std::hash::operator()

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().

Parameters

key

-

the object to be hashed

Return value

A std::size_t representing the hash value.

Exceptions

Hash functions should not throw exceptions.

Example

The following code shows how to specialize the std::hash template for a custom class. The hash function uses Fowler–Noll–Vo hash algorithm.

#include <cstdint>
#include <functional>
#include <iostream>
#include <string>

struct Employee
{
    std::string name;
    std::uint64_t ID;
};

namespace std
{
    template <>
    class hash<Employee>
    {
    public:
        std::uint64_t operator()(const Employee& employee) const
        {
             // computes the hash of an employee using a variant
             // of the Fowler-Noll-Vo hash function
             constexpr std::uint64_t prime{0x100000001B3};
             std::uint64_t result{0xcbf29ce484222325};

             for (std::uint64_t i{}, ie = employee.name.size(); i != ie; ++i)
                 result = (result * prime) ^ employee.name[i];

             return result ^ (employee.ID << 1);
         }
    };
}

int main()
{
    Employee employee;
    employee.name = "Zaphod Beeblebrox";
    employee.ID = 42;

    std::hash<Employee> hash_fn;
    std::cout << hash_fn(employee) << '\n';
}

Output:

12615575401975788567