std::ranges...py_n_result(3)

Okay so bestie, welcome to std::ranges...py_n_result(3) from the C++ Standard Libary manual 💖 — this is the group chat where uninitialized memory gets copied and nobody gets left on read.

💖 NAME 💖

The name vibe is literally std::ranges::uninitialized_copy_n,std::ranges::uninitialized_copy_n_result — then the original has a - line, then the exact same name again: std::ranges::uninitialized_copy_n,std::ranges::uninitialized_copy_n_result.

🌸 Synopsis 🌸

Defined in header <memory>.

📞 Call signature 📞

template< std::input_iterator I, no-throw-input-iterator O,
no-throw-sentinel-for<O> S >

requires std::constructible_from<std::iter_value_t<O>,
std::iter_reference_t<I>>
(1) (since C++20)
            uninitialized_copy_n_result<I, O>
            uninitialized_copy_n( I ifirst, std::iter_difference_t<I>
count,

                                  O ofirst, S olast );

🧸 Helper types 🧸

template< class I, class O >
(2) (since C++20)
using uninitialized_copy_n_result = ranges::in_out_result<I, O>;

Let \(\scriptsize N\)N be ranges::min(count, ranges::distance(ofirst, olast)), and it constructs \(\scriptsize N\)N elements in the output range [ofirst, olast), which is an uninitialized memory area, from the elements in the input range beginning at ifirst.

⚠️ The input range [ifirst, ifirst + count) must not overlap with the output range [ofirst, olast) — no cap, that part matters.

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

The function has the effect equivalent to:

auto ret = ranges::uninitialized_copy(std::counted_iterator(ifirst,
count),
                                      std::default_sentinel, ofirst,
olast);
return {std::move(ret.in).base(), ret.out};

The function-like entities described on this page are 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.

In practice, they may be implemented as function objects, or with special compiler extensions.

🎀 Parameters 🎀

• ifirst — the beginning of the range of elements to copy from
• count — the number of elements to copy
• ofirst, olast — iterator-sentinel pair denoting the destination range

🦋 Return value 🦋

{ifirst + N, ofirst + N}

🔥 Complexity 🔥

\(\scriptsize\mathcal{O}(N)\)𝓞(N).

😭 Exceptions 😭

The exception thrown on construction of the elements in the destination range, if any.

💀 Notes 💀

An implementation may improve the efficiency of the ranges::uninitialized_copy_n, by using e.g. ranges::copy_n, if the value type of the output range is TrivialType.

🌈 Possible implementation 🌈

struct uninitialized_copy_n_fn {
    template<std::input_iterator I, no-throw-input-iterator O, no-throw-
sentinel-for<O> S>
    requires std::constructible_from<std::iter_value_t<O>,
std::iter_reference_t<I>>
    ranges::uninitialized_copy_n_result<I, O>
    operator()(I ifirst, std::iter_difference_t<I> count, O ofirst, S
olast) const
    {
        O current{ofirst};
        try
        {
            for (; count > 0 && current != olast; ++ifirst, ++current,
--count)
                ranges::construct_at(std::addressof(*current), *ifirst);
            return {std::move(ifirst), std::move(current)};
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; ofirst != current; ++ofirst)
                ranges::destroy_at(std::addressof(*ofirst));
            throw;
        }
    } };

inline constexpr uninitialized_copy_n_fn uninitialized_copy_n{};

🍓 Example 🍓

// Run this code

#include <iomanip>
#include <iostream>
#include <memory>
#include <string>

int main()
{
    const char* stars[]{"Procyon", "Spica", "Pollux", "Deneb",
"Polaris"};

    constexpr int n{4};
    alignas(alignof(std::string)) char out[n * sizeof(std::string)];

    try
    {
        auto first{reinterpret_cast<std::string*>(out)};
        auto last{first + n};
        auto ret{std::ranges::uninitialized_copy_n(std::begin(stars),
n, first, last)};

        std::cout << '{';
        for (auto it{first}; it != ret.out; ++it)
            std::cout << (it == first ? "" : ", ") << std::quoted(*it);
        std::cout << "};\n";

        std::ranges::destroy(first, last);
    }
    catch (...)
    {
        std::cout << "uninitialized_copy_n exception\n";
    }
}

✨ Output ✨

{"Procyon", "Spica", "Pollux", "Deneb"};

👑 See also 👑

• ranges::uninitialized_copy — copies a range of objects to an uninitialized area of memory (C++20) (niebloid)
• uninitialized_copy_n — copies a number of objects to an uninitialized area of memory (C++11) (function template)

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Cute little sign-off, bestie: source is http://cppreference.com, version/date is 2024.06.10, and the page is std::ranges...py_n_result(3) 🫶✨