< cpp‎ | utility‎ | hash

Specializations of std::hash should define an operator() that:

  • Takes a single argument key of type Key.
  • Returns a value of type std::size_t that represents the hash value of key.
  • For two parameters k1 and k2 that are equal, std::hash<Key>()(k1) == std::hash<Key>()(k2).
  • For two different parameters k1 and k2 that are not equal, the probability that std::hash<Key>()(k1) == std::hash<Key>()(k2) should be very small, approaching 1.0/std::numeric_limits<size_t>::max().


[edit] Parameters

key - the object to be hashed

[edit] Return value

a std::size_t representing the hash value

[edit] Exceptions

Hash functions should not throw exceptions.

[edit] Example

The following code shows how to specialize the std::hash template for a custom class.

#include <functional>
#include <iostream>
#include <string>
struct Employee {
  std::string name;
  unsigned int ID;
namespace std {
template <>
class hash<Employee> {
  size_t operator()(const Employee &employee) const
    // computes the hash of an employee using a variant 
    // of the Fowler-Noll-Vo hash function
    size_t result = 2166136261;
    for (size_t i = 0, ie =; i != ie; ++i) {
      result = (result * 16777619) ^[i];
    return result ^ (employee.ID << 1);
int main()
  Employee employee; = "Zaphod Beeblebrox";
  employee.ID = 42;
  std::hash<Employee> hash_fn;
  std::cout << hash_fn(employee) << '\n';