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

NAME
       std::uninitialized_default_construct  - std::uninitialized_default_con-
       struct

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

	  1)  Constructs  objects  of	type   typename	  iterator_traits<For-
       wardIt>::value_type in the
	  uninitialized	storage	designated by the range	[first,	last) by
	  default-initialization, as if	by

	for (; first !=	last; ++first)
	  ::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, last	       -	 the range of the elements to initial-
       ize
	  policy	       -	 the execution policy to use. See exe-
       cution 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	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>
	  void	uninitialized_default_construct(ForwardIt   first,   ForwardIt
       last)
	  {
	      using	Value	  =	typename     std::iterator_traits<For-
       wardIt>::value_type;
	      ForwardIt	current	= first;
	      try {
		  for (; current != last; ++current) {
		      ::new   (const_cast<void*>(static_cast<const    volatile
       void*>(
			  std::addressof(*current)))) Value;
		  }
	      }	 catch (...) {
		  std::destroy(first, current);
		  throw;
	      }
	  }

Example
       // Run this code

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

	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 {first + n};

		std::uninitialized_default_construct(first, last);

		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
	    // generally does not zero-fill  the  given	 uninitialized	memory
       area.
	    int	v[] { 1, 2, 3, 4 };
	    const int original[] { 1, 2, 3, 4 };
	    std::uninitialized_default_construct(std::begin(v),	std::end(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  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
	  uninitialized_value_construct		  in an	uninitialized area  of
       memory, defined
	  (C++17)				  by a range
						  (function template)
						  constructs  objects  by  de-
       fault-initialization
	  ranges::uninitialized_default_construct in an	uninitialized area  of
       memory, defined
	  (C++20)				  by a range
						  (niebloid)

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

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