std::copy_backward(3)

💅 std::copy_backward(3) — the backwards-copying icon 💅

okay so bestie, this is std::copy_backward(3), straight outta the C++ Standard Libary (that’s the manual name, periodt). It’s giving “copy a range, but start from the back so the order stays cute.” 🦋

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✨ NAME ✨

std::copy_backward - std::copy_backward. That’s the name, no cap. 💅

📜 Synopsis 📜

Defined in header <algorithm>:

template< class BidirIt1, class BidirIt2 >
BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last,
(constexpr since C++20)
BidirIt2 d_last );

So what does it do? It copies the elements from the range [first, last) to another range ending at d_last. The elements are copied in reverse order — the last element is copied first — but their relative order is preserved. Yes bestie, it’s backwards but not chaotic. ✨

⚠️ The behavior is undefined if d_last is within (first, last]. std::copy must be used instead of std::copy_backward in that case. Do not play with this one, fr fr. 💀

🌸 Parameters 🌸

• first, last — the range of the elements to copy from.
• d_last — the end of the destination range.

🎀 Type requirements 🎀

• BidirIt must meet the requirements of LegacyBidirectionalIterator. That’s the vibe check for your iterator, and it has to pass. 🫶

🔥 Return value 🔥

Iterator to the last element copied. That’s the returned glow-up. ⭐

⭐ Complexity ⭐

Exactly std::distance(first, last) assignments. Not more, not less — the algorithm is literally counting its steps and staying in its lane. 💅

🧚 Notes 🧚

When copying overlapping ranges, std::copy is appropriate when copying to the left (beginning of the destination range is outside the source range), while std::copy_backward is appropriate when copying to the right (end of the destination range is outside the source range). This is the overlapping-ranges lore, iykyk. 🌈

💜 Possible implementation 💜

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

🍓 Example 🍓

Run this code, bestie — the page literally says // Run this code:

#include <algorithm>
#include <iostream>
#include <numeric>
#include <vector>

int main()
{
    std::vector<int> source(4);
    std::iota(source.begin(), source.end(), 1); // fills with 1, 2, 3, 4

    std::vector<int> destination(6);

    std::copy_backward(source.begin(), source.end(), destination.end());

    std::cout << "destination contains: ";
    for (auto i: destination)
        std::cout << i << ' ';
    std::cout << '\n';
}

🌈 Output 🌈

destination contains: 0 0 1 2 3 4

👑 Defect reports 👑

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. History really said “we fixed that,” and honestly? Slay.

• DR LWG 1206, applied to C++98:
  • Behavior as published: 1. the behavior was well-defined if d_last == last; 2. the behavior was undefined if d_last == first.
  • Correct behavior: 1. made undefined; 2. made well-defined.

🙈 See also 🙈

• copy — copies a range of elements to a new location. 👑
• copy_if — (function template), (C++11). 🎀
• ranges::copy_backward — copies a range of elements in backwards order; (C++20), (niebloid). 🦋

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made with 💖 by your Unix bestie — source: http://cppreference.com, version/date: 2024.06.10, page: std::copy_backward(3). Bye bestie, go copy backwards responsibly! 🎀