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std::uninit...construct_n(3)  C++ Standard Libary std::uninit...construct_n(3)

NAME
       std::uninitialized_default_construct_n	  -	std::uninitialized_de-
       fault_construct_n

Synopsis
	  Defined in header <memory>
	  template< class ForwardIt, class Size	>
	  ForwardIt uninitialized_default_construct_n( ForwardIt  first,  Size
       (1) (since C++17)
	  n );
	  template< class ExecutionPolicy, class ForwardIt, class Size >
	  ForwardIt    uninitialized_default_construct_n(    ExecutionPolicy&&
       (2) (since C++17)
	  policy, ForwardIt first, Size	n );

	  1)  Constructs  n  objects  of  type	typename  iterator_traits<For-
       wardIt>::value_type in
	  the  uninitialized  storage starting at first	by default-initializa-
       tion, as	if by

	for (; n > 0; (void) ++first, --n)
	  ::new	(/*VOIDIFY*/(*first))
	      typename std::iterator_traits<ForwardIt>::value_type;

	  where	/*VOIDIFY*/(e) is:

	     static_cast<void*>(std::addressof(e))
       (until C++20)
	     const_cast<void*>(static_cast<const		      volatile
       (since C++20)
	     void*>(std::addressof(e)))

	  If an	exception is thrown during the initialization, the objects al-
       ready 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>>
	  (until C++20)
	  std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
	  (since C++20)	is true.

Parameters
	  first		     -		the beginning of the range of elements
       to initialize
	  n		    -	       the number of elements to construct
	  policy	    -	       the execution policy to use. See	execu-
       tion 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
	  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	standard policies, std::termi-
       nate 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   (const_cast<void*>(static_cast<const    volatile
       void*>(
			  std::addressof(*current)))) T;
		  }
		  return current;
	      }	 catch (...) {
		  std::destroy(first, current);
		  throw;
	      }
	  }

Example
       // Run this code

	#include <iostream>
	#include <memory>
	#include <string>

	int main()
	{
	    struct S { std::string m{ "Default value" }; };

	    constexpr int n {3};
	    alignas(alignof(S))	unsigned char mem[n * sizeof(S)];

	    try
	    {
		auto first {reinterpret_cast<S*>(mem)};
		auto last = std::uninitialized_default_construct_n(first, n);

		for (auto it {first}; it != last; ++it)	{
		    std::cout << it->m << '\n';
		}

		std::destroy(first, last);
	    }
	    catch(...)
	    {
		std::cout << "Exception!\n";
	    }

	    //	Notice that for	"trivial types"	the uninitialized_default_con-
       struct_n
	    // generally does not zero-initialize the given uninitialized mem-
       ory area.
	    int	v[] { 1, 2, 3, 4 };
	    const int original[] { 1, 2, 3, 4 };
	    std::uninitialized_default_construct_n(std::begin(v),
       std::size(v));
	    // for (const int i	: v) { std::cout << i << ' '; }
	    // Maybe undefined behavior, pending CWG 1997.
	    std::cout <<
		(std::memcmp(v,	original, sizeof(v)) ==	0 ?  "Unmodified\n"  :
       "Modified\n");
	    // The result is unspecified.
	}

Possible output:
	Default	value
	Default	value
	Default	value
	Unmodified

See also
						    constructs objects by
	  uninitialized_default_construct	     default-initialization in
       an uninitialized
	  (C++17)				    area of memory, defined by
       a range
						    (function template)
						    constructs	 objects    by
       value-initialization
	  uninitialized_value_construct_n	     in	 an uninitialized area
       of memory,
	  (C++17)				    defined by a start	and  a
       count
						    (function template)
						    constructs objects by
	  ranges::uninitialized_default_construct_n  default-initialization in
       an uninitialized
	  (C++20)				    area of memory, defined by
       a start and
						    count
						    (niebloid)

http://cppreference.com		  2022.07.31	  std::uninit...construct_n(3)

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