std::move_backward(3)

okay bestie, you clicked into std::move_backward(3) and honestly? taste. this one lives in the C++ Standard Libary (manual name and all — yes, the header spells it Libary, and we are not fixing her 💅).

✨ NAME ✨

std::move_backward — std::move_backward (the name section said what it said, and honestly? slay).

🌸 SYNOPSIS 🌸

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

timeline check: (since C++11), and (constexpr since C++20). both details are important, bestie, don't skip them.

here's the vibe: 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. cute, right? 🎀

🚨 okay but for real: if d_last is within (first, last], the behavior is undefined. In this case, std::move may be used instead. undefined behavior is not cute, do not do it.

🎀 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 std::distance(first, last) 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). that's the directional tea ☕

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

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source: http://cppreference.com • version/date: 2024.06.10 • bye bestie, go move some ranges 💖