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 calling 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 inhibited.
   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-initialization 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 location
   (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)