std::uninitialized_default_construct_n

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 )
{
    typedef typename std::iterator_traits<ForwardIt>::value_type Value;
    ForwardIt current = first;
    try {
        for (; n > 0 ; (void) ++current, --n) {
            ::new (static_cast<void*>(std::addressof(*current))) Value;
        }
        return current;
    }  catch (...) {
        for (; first != current; ++first) {
            first->~Value();
        }
        throw;
    }
}

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
Relational operators
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
`optional`, `any` and `variant` (C++17)
optional
any
variant
in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t
Pairs and tuples
pair
tuple (C++11)
apply (C++17)
make_from_tuple (C++17)
piecewise_construct_t (C++11)
piecewise_construct (C++11)
integer_sequence (C++14)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
Elementary string conversions
to_chars (C++17)
from_chars (C++17)
chars_format (C++17)
Type operations
declval (C++11)
as_const (C++17)

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