std::uninitialized_default_construct

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Defined in header <memory>
template< class ForwardIt >
void uninitialized_default_construct( ForwardIt first, ForwardIt last);
 (1) (since C++17)
template< class ExecutionPolicy, class ForwardIt >
void uninitialized_default_construct( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );
 (2) (since C++17)
1) Constructs objects of type typename iterator_traits<ForwardIt>::value_type in the uninitialized storage designated by the range [first, last) by default-initialization, as if by 
for (; first != last; ++first)
  ::new (static_cast<void*>(std::addressof(*first)))
      typename std::iterator_traits<ForwardIt>::value_type;
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
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, last - the range of the elements to initialize
policy - the execution policy to use. See execution policy for details.
Type requirements
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
No increment, assignment, comparison, or indirection through valid instances of ForwardIt may throw exceptions.

Return value

(none)

Complexity

Linear in the distance between first and last

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>
void uninitialized_default_construct(ForwardIt first, ForwardIt last)
{
    using Value = typename std::iterator_traits<ForwardIt>::value_type;
    ForwardIt current = first;
    try {
        for (; current != last; ++current) {
            ::new (static_cast<void*>(std::addressof(*current))) Value;
        }
    }  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 start and a count
(function template)
(C++17)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a range
(function template)
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