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# std::uninitialized_default_construct_n

From cppreference.com
< cpp‎ | memory

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

### [edit] 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.

### [edit] Return value

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

### [edit] Complexity

Linear in `n`.

### [edit] 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.

### [edit] 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;
        }
    }

### [edit] Example

| This section is incomplete Reason: no example

### [edit] See also

uninitialized_default_construct (C++17) | constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template) [edit]
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) [edit]

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- Todo no example