sort by data member in class

sort_by.cpp

#include <algorithm>
#include <iterator>
#include <tuple>

template <
  typename Iterator,  // RandomAccessIterator
  typename... Ts,
  typename U = typename std::iterator_traits<Iterator>::value_type>
void sort_by(Iterator begin, Iterator end, Ts U::*... pm)
{
  static_assert(0 < sizeof...(Ts), "empty sort key");
  std::sort(begin, end, [&pm...](const U& x, const U& y){
    return std::tie(x.*pm...) < std::tie(y.*pm...);
  });
}


//----
#include <iostream>
#include <vector>

struct S { int x,y,z; };

int main()
{
  std::vector<S> vs = {{2,4,6}, {2,3,7}, {1,5,6}, {1,3,8}, {0,5,7}, {0,4,8}};

  std::cout << "--sort vector by x" << std::endl;
  sort_by(std::begin(vs), std::end(vs), &S::x);
  for (auto& e : vs)
    std::cout << e.x << e.y << e.z << std::endl;

  S as[] = {{2,4,6}, {2,3,7}, {1,5,6}, {1,3,8}, {0,5,7}, {0,4,8}};
  std::cout << "--sort array by (z,y)" << std::endl;
  sort_by(std::begin(as), std::end(as), &S::z, &S::y);
  for (auto& e : as)
    std::cout << e.x << e.y << e.z << std::endl;
}