std::ranges::generate(3)

NAME
std::ranges::generate - std::ranges::generate

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_or_output_iterator O, std::sentinel_for<O> S,
             std::copy_constructible F >
   requires std::invocable<F&> && std::indirectly_writable<O, (1) (since C++20)
   std::invoke_result_t<F&>>
   constexpr O
       generate( O first, S last, F gen );
   template< class R, std::copy_constructible F >
   requires std::invocable<F&> && ranges::output_range<R,
   std::invoke_result_t<F&>> (2) (since C++20)
   constexpr ranges::borrowed_iterator_t<R>
       generate( R&& r, F gen );
   1) Assigns the result of successive invocations of the function object gen to each
   element in the range [first, last).
   2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first and
   ranges::end(r) as last.
   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, last - the range of elements to modify
   r - the range of elements to modify
   gen - the generator function object

Return value
   An output iterator that compares equal to last.

Complexity
   Exactly ranges::distance(first, last) invocations of gen() and assignments.

Possible implementation
  struct generate_fn
  {
      template<std::input_or_output_iterator O, std::sentinel_for<O> S,
               std::copy_constructible F>
      requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>>
      constexpr O operator()(O first, S last, F gen) const
      {
          for (; first != last; *first = std::invoke(gen), ++first)
          {}
          return first;
      }
      template<class R, std::copy_constructible F>
      requires std::invocable<F&> && ranges::output_range<R, std::invoke_result_t<F&>>
      constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, F gen) const
      {
          return (*this)(ranges::begin(r), ranges::end(r), std::move(gen));
      }
  };
  inline constexpr generate_fn generate {};

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 device;
     static std::mt19937 engine {device()};
     return distr(engine);
 }
 void iota(auto& v, int n)
 {
     std::ranges::generate(v, [&n]() mutable { return n++; });
 }
 void print(std::string_view comment, const auto& v)
 {
     for (std::cout << comment; int i : v)
         std::cout << i << ' ';
     std::cout << '\n';
 }
 int main()
 {
     std::array<int, 8> v;
     std::ranges::generate(v.begin(), v.end(), dice);
     print("dice: ", v);
     std::ranges::generate(v, dice);
     print("dice: ", v);
     iota(v, 1);
     print("iota: ", v);
 }

Possible output:
 dice: 4 3 1 6 6 4 5 5
 dice: 4 2 5 3 6 2 6 2
 iota: 1 2 3 4 5 6 7 8

See also
   ranges::generate_n saves the result of N applications of a function
   (C++20) (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)
   ranges::generate_random fills a range with random numbers from a uniform random bit
   (C++26) generator
                        (niebloid)
                        assigns the results of successive function calls to every
   generate element in a range
                        (function template)