std::uninitialized_copy_n() algorithm

od C++17

// (1)
template< class InputIt, class Size, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_copy_n( InputIt first, Size count, NoThrowForwardIt d_first );

// (2)
template< class ExecutionPolicy, class ForwardIt, class Size, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_copy_n(
  ExecutionPolicy&& policy,
  ForwardIt first,
  Size count,
  NoThrowForwardIt d_first
);

od C++11

// (1)
template< class InputIt, class Size, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_copy_n( InputIt 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);
  uwaga

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

  Undefined Behaviour

  If d_first + [0, n) overlaps with [first; last), the behavior is undefined. (od C++20)

• (2) Same as (1), but executed according to policy.

Overload Resolution

These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (od C++20) is true.

Parameters

first	The beginning of the range of the elements to copy.
copy	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	LegacyInputIterator
ForwardIt NoThrowForwardIt	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 overloads with a template parameter named ExecutionPolicy report 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 none other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

uninitialized_copy_n (1)

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;
}

Examples

Main.cpp

#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);
}

Output

This is an example