user defined hash function in C++ STL unordered_map and unordered_set

Reference :
http://mikecvet.wordpress.com/2011/01/28/customizing-tr1-unordered_map-hashing-and-equality-functions/
http://en.cppreference.com/w/cpp/utility/hash
http://stackoverflow.com/questions/17016175/c-unordered-map-using-a-custom-class-type-as-the-key

template<class Key,          class T,          class Hash = hash<Key>,          class Pred = std::equal_to<Key>,          class Alloc = std::allocator<std::pair<const Key, T> > > typedef struct {   long operator() (const AggregateKey &k) const { return my_hash_fnct (k); } } AggregateKeyHash; typedef struct {   bool operator() (const AggregateKey &x, const AggregateKey &y) const { return my_eq_test (x, y); } } AggregateKeyEq; AggregateKey k; {...} // Now, hash value generation and equality&nbsp;testing are // defined for the AggregateKeyHash&nbsp;type, so declare the // map using the functors&nbsp;above in the object's template list std::tr1::unordered_map<AggregateKey, int, AggregateKeyHash, AggregateKeyEq> M; M[k] = 1; {...} std::tr1::unordered_map<int, int> M; const std::tr1::unordered_map<int, int>::hasher &hfn = M.hash_function (); const std::tr1::unordered_map<int, int>::key_equal &eqfn = M.key_eq (); long h = hfn (123); bool b = eqfn (1, 2);

struct Key{ std::string first; std::string second; int third; bool operator==(const Key &other) const { return (first == other.first && second == other.second && third == other.third); } }; namespace std { template <> struct hash<Key> { std::size_t operator()(const Key& k) const { using std::size_t; using std::hash; using std::string; // Compute individual hash values for first, // second and third and combine them using XOR // and bit shifting: return ((hash<string>()(k.first) ^ (hash<string>()(k.second) << 1)) >> 1) ^ (hash<int>()(k.third) << 1); } }; } int main() { std::unordered_map<Key,std::string> m6 = { { {"John", "Doe", 12}, "example"}, { {"Mary", "Sue", 21}, "another"} }; }

struct Key{ std::string first; std::string second; int third; bool operator==(const Key &other) const { return (first == other.first && second == other.second && third == other.third); } }; struct KeyHasher { std::size_t operator()(const Key& k) const { using std::size_t; using std::hash; using std::string; return ((hash<string>()(k.first) ^ (hash<string>()(k.second) << 1)) >> 1) ^ (hash<int>()(k.third) << 1); } }; int main() { std::unordered_map<Key,std::string,KeyHasher> m6 = { { {"John", "Doe", 12}, "example"}, { {"Mary", "Sue", 21}, "another"} }; }