", a);
        print("b", b);

        std::ranges::move_backward(a.begin(), a.begin()+3, a.end());
        print("\n" "Overlapping move a[0, 3) >> a[5, 8):\n" "a", a);
    }

Possible output:

    Before move:
    a[8]: ▁ ▂ ▃ ▄ ▅ ▆ ▇ █
    b[8]: · · · · · · · ·

    Move a >> b:
    a[8]: · · · · · · · ·
    b[8]: ▁ ▂ ▃ ▄ ▅ ▆ ▇ █

    Move b >> a:
    a[8]: ▁ ▂ ▃ ▄ ▅ ▆ ▇ █
    b[8]: · · · · · · · ·

    Overlapping move a[0, 3) >> a[5, 8):
    a[8]: · · · ▄ ▅ ▁ ▂ ▃

### See also

ranges::move (C++20) | moves a range of elements to a new location (algorithm function object)
ranges::copyranges::copy_if (C++20)(C++20) | copies a range of elements to a new location (algorithm function object)
ranges::copy_backward (C++20) | copies a range of elements in backwards order (algorithm function object)
move (C++11) | moves a range of elements to a new location (function template)
move (C++11) | converts the argument to an xvalue (function template)

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