std::hash<Key>::operator()
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std::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
k1andk2that are equal, std::hash <Key>()(k1) == std::hash <Key>()(k2). - For two different parameters
k1andk2that 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().
Contents
[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. 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 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