# std::uninitialized_copy_n

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class InputIt, class Size, class NoThrowForwardIt > NoThrowForwardIt uninitialized_copy_n( InputIt first, Size count, NoThrowForwardIt d_first ); | (1) | (since C++11)
template< class ExecutionPolicy, class ForwardIt, class Size, class NoThrowForwardIt > NoThrowForwardIt uninitialized_copy_n( ExecutionPolicy&& policy, ForwardIt first, Size count, NoThrowForwardIt d_first ); | (2) | (since C++17)

1) Copies 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(*first);
If an exception is thrown during the initialization, 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 copy
count | - | the number of elements to copy
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 copied.

### 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>
    NoThrowForwardIt uninitialized_copy_n(InputIt first, Size count, NoThrowForwardIt d_first)
    {
        using T = 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))) T(*first);
        }
        catch (...)
        {
            for (; d_first != current; ++d_first)
                d_first->~T();
            throw;
        }
        return current;
    }

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <memory>
    #include <string>
    #include <tuple>
    #include <vector>

    int main()
    {
        std::vector<std::string> v = {"This", "is", "an", "example"};

        std::string* p;
        std::size_t sz;
        std::tie(p, sz) = std::get_temporary_buffer<std::string>(v.size());
        sz = std::min(sz, v.size());

        std::uninitialized_copy_n(v.begin(), sz, p);

        for (std::string* i = p; i != p + sz; ++i)
        {
            std::cout << *i << ' ';
            i->~basic_string<char>();
        }
        std::cout << '\n';

        std::return_temporary_buffer(p);
    }

Possible output:

    This is an example

### 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 2433 | C++11 | this algorithm might be hijacked by overloaded operator& | uses std::addressof
LWG 3870 | C++20 | this algorithm might create objects on a const storage | kept disallowed

### See also

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

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