pair<int,int> pair as key of unordered_map issue

Question

My code:

 typedef pair<int,int> Pair
  tr1::unordered_map<Pair,bool> h;
  h.insert(make_pair(Pair(0,0),true));

Erorr

 undefined reference to `std::tr1::hash<std::pair<int, int> >::operator()(std::pair<int, int>) const'

Something I need to fix?

thanks

Answer 1

This happens because there is no specialization for std::tr1::hash<Key> with Key = std::pair<int, int>. You must to specialize std::tr1::hash<Key> with Key = std::pair<int, int> before declaring tr1::unordered_map<Pair,bool> h;. This happens because std don't know how to hash a pair<int, int>.

Following there is a example of how to specialize std::tr1::hash<>

template <>
struct std::tr1::hash<std::pair<int, int> > {
public:
        size_t operator()(std::pair<int, int> x) const throw() {
             size_t h = SOMETHING;//something with x   
             return h;
        }
};