C++ hash

Table of contents

A basic struct for demonstration purpose:

struct S {
  std::string first_name;
  std::string last_name;
};
bool operator==(const S& lhs, const S& rhs) {
  return lhs.first_name == rhs.first_name && lhs.last_name == rhs.last_name;
}

struct MyHash {
  std::size_t operator()(S const& s) const noexcept {
    std::size_t h1 = std::hash<std::string>{}(s.first_name);
    std::size_t h2 = std::hash<std::string>{}(s.last_name);
    return h1 ^ (h2 << 1);  // combine hashes
  }
};

template <>
struct std::hash<S> {
  std::size_t operator()(S const& s) const noexcept {
    std::size_t h1 = std::hash<std::string>{}(s.first_name);
    std::size_t h2 = std::hash<std::string>{}(s.last_name);
    return h1 ^ (h2 << 1);  // combine hashes
  }
};

  // size_t will take 32bits on 32bits systems
  // size_t will take 64bits on 64bits systems

  size_t h1 = hash<string>{}("Geraldo");
  size_t h2 = hash<string>{}("geraldo");
  size_t h3 = h1 ^ (h2 << 1);
  size_t h4 = hash<char>{}('\n');
  size_t h5 = hash<int>{}(10);
  size_t h6 = hash<long>{}(10);

  S obj = {"Geraldo", "Ribeiro"};
  size_t h7 = hash<S>{}(obj);

  cout << "h1 = " << h1 << '\n';
  cout << "h2 = " << h2 << '\n';
  cout << "h3 = " << h3 << '\n';
  cout << "h4 = " << h4 << '\n';
  cout << "h5 = " << h5 << '\n';
  cout << "h6 = " << h6 << '\n';
  cout << "h7 = " << h7 << '\n';

Possible output

h1 = 16517155802914804667
h2 = 15968437481030298971
h3 = 6777401911469705485
h4 = 10
h5 = 10
h6 = 10
h7 = 7993227047372965953