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

 

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;unsignedint 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