# std::uninitialized_copy

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class InputIt, class NoThrowForwardIt > NoThrowForwardIt uninitialized_copy( InputIt first, InputIt last, NoThrowForwardIt d_first ); | (1) |
template< class ExecutionPolicy, class ForwardIt, class NoThrowForwardIt > NoThrowForwardIt uninitialized_copy( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, NoThrowForwardIt 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 (/* VOIDIFY */(*d_first))
typename std::iterator_traits<NoThrowForwardIt>::value_type(*first);
where /* VOIDIFY */(e) is:

static_cast<void*>(&e) | (until C++11)
static_cast<void*>(std::addressof(e)) | (since C++11)

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

If d_first`+` `[`​0​`,` std::distance(first, last)`)` overlaps with `[`first`,` last`)`, 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, 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.
- `NoThrowForwardIt` must meet the requirements of LegacyForwardIterator.
- No increment, assignment, comparison, or indirection through valid instances of `NoThrowForwardIt` may throw exceptions. Applying &* to a `NoThrowForwardIt` value must yield a pointer to its value type. (until C++11)

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

### Example

Run this code

    #include <cstdlib>
    #include <iostream>
    #include <memory>
    #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::cout << '\n';

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

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 866 | C++98 | given `T` as the value type of `NoThrowForwardIt`, if T::operator new exists, the program might be ill-formed | uses global replacement- new instead
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_n (C++11) | copies a number of objects to an uninitialized area of memory (function template)
ranges::uninitialized_copy (C++20) | copies a range of objects to an uninitialized area of memory (niebloid)

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