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

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C++

Linguaggio

Concetti

Libreria di Utilità

Stringhe

Contenitori

Algoritmi

Iteratori

Libreria matematca

Input/output

Localizzazione

Espressioni regolari (C++11)

Operazioni atomicche (C++11)

Multithreading (C++11)

 

Utilità libreria

Tipo di supporto (basic types, RTTI, type traits)

Gestione della memoria dinamica

La gestione degli errori

Programma di utilità

Funzioni variadic

Data e ora

Funzione oggetti

initializer_list (C++11)

bitset

hash (C++11)

Gli operatori relazionali Original: Relational operators The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

rel_ops::operator!= rel_ops::operator> rel_ops::operator<= rel_ops::operator>=

Coppie e tuple Original: Pairs and tuples The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

pair

tuple (C++11)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

Swap, in avanti e spostare Original: Swap, forward and move The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

swap

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

declval (C++11)

 

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 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().

Parametri

key

-

the object to be hashed

Valore di ritorno

a size_t representing the hash value

Eccezioni

Hash functions should not throw exceptions.

Esempio

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> {
 public:
  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 = employee.name.size(); i != ie; ++i) {
      result = (result * 16777619) ^ 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:

177237019