# std::uninitialized_copy

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class InputIt, class ForwardIt > ForwardIt uninitialized_copy( InputIt first, InputIt last, ForwardIt d_first ); | (1) |
template< class ExecutionPolicy, class InputIt, class ForwardIt > ForwardIt uninitialized_copy( ExecutionPolicy&& policy, InputIt first, InputIt last, ForwardIt d_first ); | (2) | (since C++17)

1) Copies elements from the range `[first, last)` to an uninitialized memory area beginning at `d_first` as if by

    for (; first != last; ++d_first, (void) ++first)
       ::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, last | - | 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 the distance between `first` and `last`

### 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 ForwardIt>
    ForwardIt uninitialized_copy(InputIt first, InputIt last, ForwardIt d_first)
    {
        typedef typename std::iterator_traits<ForwardIt>::value_type Value;
        ForwardIt current = d_first;
        try {
            for (; first != last; ++first, (void) ++current) {
                ::new (static_cast<void*>(std::addressof(*current))) Value(*first);
            }
            return current;
        } catch (...) {
            for (; d_first != current; ++d_first) {
                d_first->~Value();
            }
            throw;
        }
    }

### Example

Run this code

    #include <iostream>
    #include <memory>
    #include <cstdlib>
    #include <string>

    int main()
    {
        const char *v[] = {"This", "is", "an", "example"};

        auto sz = std::size(v);

        if(void *pbuf = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz))
        {
            try
            {
                auto first = static_cast<std::string*>(pbuf);
                auto last = std::uninitialized_copy(std::begin(v), std::end(v), first);

                for (auto it = first; it != last; ++it)
                    std::cout << *it << '_';

                std::destroy(first, last);
            }
            catch(...) {}
            std::free(pbuf);
        }
    }

Output:

    This_is_an_example_

### See also

uninitialized_copy_n (C++11) | copies a number of objects to an uninitialized area of memory (function template)

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