std::hash<Key>::operator()

 

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std::hash

hash::hash

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.

Run this code

#include <cstddef>
#include <functional>
#include <iostream>
#include <string>

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

static_assert([](auto... sz) { return ((sizeof(std::size_t) == sz) or ...); }(4, 8));

namespace std
{
    template <>
    class hash<Employee>
    {
    public:
        static constexpr std::size_t operator()(const Employee& employee)
        {
             // Computes the hash of an employee using Fowler-Noll-Vo-1a hash function.
             constexpr std::size_t prime{sizeof(size_t) < 8 ? 0x01000193 : 0x100000001b3};
             std::size_t result{sizeof(size_t) < 8 ? 0x811c9dc5 : 0xcbf29ce484222325};

             for (auto ch : employee.name)
                 result = (result ^ ch) * prime;

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

int main()
{
    std::hash<Employee> hash_fn;
    std::cout << std::hex << hash_fn(Employee{"Zaphod Beeblebrox", 42}) << '\n';
}

Output:

6fbb35ba06f7b013