sort-functional-style.cpp

#include <vector>
#include <functional>
#include <algorithm>
#include <iterator>
#include <iostream>
#include <utility>
#include <type_traits>

template<typename T> auto read_vector() {
    return std::vector<T>{std::istream_iterator<T>{std::cin}, std::istream_iterator<T>{}};
}

template<typename T> auto operator+(std::vector<T> lhs, const std::vector<T>& rhs) {
    std::copy(std::begin(rhs), std::end(rhs), std::back_inserter(lhs));
    return lhs;
}

template<typename T> auto operator+(std::vector<T> lhs, const T& rhs) {
    lhs.push_back(rhs);
    return lhs;
}

template<typename T, typename F> auto filter(const std::vector<T>& v, F&& f) {
    auto ret = std::vector<T>{};
    std::copy_if(std::begin(v), std::end(v), std::back_inserter(ret), std::forward<F>(f));
    return ret;
}

template<typename T> std::vector<T> quicksort(const std::vector<T>& v) {
    return v.size() < 2 ? v :
        quicksort(filter(v, std::bind(std::greater<T>{}, v.front(), std::placeholders::_1))) +
        v.front() +
        quicksort(filter(v, std::bind(std::less<T>{}, v.front(), std::placeholders::_1)));
}

int main() {
    const auto v = quicksort(read_vector<int>());

    std::for_each(std::begin(v), std::end(v), [] (auto item) { std::cout << item << ' '; });    
    std::cout << std::endl;
}