Add the missing hash function for pairs in the standard.

hash_pair.cpp

#include <string>
#include <unordered_map>

namespace std {

template <typename F, typename S>
struct hash<std::pair<F, S>>
{
  std::size_t operator()(std::pair<F, S> const &p) const
  {
    return std::hash<F>{}(p.first) ^ (std::hash<S>{}(p.second) << 1);

    // other hashing options:
    // return std::hash<F>{}(p.first) ^ std::hash<S>{}(p.second);
    // return std::hash<F>{}(p.first) + std::hash<S>{}(p.second) + 1;

    // further information about hash function:
    // https://stackoverflow.com/questions/5889238/why-is-xor-the-default-way-to-combine-hashes
  }
};

};

int main()
{
  typedef std::pair<int, std::string> Key;
  typedef short Value;

  auto p1 = Key(33, "France");
  auto p2 = Key(44, "England");
  auto p3 = Key(852, "Hong Kong");

  std::hash<Key>{}(p1);
  std::hash<Key>{}(p2);
  std::hash<Key>{}(p3);

  std::unordered_map<Key, Value> container;

  container.insert(std::make_pair<Key, Value>(std::move(p1), 100));
  container.insert(std::make_pair<Key, Value>(std::move(p2), 200));
  container.insert(std::make_pair<Key, Value>(std::move(p3), 300));
}