Question

I need to define an unordered_map like this unordered_map<pair<int, int>, *Foo>, what is the syntax for defining and passing a hash and equal functions to this map?

I’ve tried passing to it this object:

class pairHash{
public:
    long operator()(const pair<int, int> &k) const{
        return k.first * 100 + k.second;
    }
};

and no luck:

unordered_map<pair<int, int>, int> map = unordered_map<pair<int, int>, int>(1,
*(new pairHash()));

I have no Idea what is the size_type_Buskets means so I gave it 1.
What is the right way to do it?
Thanks.

Answer 1

This is an unfortunate omission in C++11; Boost has the answer in terms of hash_combine. Feel free to just paste it from them! Here’s how I hash pairs:

template <class T>
inline void hash_combine(std::size_t & seed, const T & v)
{
  std::hash<T> hasher;
  seed ^= hasher(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}

namespace std
{
  template<typename S, typename T> struct hash<pair<S, T>>
  {
    inline size_t operator()(const pair<S, T> & v) const
    {
      size_t seed = 0;
      ::hash_combine(seed, v.first);
      ::hash_combine(seed, v.second);
      return seed;
    }
  };
}

You can use hash_combine as the basis for many other things, like tuples and ranges, so you could hash an entire (ordered) container, for example, as long as each member is individually hashable.

Now you can just declare a new map:

std::unordered_map<std::pair<int, int>, my_mapped_type> mymap;

---

If you want to use your homebrew hasher (which hasn’t got good statistical properties), you have to specify the template parameters explicitly:

std::unordered_map<std::pair<int,int>, int, pairHash> yourmap;

Note that there’s no need to specify a copy of a hasher object, as the default is to default-construct one for you.