std::uninitialized_copy_n

 

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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 iterator_traits<ForwardIt>::value_type(*first);

If an exception is thrown during the initialization, the function has no effects.

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
- 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)