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std::ranges::fill_n(3)	      C++ Standard Libary	std::ranges::fill_n(3)

NAME
       std::ranges::fill_n - std::ranges::fill_n

Synopsis
	  Defined in header <algorithm>
	  Call signature
	  template<    class	T,    std::output_iterator<const   T&>	 O   >
       (since C++20)
	  constexpr O fill_n( O	first, std::iter_difference_t<O> n,  const  T&
       (until C++26)
	  value	);
	  template< class O, class T = std::iter_value_t<O> >

	  requires	    std::output_iterator<O,	     const	   T&>
       (since C++26)

	  constexpr O fill_n( O	first, std::iter_difference_t<O> n, const T&
	  value	);

	  Assigns the given value to all elements in the range [first, first +
       n).

	  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	elements to modify
	  n	- number of elements to	modify
	  value	- the value to be assigned

Return value
	  An output iterator that compares equal to first + n.

Complexity
	  Exactly n assignments.

Possible implementation
	  struct fill_n_fn
	  {
	      template<class O,	class T	= std::iter_value_t<O>>
	      requires std::output_iterator<O, const T&>
	      constexpr	 O  operator()(O  first,  std::iter_difference_t<O> n,
       const T&	value) const
	      {
		  for (std::iter_difference_t<O> i {}; i != n; ++first,	++i)
		      *first = value;
		  return first;
	      }
	  };

	  inline constexpr fill_n_fn fill_n {};

Notes
		    Feature-test      macro		    Value	   Std
       Feature
	  __cpp_lib_algorithm_default_value_type  202403 (C++26) List-initial-
       ization for
								algorithms

Example
       // Run this code

	#include <algorithm>
	#include <complex>
	#include <iostream>
	#include <string>
	#include <vector>

	void println(const auto& v)
	{
	    for	(const auto& elem : v)
		std::cout << ' ' << elem;
	    std::cout << '\n';
	}

	int main()
	{
	    constexpr auto n{8};

	    std::vector<std::string> v(n, "");
	    println(v);

	    std::ranges::fill_n(v.begin(), n, "");
	    println(v);

	    std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
	    println(nums);
	    #ifdef __cpp_lib_algorithm_default_value_type
		std::ranges::fill_n(nums.begin(), 2, {4, 2});
	    #else
		std::ranges::fill_n(nums.begin(),  2,  std::complex<double>{4,
       2});
	    #endif
	    println(nums);
	}

Output:

	 (1,3) (2,2) (4,8)
	 (4,2) (4,2) (4,8)

See also
	  ranges::fill		  assigns a range of elements a	certain	value
	  (C++20)		  (niebloid)
	  ranges::copy_n	   copies  a number of elements	to a new loca-
       tion
	  (C++20)		  (niebloid)
	  ranges::generate	  saves	the result of a	function in a range
	  (C++20)		  (niebloid)
	  ranges::transform	  applies a function to	a range	of elements
	  (C++20)		  (niebloid)
	  ranges::generate_random fills	a range	with  random  numbers  from  a
       uniform random bit
	  (C++26)		  generator
				  (niebloid)
	  fill_n		   copy-assigns	 the given value to N elements
       in a range
				  (function template)

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

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