std::ranges::shuffle(3)

NAME
std::ranges::shuffle - std::ranges::shuffle

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::random_access_iterator I, std::sentinel_for<I> S, class
   Gen >
                        (since
   requires std::permutable<I> && (1) C++20)
            std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
       I shuffle( I first, S last, Gen&& gen );
   template< ranges::random_access_range R, class Gen >
   requires std::permutable<ranges::iterator_t<R>> && (since
            std::uniform_random_bit_generator<std::remove_reference_t<Gen>> (2) C++20)
   ranges::borrowed_iterator_t<R>
       shuffle( R&& r, Gen&& gen );
   1) Reorders the elements in the given range [first, last) such that each possible
   permutation of those elements has equal probability of appearance.
   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 shuffle randomly
   r - the range of elements to shuffle randomly
   gen - the random number generator

Return value
   An iterator equal to last.

Complexity
   Exactly (last - first) - 1 swaps.

Possible implementation
   struct shuffle_fn
   {
       template<std::random_access_iterator I, std::sentinel_for<I> S, class Gen>
       requires std::permutable<I> &&
                std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
       I operator()(I first, S last, Gen&& gen) const
       {
           using diff_t = std::iter_difference_t<I>;
           using distr_t = std::uniform_int_distribution<diff_t>;
           using param_t = typename distr_t::param_type;
           distr_t D;
           const auto n {last - first};
           for (diff_t i {1}; i < n; ++i)
               ranges::iter_swap(first + i, first + D(gen, param_t(0, i)));
           return ranges::next(first, last);
       }
       template<ranges::random_access_range R, class Gen>
       requires std::permutable<ranges::iterator_t<R>> &&
                std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
       ranges::borrowed_iterator_t<R> operator()(R&& r, Gen&& gen) const
       {
           return (*this)(ranges::begin(r), ranges::end(r), std::forward<Gen>(gen));
       }
   };
   inline constexpr shuffle_fn shuffle {};

Example
// Run this code
 #include <algorithm>
 #include <array>
 #include <iostream>
 #include <random>
 void print(const auto& a)
 {
     for (const auto e : a)
         std::cout << e << ' ';
     std::cout << '\n';
 }
 int main()
 {
     std::array a {'A', 'B', 'C', 'D', 'E', 'F'};
     print(a);
     std::random_device rd;
     std::mt19937 gen {rd()};
     for (int i {}; i != 3; ++i)
     {
         std::ranges::shuffle(a, gen);
         print(a);
     }
 }

Possible output:
 A B C D E F
 F E A C D B
 E C B F A D
 B A E C F D

See also
   ranges::next_permutation generates the next greater lexicographic permutation of a
   (C++20) range of elements
                        (niebloid)
   ranges::prev_permutation generates the next smaller lexicographic permutation of a
   (C++20) range of elements
                        (niebloid)
   random_shuffle
   shuffle randomly re-orders elements in a range
   (until C++17) (function template)
   (C++11)