std::ranges::generate_n(3)

NAME
std::ranges::generate_n - std::ranges::generate_n

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_or_output_iterator O, std::copy_constructible F
   >
   requires std::invocable<F&> && std::indirectly_writable<O, (since C++20)
   std::invoke_result_t<F&>>
   constexpr O
       generate_n( O first, std::iter_difference_t<O> n, F gen );
   Assigns the result of successive invocations of the function object gen to each
   element in the range [first, first + n), if 0 < n. Does nothing otherwise.
   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
   gen - the generator function object.

Return value
   Iterator one past the last element assigned if 0 < count, first otherwise.

Complexity
   Exactly n invocations of gen() and assignments.

Possible implementation
  struct generate_n_fn
  {
      template<std::input_or_output_iterator O, std::copy_constructible F>
      requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>>
      constexpr O operator()(O first, std::iter_difference_t<O> n, F gen) const
      {
          for (; n-- > 0; *first = std::invoke(gen), ++first)
          {}
          return first;
      }
  };
  inline constexpr generate_n_fn generate_n {};

Example
// Run this code
 #include <algorithm>
 #include <array>
 #include <iostream>
 #include <random>
 #include <string_view>
 auto dice()
 {
     static std::uniform_int_distribution<int> distr {1, 6};
     static std::random_device engine;
     static std::mt19937 noise {engine()};
     return distr(noise);
 }
 void print(const auto& v, std::string_view comment)
 {
     for (int i : v)
         std::cout << i << ' ';
     std::cout << '(' << comment << ")\n";
 }
 int main()
 {
     std::array<int, 8> v;
     std::ranges::generate_n(v.begin(), v.size(), dice);
     print(v, "dice");
     std::ranges::generate_n(v.begin(), v.size(), [n {0}] mutable { return n++; });
     // same effect as std::iota(v.begin(), v.end(), 0);
     print(v, "iota");
 }

Possible output:
 5 5 2 2 6 6 3 5 (dice)
 0 1 2 3 4 5 6 7 (iota)

See also
   ranges::generate saves the result of a function in a range
   (C++20) (niebloid)
   ranges::generate_random fills a range with random numbers from a uniform random bit
   (C++26) generator
                        (niebloid)
   ranges::fill assigns a range of elements a certain value
   (C++20) (niebloid)
   ranges::fill_n assigns a value to a number of elements
   (C++20) (niebloid)
   ranges::transform applies a function to a range of elements
   (C++20) (niebloid)
                        assigns the results of successive function calls to N
   generate_n elements in a range
                        (function template)