std::uninitialized_default_construct_n

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Low level memory management
Defined in header <memory>
template< class ForwardIt, class Size >
ForwardIt uninitialized_default_construct_n( ForwardIt first, Size n );
 (1) (since C++17)
template< class ExecutionPolicy, class ForwardIt, class Size >
ForwardIt uninitialized_default_construct_n( ExecutionPolicy&& policy, ForwardIt first, Size n );
 (2) (since C++17)
1) Constructs n objects of type typename iterator_traits<ForwardIt>::value_type in the uninitialized storage starting at first by default-initialization, as if by 
for (; n > 0; (void) ++first, --n)
  ::new (static_cast<void*>(std::addressof(*first)))
      typename std::iterator_traits<ForwardIt>::value_type;
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 elements to initialize
n - the number of elements to initialize
policy - the execution policy to use. See execution policy for details.
Type requirements
-
ForwardIt must meet the requirements of ForwardIterator.
-
No increment, assignment, comparison, or indirection through valid instances of ForwardIt may throw exceptions.

Return value

The end of the range of objects (i.e., std::next(first, n)).

Complexity

Linear in n.

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 three 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 ForwardIt, class Size>
ForwardIt uninitialized_default_construct_n( ForwardIt first, Size n )
{
    using T = typename std::iterator_traits<ForwardIt>::value_type;
    ForwardIt current = first;
    try {
        for (; n > 0 ; (void) ++current, --n) {
            ::new (static_cast<void*>(std::addressof(*current))) T;
        }
        return current;
    }  catch (...) {
        std::destroy(first, current);
        throw;
    }
}

Example

This section is incomplete
Reason: no example

See also

(C++17)
 constructs objects by default-initialization in an uninitialized area of memory, defined by a range
(function template)
(C++17)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
(function template)
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