std::ranges::partition

#include <algorithm>
#include <forward_list>
#include <functional>
#include <iostream>
#include <iterator>
#include <ranges>
#include <vector>
namespace ranges = std::ranges;
template<class I, std::sentinel_for<I> S, class Cmp = ranges::less>
requires std::sortable<I, Cmp>
void quicksort(I first, S last, Cmp cmp = Cmp {})
{
    using reference = std::iter_reference_t<I>;
    if (first == last)
        return;
    auto size = ranges::distance(first, last);
    auto pivot = ranges::next(first, size - 1);
    ranges::iter_swap(pivot, ranges::next(first, size / 2));
    auto tail = ranges::partition(first, pivot, [=](reference em)
    {
        return std::invoke(cmp, em, *pivot); // em < pivot
    });
    ranges::iter_swap(pivot, tail.begin());
    quicksort(first, tail.begin(), std::ref(cmp));
    quicksort(ranges::next(tail.begin()), last, std::ref(cmp));
}
int main()
{
    std::ostream_iterator<int> cout {std::cout, " "};
    std::vector<int> v {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    std::cout << "元のベクトル:  \t";
    ranges::copy(v, cout);
    auto tail = ranges::partition(v, [](int i) { return i % 2 == 0; });
    std::cout << "\n分割済みベクトル: \t";
    ranges::copy(ranges::begin(v), ranges::begin(tail), cout);
    std::cout << "│ ";
    ranges::copy(tail, cout);
    std::forward_list

（注：指示に従い、HTMLタグ・属性は翻訳せず、C++固有用語も保持しました。表示内容は元のまま維持されています）<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
    std::cout << "\n未ソートリスト: \t\t";
    ranges::copy(fl, cout);
    quicksort(ranges::begin(fl), ranges::end(fl), ranges::greater {});
    std::cout << "\nクイックソート済みリスト: \t";
    ranges::copy(fl, cout);
    std::cout << '\n';
}