Building an unordered map with tuples as keys

Question

In a C++ program with Boost, I am trying to build an unordered map whose keys are tuples of doubles:

typedef boost::tuples::tuple

        

Answer 1

Actually, you could perfectly define a general hash function for boost::tuple. The only requirement is that it lives within the same namespace so that it is picked up by ADL.

I am actually surprised that they did not already write one.

namespace boost { namespace tuples {

  namespace detail {

    template ::value - 1>
    struct HashValueImpl
    {
      static void apply(size_t& seed, Tuple const& tuple)
      {
        HashValueImpl::apply(seed, tuple);
        boost::hash_combine(seed, tuple.get());
      }
    };

    template 
    struct HashValueImpl
    {
      static void apply(size_t& seed, Tuple const& tuple)
      {
        boost::hash_combine(seed, tuple.get<0>());
      }
    };
  } // namespace detail

  template 
  size_t hash_value(Tuple const& tuple)
  {
    size_t seed = 0;
    detail::HashValueImpl::apply(seed, tuple);
    return seed;
  }

} }

Note: I have only proved it correct, I haven't tested it.