std::ranges...ve_n_result(3)

okay so bestie, you're reading the C++ Standard Libary manual page — the one named std::ranges...ve_n_result(3) — and it's all about std::ranges::uninitialized_move_n and std::ranges::uninitialized_move_n_result. bucket hat on, we're doing this 💅✨

✨ NAME ✨

std::ranges::uninitialized_move_n,std::ranges::uninitialized_move_n_result — std::ranges::uninitialized_move_n,std::ranges::uninitialized_move_n_result

🌸 Synopsis 🌸

Defined in header <memory>.

Call signature:

template< std::input_iterator I, no-throw-forward-iterator O,
          no-throw-sentinel-for<O> S >
requires std::constructible_from<std::iter_value_t<O>,
         std::iter_rvalue_reference_t<I>>
(1) (since C++20)
uninitialized_move_n_result<I, O>
uninitialized_move_n( I ifirst, std::iter_difference_t<I> n, O ofirst, S olast );

🧚 Helper types 🧚

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

Moves N elements from the input range beginning at ifirst to the uninitialized storage designated by the range [ofirst, olast), where N is min(n, ranges::distance(ofirst, olast)).

The effect is equivalent to:

for (; n-- > 0 && ofirst != olast; ++ifirst, ++ofirst)
    ::new (static_cast<void*>(std::addressof(*first)))
        std::remove_reference_t<std::iter_reference_t<O>>(ranges::iter_move(ifirst));

If an exception is thrown during the initialization then the objects that already constructed in [ofirst, olast) are destroyed in an unspecified order. Also, the objects in the input range beginning at ifirst, that were already moved, are left in a valid but unspecified state.

bestie, “valid but unspecified state” is literally not a promise that your moved-from objects still say what they said before 😭

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 input range of elements to move from
• ofirst, olast — iterator-sentinel pair denoting the output range of elements to initialize
• n — the number of elements to move

⭐ Return value ⭐

{ifirst + N, ofirst + N}.

🦋 Complexity 🦋

Linear in 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_move_n, e.g. by using ranges::copy_n, if the value type of the output range is TrivialType.

🎀 Possible implementation 🎀

struct uninitialized_move_n_fn
{
    template<std::input_iterator I, no-throw-forward-iterator O,
             no-throw-sentinel-for<O> S>
    requires std::constructible_from<std::iter_value_t<O>,
             std::iter_rvalue_reference_t<I>>
    ranges::uninitialized_move_n_result<I, O>
    operator()(I ifirst, std::iter_difference_t<I> n, O ofirst, S olast) const
    {
        O current{ofirst};
        try
        {
            for (; n-- > 0 && current != olast; ++ifirst, ++current)
                ::new (const_cast<void*>(static_cast<const volatile void*>
                    (std::addressof(*current))))
                    std::remove_reference_t<
                        std::iter_reference_t<O>>(ranges::iter_move(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_move_n_fn uninitialized_move_n{};

🍓 Example 🍓

// Run this code

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

void print(auto rem, auto first, auto last)
{
    for (std::cout << rem; first != last; ++first)
        std::cout << std::quoted(*first) << ' ';
    std::cout << '\n';
}

int main()
{
    std::string in[]{ "No", "Diagnostic", "Required", };
    print("initially, in: ", std::begin(in), std::end(in));

    if (
        constexpr auto sz = std::size(in);
        void* out = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz)
    )
    {
        try
        {
            auto first{static_cast<std::string*>(out)};
            auto last{first + sz};
            std::ranges::uninitialized_move_n(std::begin(in), sz, first, last);

            print("after move, in: ", std::begin(in), std::end(in));
            print("after move, out: ", first, last);

            std::ranges::destroy(first, last);
        }
        catch (...)
        {
            std::cout << "Exception!\n";
        }
        std::free(out);
    }
}

💜 Possible output 💜

initially, in: "No" "Diagnostic" "Required"
after move, in: "" "" ""
after move, out: "No" "Diagnostic" "Required"

🌟 Defect reports 🌟

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• DR LWG 3870 — applied to C++20; behavior as published: this algorithm might create objects on a const storage; correct behavior: kept disallowed

👑 See also 👑

• ranges::uninitialized_move (C++20) — moves a range of objects to an uninitialized area of memory (niebloid)
• uninitialized_move_n (C++17) — moves a number of objects to an uninitialized area of memory (function template)

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xoxo, your manual-page bestie 💖 source http://cppreference.com, version 2024.06.10, page std::ranges...ve_n_result(3) — mwah 💋🎀