std::uninitialized_copy_n

 

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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 participates in overload resolution only if

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 2133	C++98	the effect description used a for loop with the iteration expression ++d_first, ++first, --n, which results in argument-dependent lookups of operator,	discards the value of one operand to disable those ADLs
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)