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 <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 functionconstexprstd::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