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

NAME
       std::uninitialized_value_construct_n   -	 std::uninitialized_value_con-
       struct_n

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

	  ForwardIt uninitialized_value_construct_n( ExecutionPolicy&&
	  policy,
       (2) (since C++17)

						     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 value-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 objects al-
       ready constructed
	  are destroyed	in an unspecified order.
	  2) Same as (1), but executed according to policy. This overload par-
       ticipates in
	  overload resolution only if

	  std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  is	 true.
       (until
										    C++20)
	  std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  is
       true. (since
										    C++20)

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	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_value_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_value_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_value_con-
       struct_n
	    // zero-initializes	the given uninitialized	memory area.
	    int	v[]{1, 2, 3, 4};
	    for	(const int i : v)
		std::cout << i << ' ';
	    std::cout << '\n';
	    std::uninitialized_value_construct_n(std::begin(v),	std::size(v));
	    for	(const int i : v)
		std::cout << i << ' ';
	    std::cout << '\n';
	}

Output:
	Default	value
	Default	value
	Default	value
	1 2 3 4
	0 0 0 0

	  Defect reports

	  The following	behavior-changing defect reports were applied retroac-
       tively to
	  previously published C++ standards.

	     DR	      Applied	 to		    Behavior	as   published
       Correct behavior
	  LWG 3870 C++20      this algorithm might create objects on  a	 const
       kept disallowed
			      storage

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

http://cppreference.com		  2024.06.10	  std::uninit...construct_n(3)

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