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std::ranges...lized_fill_n(3) C++ Standard Libarystd::ranges...lized_fill_n(3)

NAME
       std::ranges::uninitialized_fill_n - std::ranges::uninitialized_fill_n

Synopsis
	  Defined in header <memory>
	  Call signature
	  template< no-throw-forward-range I, class T >

	  requires   std::constructible_from<std::iter_value_t<I>,  const  T&>
       (since C++20)

	  I uninitialized_fill_n( I first, std::iter_difference_t<I> n,	const
	  T& x );

	  Constructs n copies of the given value x in an uninitialized	memory
       area, designated
	  by the range [first, first + n), as if by

	for (; n--; ++first) {
	    ::new (static_cast<void*>(std::addressof(*first)))
		std::remove_reference_t<std::iter_reference_t<I>>(x);
	}

	  If an	exception is thrown during the initialization, the objects al-
       ready constructed
	  are destroyed	in an unspecified order.

	  The  function-like  entities	described  on this page	are niebloids,
       that is:

	    * Explicit template	argument lists cannot be specified when	 call-
       ing any of them.
	    * None of them are visible to argument-dependent lookup.
	    *  When  any of them are found by normal unqualified lookup	as the
       name to the left
	      of the function-call operator, argument-dependent	lookup is  in-
       hibited.

	  In  practice,	 they  may be implemented as function objects, or with
       special compiler
	  extensions.

Parameters
	  first	- the beginning	of the range of	the elements to	initialize
	  n	- number of elements to	construct
	  x	- the value to construct the elements with

Return value
	  An iterator equal to first + n.

Complexity
	  Linear in n.

Exceptions
	  The exception	thrown on construction of the elements in the destina-
       tion range, if
	  any.

Notes
	  An implementation may	improve	the efficiency of  the	ranges::unini-
       tialized_fill_n,
	  e.g.	by using ranges::fill_n, if the	value type of the output range
       is TrivialType.

Possible implementation
	  struct uninitialized_fill_n_fn
	  {
	      template<no-throw-forward-range I, class T>
	      requires std::constructible_from<std::iter_value_t<I>, const T&>
	      I	operator()(I first, std::iter_difference_t<I> n, const	T&  x)
       const
	      {
		  I rollback{first};
		  try
		  {
		      for (; n-- > 0; ++first)
			  ranges::construct_at(std::addressof(*first), x);
		      return first;
		  }
		  catch	(...) // rollback: destroy constructed elements
		  {
		      for (; rollback != first;	++rollback)
			  ranges::destroy_at(std::addressof(*rollback));
		      throw;
		  }
	      }
	  };

	  inline constexpr uninitialized_fill_n_fn uninitialized_fill_n{};

Example
       // Run this code

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

	int main()
	{
	    constexpr int n{3};
	    alignas(alignof(std::string)) char out[n * sizeof(std::string)];

	    try
	    {
		auto first{reinterpret_cast<std::string*>(out)};
		auto  last  = std::ranges::uninitialized_fill_n(first, n, "cp-
       preference");

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

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

Output:
	cppreference
	cppreference
	cppreference

	  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
	  ranges::uninitialized_fill copies an object to an uninitialized area
       of memory,
	  (C++20)		     defined by	a range
				     (niebloid)
				     copies an object to an uninitialized area
       of memory,
	  uninitialized_fill_n	     defined by	a start	and a count
				     (function template)

http://cppreference.com		  2024.06.10	 std::ranges...lized_fill_n(3)

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