std::move_backward() algorithm

od C++20

// (1)
template< class BidirIt1, class BidirIt2 >
constexpr BidirIt2 move_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );

od C++11

// (1)
template< class BidirIt1, class BidirIt2 >
BidirIt2 move_backward( BidirIt1 first, BidirIt1 last, BidirIt2 d_last );

• (1) 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.

  Undefined Behaviour

  The behavior is undefined if d_first is within [first; last). In this case, std::move may be used instead.

Parameters

first last	The range of elements to move.
d_first	The beginning of the destination range.

Type requirements

BidirIt1 BidirIt2

LegacyBidirectionalIterator

Return value

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

Complexity

Exactly last - first assignments.

Exceptions

(none)

Possible implementation

move_backward (1)

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).
• std::move_backward is appropriate when moving to the right (end of the destination range is outside the source range).

Examples

Main.cpp

#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