# std::ranges::uninitialized_copy_n, std::ranges::uninitialized_copy_n_result

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< cpp‎ | memory

Defined in header `<memory>` |  |
Call signature |  |
template< std::input_iterator I, no-throw-input-iterator O, no-throw-sentinel-for<O> S > requires std::constructible_from<std::iter_value_t<O>, std::iter_reference_t<I>> uninitialized_copy_n_result<I, O> uninitialized_copy_n( I ifirst, std::iter_difference_t<I> count, O ofirst, S olast ); | (1) | (since C++20)
Helper types |  |
template< class I, class O > using uninitialized_copy_n_result = ranges::in_out_result<I, O>; | (2) | (since C++20)

Let \(\scriptsize N\)N be ranges::min(count, ranges::distance(ofirst, olast)), constructs \(\scriptsize N\)N elements in the output range `[`ofirst`,` olast`)`, which is an uninitialized memory area, from the elements in the input range beginning at ifirst.

The input range `[`ifirst`,` ifirst + count`)` must not overlap with the output range `[`ofirst`,` olast`)`.

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

The function has the effect equivalent to:

    auto ret = ranges::uninitialized_copy(std::counted_iterator(ifirst, count),
                                          std::default_sentinel, ofirst, olast);
    return {std::move(ret.in).base(), ret.out};

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 range of elements to copy from
count | - | the number of elements to copy
ofirst, olast | - | iterator-sentinel pair denoting the destination range

### Return value

{ifirst + N, ofirst + N}.

### Complexity

\(\scriptsize\mathcal{O}(N)\)𝓞(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_copy_n`, by using e.g. ranges::copy_n, if the value type of the output range is TrivialType.

### Possible implementation

    struct uninitialized_copy_n_fn
    {
        template<std::input_iterator I, no-throw-input-iterator O, no-throw-sentinel-for<O> S>
        requires std::constructible_from<std::iter_value_t<O>, std::iter_reference_t<I>>
        ranges::uninitialized_copy_n_result<I, O>
        operator()(I ifirst, std::iter_difference_t<I> count, O ofirst, S olast) const
        {
            O current{ofirst};
            try
            {
                for (; count > 0 && current != olast; ++ifirst, ++current, --count)
                    ranges::construct_at(std::addressof(*current), *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_copy_n_fn uninitialized_copy_n{};

### Example

Run this code

    #include <iomanip>
    #include <iostream>
    #include <memory>
    #include <string>

    int main()
    {
        const char* stars[]{ "Procyon", "Spica", "Pollux", "Deneb", "Polaris", };

        constexpr int n{4};
        alignas(alignof(std::string)) char out[n * sizeof(std::string)];

        try
        {
            auto first{reinterpret_cast<std::string*>(out)};
            auto last{first + n};
            auto ret{std::ranges::uninitialized_copy_n(std::begin(stars), n, first, last)};

            std::cout << "{ ";
            for (auto it{first}; it != ret.out; ++it)
                std::cout << std::quoted(*it) << ", ";
            std::cout << "};\n";

            std::ranges::destroy(first, last);
        }
        catch (...)
        {
            std::cout << "uninitialized_copy_n exception\n";
        }
    }

Output:

    { "Procyon", "Spica", "Pollux", "Deneb", };

### See also

ranges::uninitialized_copy (C++20) | copies a range of objects to an uninitialized area of memory (niebloid)
uninitialized_copy_n (C++11) | copies a number of objects to an uninitialized area of memory (function template)

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