std::uninitialized_move(3)

💖 std::uninitialized_move 💖

okay so bestie, welcome to the C++ Standard Libary page for std::uninitialized_move — yes, that's the manual name, and yes, this is page (3). We are moving objects into raw uninitialized memory and we are going to understand every single detail. No cap. 🎀

---

🎀 NAME 🎀

std::uninitialized_move — std::uninitialized_move. That's it, that's the name, main character energy only. 💅

---

✨ Synopsis ✨

Defined in header <memory>.

template< class InputIt, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_move( InputIt first, InputIt last,
                                     NoThrowForwardIt d_first );

(1) (since C++17)

template< class ExecutionPolicy, class ForwardIt, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_move( ExecutionPolicy&& policy,
                                     ForwardIt first, ForwardIt last,
                                     NoThrowForwardIt d_first );

(2) (since C++17)

1) Moves elements from the range [first, last) to an uninitialized memory area beginning at d_first as if by:

for (; first != last; ++d_first, (void) ++first)
    ::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, last) are left in a valid but unspecified state, and the objects already constructed are destroyed in an unspecified order.

If d_first + [0, std::distance(first, last)) overlaps with [first, last), the behavior is undefined. (since C++20)

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, last — the range of the elements to move.
• d_first — the beginning of the destination range.
• 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 🌈

Iterator to the element past the last element moved.

---

🍓 Complexity 🍓

Linear in the distance between first and last.

---

💀 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 NoThrowForwardIt> NoThrowForwardIt
uninitialized_move(InputIt first, InputIt last, NoThrowForwardIt
d_first) {
    using Value = typename
std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = d_first;
    try
    {
        for (; first != last; ++first, (void) ++current)
            ::new (static_cast<void*>(std::addressof(*current)))
Value(std::move(*first));
        return current;
    }
    catch (...)
    {
        std::destroy(d_first, current);
        throw;
    } }

---

🌸 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[]{"Home", "Work!"};
    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(std::begin(in), std::end(in), 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: "Home" "Work!"
after move, in: "" ""
after move, out: "Home" "Work!"

---

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

---

okay bestie that's the whole page. big kiss from http://cppreference.com, version/date 2024.06.10, std::uninitialized_move(3) — now go move some objects into uninitialized memory responsibly. 💋✨