HashTuple.h

Go to the documentation of this file.

// From https://stackoverflow.com/questions/7110301/generic-hash-for-tuples-in-unordered-map-unordered-set
// Maybe we should put something like this in a standard header?
namespace std {
  namespace {
    template <class T>
    inline void hash_combine(std::size_t& seed, const T& v)
    {
      seed ^= std::hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
    }

    // Recursive template code derived from Matthieu M.
    template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
    struct HashValueImpl {
      static void apply(size_t& seed, const Tuple& tuple)
      {
        HashValueImpl<Tuple, Index - 1>::apply(seed, tuple);
        hash_combine(seed, std::get<Index>(tuple));
      }
    };

    template <class Tuple>
    struct HashValueImpl<Tuple, 0> {
      static void apply(size_t& seed, const Tuple& tuple)
      {
        hash_combine(seed, std::get<0>(tuple));
      }
    };
  }

  template <class... TT>
  struct hash<std::tuple<TT...>> {
    size_t operator()(const std::tuple<TT...>& tt) const
    {
      size_t seed = 0;
      HashValueImpl<std::tuple<TT...>>::apply(seed, tt);
      return seed;
    }
  };
}