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std::uninitialized_move_n - cppreference.com

Copies the elements from

(using move semantics if supported) to an uninitialized memory area beginning at

as if by

for (; count > 0; ++d_first, (void) ++first, --count)
    ::new (voidify(*d_first))
        typename std::iterator_traits<NoThrowForwardIt>::value_type(/* value */);
return {first, d_first};

where /* value */ is std::move(*first) if *first is of an lvalue reference type, or *first otherwise.

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

2) Same as (1), but executed according to policy.

This overload participates in overload resolution only if all following conditions are satisfied:

If d_first + [​0​, count) overlaps with first + [​0​, count), the behavior is undefined.

As described above.

Linear in count.

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.

When the input iterator deferences to an rvalue, the behavior of std::uninitialized_move_n is same as std::uninitialized_copy_n.

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

#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"

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

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