NAME¶

std::uninitialized_copy_n - std::uninitialized_copy_n

Synopsis¶

Defined in header <memory>
template< class InputIt, class Size, class NoThrowForwardIt >

NoThrowForwardIt uninitialized_copy_n( InputIt first, Size count, (1) (since C++11)

NoThrowForwardIt d_first );
template< class ExecutionPolicy, class ForwardIt,

class Size, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_copy_n( ExecutionPolicy&& policy, (2) (since C++17)
ForwardIt first, Size
count,

NoThrowForwardIt d_first );

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 (since C++20)
undefined.

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
the effect description used a for loop with
the iteration discards the value
LWG 2133 C++98 expression ++d_first, ++first, --n, which of one operand to
results disable those ADLs
in argument-dependent lookups of operator,
LWG 2433 C++11 this algorithm might be hijacked by uses std::addressof
overloaded operator&
LWG 3870 C++20 this algorithm might create objects on a kept disallowed
const storage

See also¶

copies a range of objects to an uninitialized area of
uninitialized_copy memory
(function template)
ranges::uninitialized_copy_n copies a number of objects to an uninitialized area of
(C++20) memory
(niebloid)