unordered_map.sort_by_key.cxx

#include <unordered_map>
#include <map>
#include <vector>
#include <algorithm>
#include <type_traits>
#include <string>
#include <iostream>

int main()
{
  using namespace std;
  
  std::unordered_map<std::string, const int> master =
  {{ {"ABC", 123}
  ,  {"GHI", 345}
  ,  {"DEF", 123}
  ,  {"XYZ", 234}
  ,  {"UVW", 234}
  }};
  
  std::cout << "=== master data(unordered_map) ===\n";
  for(const auto& p : master)
    std::cout << p.first << " : " << p.second << "\n";
  
  auto sorted_by_key =
    std::map<decltype(master)::key_type, decltype(master)::mapped_type>
    (std::begin(master), std::end(master))
    ;
  
  std::cout << "=== sort by key ===\n";
  for(const auto& p : sorted_by_key)
    std::cout << p.first << " : " << p.second << "\n";
  
  std::vector
  < std::pair
    < std::remove_const<decltype(sorted_by_key)::key_type>::type
    , std::remove_const<decltype(sorted_by_key)::mapped_type>::type
    >
  > sorted_by_mapped(std::begin(sorted_by_key), std::end(sorted_by_key));
  
  std::stable_sort
  ( std::begin(sorted_by_mapped), std::end(sorted_by_mapped)
  , [](const decltype(sorted_by_mapped)::value_type& a, const decltype(sorted_by_mapped)::value_type& b)
    { return a.second < b.second; }
  );
  
  std::cout << "=== sort by mapped (with stable key)\n";
  for(const auto& p : sorted_by_mapped)
    std::cout << p.first << " : " << p.second << "\n";
}

result:

=== master data(unordered_map) ===
UVW : 234
XYZ : 234
DEF : 123
GHI : 345
ABC : 123
=== sort by key ===
ABC : 123
DEF : 123
GHI : 345
UVW : 234
XYZ : 234
=== sort by mapped (with stable key)
ABC : 123
DEF : 123
UVW : 234
XYZ : 234
GHI : 345

wandbox: http://melpon.org/wandbox/permlink/IOjquy9kaC65Fpgo
ideone: http://ideone.com/F5Tdb2