Hash function for a vector of a pair <int, int>

Question

I am trying to implement unordered_map for a vector <pair <int, int →. Since there is no such hash function by default , I tried to imagine my own function:

struct ObjectHasher
{
  std::size_t operator()(const Object& k) const
  {
    std::string h_string("");
    for (auto i = k.vec.begin(); i != k.vec.end(); ++i)
    {
      h_string.push_back(97+i->first);
      h_string.push_back(47); // '-'
      h_string.push_back(97+i->second);
      h_string.push_back(43); // '+'
    }
    return std::hash<std::string>()(h_string);
  }
};

The main idea is to change the list of integers, for example ((97, 98), (105, 107)) into a formatted string such as "ab + ik" and calculate its hash thanks to the hash, string> () . I chose numbers 97, 48, and 43 only to allow the hash string to be displayed easily in the terminal during my tests.

, , - . , , push_back(), 255, , ... , :

• (1) - ok ?
• (2) - ok /?
• (3) , -?
• (4)... - ?

Answer 1

, , "hash in" integer. ​​ boost:

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

:

struct ObjectHasher
{
  std::size_t operator()(const Object& k) const
  {
    std::size_t hash = 0;
    for (auto i = k.vec.begin(); i != k.vec.end(); ++i)
    {
      hash_combine(hash, i->first);
      hash_combine(hash, i->second);
    }
    return hash;
  }
};

Answer 2

, , -, . std::hash<std::string> -, . , XOR int std::hash<int>.

Answer 3

This is absolutely the right decision. All functions A hash function is a sequence of bytes and combining the contours of your elements as a string that provides a unique representation of the bytes of the map.

Of course, this can become naughty if your map contains a large number of elements.