# std::ranges::uninitialized_move_n, std::ranges::uninitialized_move_n_result

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< cpp‎ | memory

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>> uninitialized_move_n_result<I, O> uninitialized_move_n( I ifirst, std::iter_difference_t<I> n, O ofirst, S olast ); | (1) | (since C++20)
Helper types |  |
template< class I, class O > using uninitialized_move_n_result = ranges::in_out_result<I, O>; | (2) | (since C++20)

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.

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