# std::uninitialized_move_n

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class InputIt, class Size, class NoThrowForwardIt > std::pair<InputIt, NoThrowForwardIt> uninitialized_move_n( InputIt first, Size count, NoThrowForwardIt d_first ); | (1) | (since C++17)
template< class ExecutionPolicy, class ForwardIt, class Size, class NoThrowForwardIt > std::pair<ForwardIt, NoThrowForwardIt> uninitialized_move_n( ExecutionPolicy&& policy, ForwardIt first, Size count, NoThrowForwardIt d_first ); | (2) | (since C++17)

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)

2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless

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 | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3870 | C++20 | this algorithm might create objects on a const storage | 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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