# std::uninitialized_copy_n

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< cpp‎ | memory

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

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

### Parameters

first | - | the beginning of the range of the 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.
- No increment, assignment, comparison, or indirection through valid instances of `ForwardIt` 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 ForwardIt>
    ForwardIt uninitialized_copy_n(InputIt first, Size count, ForwardIt d_first)
    {
        typedef typename std::iterator_traits<ForwardIt>::value_type Value;
        ForwardIt current = d_first;
        try {
            for (; count > 0; ++first, (void) ++current, --count) {
                ::new (static_cast<void*>(std::addressof(*current))) Value(*first);
            }
        } catch (...) {
            for (; d_first != current; ++d_first) {
                d_first->~Value();
            }
            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::return_temporary_buffer(p);
    }

Output:

    This is an example

### See also

uninitialized_copy | copies a range of objects to an uninitialized area of memory (function template)

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