std::unordered_map with custom key type

build.sh

g++ -std=c++11 custom_hash.cc -o custom_hash

custom_hash.cc

#include <iostream>
#include <unordered_map>

#include <boost/functional/hash.hpp>

struct KeyData
{
  int id;
  int age;

  // equality
  bool operator == (const KeyData &other) const {
    return (id == other.id) && (age == other.age);
  }
};

struct KeyDataHasher
{
  std::size_t operator () (const KeyData &key) const 
  {
    // The following line is a stright forward implementation. But it can be
    // hard to design a good hash function if KeyData is complex.

    //return (key.id << 32 | key.age); // suppose size_t is 64-bit and int is 32-bit

    // A commonly used way is to use boost
    std::size_t seed = 0;
    boost::hash_combine(seed, boost::hash_value(key.id));
    boost::hash_combine(seed, boost::hash_value(key.age));
    return seed;
  }
};

using namespace std;

int main(int argc, const char *argv[])
{
  KeyData k1{0, 30}, k2{1, 1}, k3{2, 0};

  // Print the hash results
  KeyDataHasher hasher;
  cout << hasher(k1) << endl 
    << hasher(k2) << endl
    << hasher(k3) << endl;

  // Construct an unordered_map using KeyData, which maps
  // KeyData -> int, using KeyDataHasher as hash function
  typedef std::unordered_map<KeyData, int, KeyDataHasher> KeyDataHashMap;
  KeyDataHashMap mapping{ {k1, 1}, {k2, 2}, {k2, 3} };

  for (auto &kv : mapping) {
    cout << "id:" << kv.first.id 
      << " age:" << kv.first.age 
      << " value:" << kv.second << endl;
  }

  return 0;
}