std::uninitialized_move_n(3)

std::uninitialized_move_n 💅✨

okay bestie, manual-name check: std::uninitialized_move_n(3) — that's the one. It lives in the C++ Standard Libary pages (yes, the header really says "Libary", not me, I promise 😭).

🌸 NAME 🌸

std::uninitialized_move_n - std::uninitialized_move_n

Self-titled icon behavior. We love a function that knows exactly who she is. 💖

✨ Synopsis ✨

Defined in header <memory>

template< class InputIt, class Size, class NoThrowForwardIt >

std::pair<InputIt, NoThrowForwardIt>
(1) (since C++17)
       uninitialized_move_n( InputIt first, Size count,

                             NoThrowForwardIt d_first );
template< class ExecutionPolicy, class ForwardIt,

          class Size, class NoThrowForwardIt >
std::pair<ForwardIt, NoThrowForwardIt>
(2) (since C++17)
       uninitialized_move_n( ExecutionPolicy&& policy, ForwardIt first,

                             Size count, NoThrowForwardIt d_first );

1. Moves count elements from a range beginning at first to an uninitialized memory area beginning at d_first as if by:

for (; n > 0; ++d_first, (void) ++first, --n)
    ::new (static_cast<void*>(std::addressof(*d_first)))
        typename std::iterator_traits<NoThrowForwardIt>::value_type(std::move(*first));

If an exception is thrown during the initialization, some objects in first + [0, n) are left in a valid but unspecified state, and the objects already constructed are destroyed in an unspecified order.

🚨 If d_first + [0, n) overlaps with first + [0, n), the behavior is undefined. (since C++20) Do not do this, bestie.

2. Same as (1), but executed according to policy. This overload participates in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since C++20)

🎀 Parameters 🎀

• first — the beginning of the range of the elements to move.
• d_first — the beginning of the destination range.
• count — the number of elements to move.
• policy — the execution policy to use. See execution policy for details.

🧚 Type requirements 🧚

• InputIt must meet the requirements of LegacyInputIterator.
• ForwardIt must meet the requirements of LegacyForwardIterator.
• NoThrowForwardIt must meet the requirements of LegacyForwardIterator.
• No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions.

🎁 Return value 🎁

A pair whose first element is an iterator to the element past the last element moved in the source range, and whose second element is an iterator to the element past the last element moved in the destination range.

⚡ Complexity ⚡

Linear in count.

💀 Exceptions 💀

The overload with a template parameter named ExecutionPolicy reports errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

🛠️ Possible implementation 🛠️

template<class InputIt, class Size, class NoThrowForwardIt>
std::pair<InputIt, NoThrowForwardIt>
    uninitialized_move_n(InputIt first, Size count, NoThrowForwardIt
d_first) {
    using Value = typename
std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = d_first;
    try
    {
        for (; count > 0; ++first, (void) ++current, --count)
            ::new (static_cast<void*>(std::addressof(*current)))
Value(std::move(*first));
    }
    catch (...)
    {
        std::destroy(d_first, current);
        throw;
    }
    return {first, current}; }

🍓 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[]{"One", "Definition", "Rule"};
    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::uninitialized_move_n(std::begin(in), sz, first);

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

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

🌈 Possible output 🌈

initially, in: "One" "Definition" "Rule"
after move, in: "" "" ""
after move, out: "One" "Definition" "Rule"

🐞 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 👀

• uninitialized_move (C++17) — moves a range of objects to an uninitialized area of memory (function template).
• uninitialized_copy_n (C++11) — copies a number of objects to an uninitialized area of memory (function template).
• ranges::uninitialized_move_n (C++20) — moves a number of objects to an uninitialized area of memory (niebloid).

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💖 source/version: http://cppreference.com, dated 2024.06.10, for manual std::uninitialized_move_n(3). Now go move some objects into uninitialized memory and slay, bestie! ✨🫶