Namespace
Varianti

std::hash

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C++
 
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
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rel_ops::operator!=
rel_ops::operator>
rel_ops::operator<=
rel_ops::operator>=
Coppie e tuple
Original:
Pairs and tuples
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pair
tuple(C++11)
piecewise_construct_t(C++11)
piecewise_construct(C++11)
Swap, in avanti e spostare
Original:
Swap, forward and move
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swap
forward(C++11)
move(C++11)
move_if_noexcept(C++11)
declval(C++11)
 
std::hash
 
Elemento definito nell'header <functional>
template< class Key >
struct hash; // not defined
 (dal C++11)
Il modello hash definisce un oggetto che implementa una funzione funzione di hash. Le istanze di questo oggetto funzione di definire un operator() che:
Original:
The hash template defines a function object that implements a funzione di hash. Instances of this function object define an operator() that:
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1. Accetta un singolo parametro di tipo Key.
Original:
1. Accepts a single parameter of type Key.
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2. Restituisce un valore di size_t tipo che rappresenta il valore hash del parametro.
Original:
2. Returns a value of type size_t that represents the hash value of the parameter.
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3. Non generare eccezioni quando viene chiamato.
Original:
3. Does not throw exceptions when called.
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4. Per due parametri k1 e k2 che sono uguali, std::hash<Key>()(k1) == std::hash<Key>()(k2).
Original:
4. For two parameters k1 and k2 that are equal, std::hash<Key>()(k1) == std::hash<Key>()(k2).
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5. Per due parametri diversi k1 e k2 che non sono uguali, la probabilità che std::hash<Key>()(k1) == std::hash<Key>()(k2) dovrebbe essere molto piccolo, avvicinando 1.0/std::numeric_limits<size_t>::max().
Original:
5. 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().
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Il modello di hash è sia CopyConstructible e Destructible.
Original:
The hash template is both CopyConstructible and Destructible.
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L'ordinata contenitori associativi std::unordered_set, std::unordered_multiset, std::unordered_map, std::unordered_multimap utilizzare specializzazioni della std::hash modello come la funzione di hash predefinito.
Original:
The unordered associative containers std::unordered_set, std::unordered_multiset, std::unordered_map, std::unordered_multimap use specializations of the template std::hash as the default hash function.
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Indice

[modifica] Membri tipi

argument_type Key
result_type std::size_t

[modifica] Membri funzioni
costruisce un oggetto funzione hash
Original:
constructs a hash function object
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(metodo pubblico)
calcolare l'hash dell'argomento
Original:
calculate the hash of the argument
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(metodo pubblico)

[modifica] Specializzazioni standard per tipi di base

Elemento definito nell'header <functional>
template<> struct hash<bool>;

template<> struct hash<char>;
template<> struct hash<signed char>;
template<> struct hash<unsigned char>;
template<> struct hash<char16_t>;
template<> struct hash<char32_t>;
template<> struct hash<wchar_t>;
template<> struct hash<short>;
template<> struct hash<unsigned short>;
template<> struct hash<int>;
template<> struct hash<unsigned int>;
template<> struct hash<long>;
template<> struct hash<long long>;
template<> struct hash<unsigned long>;
template<> struct hash<unsigned long long>;
template<> struct hash<float>;
template<> struct hash<double>;
template<> struct hash<long double>;

template< class T > struct hash<T*>;

[modifica] Specializzazioni standard per i tipi di libreria

(C++11)
(C++11)
(C++11)
(C++11)
 supporto per l'hash di stringhe
(classe modello di specializzazione) [modifica]
(C++11)
sostegno hash per std::error_code
Original:
hash support for std::error_code
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(classe modello di specializzazione) [modifica]
(C++11)
sostegno hash per std::bitset
Original:
hash support for std::bitset
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(classe modello di specializzazione) [modifica]
(C++11)
sostegno hash per std::unique_ptr
Original:
hash support for std::unique_ptr
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(classe modello di specializzazione) [modifica]
(C++11)
sostegno hash per std::shared_ptr
Original:
hash support for std::shared_ptr
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(classe modello di specializzazione) [modifica]
(C++11)
sostegno hash per std::type_index
Original:
hash support for std::type_index
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(classe modello di specializzazione) [modifica]
(C++11)
sostegno hash per std::vector<bool>
Original:
hash support for std::vector<bool>
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(classe modello di specializzazione)
(C++11)
sostegno hash per std::thread::id
Original:
hash support for std::thread::id
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(classe modello di specializzazione)

[modifica] Esempi

Viene illustrato il calcolo di un hash per std::string, un tipo che ha già una specializzazione hash .

Original:

Demonstrates the computation of a hash for std::string, a type that already has a hash specialization.

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#include <iostream>
#include <functional>
#include <string>
 
int main()
{
    std::string str = "Meet the new boss...";
    std::hash<std::string> hash_fn;
    size_t str_hash = hash_fn(str);
 
    std::cout << str_hash << '\n';
}

Output: 

391070135

Viene illustrata la creazione di una funzione hash per un tipo definito dall'utente. Usando questo come parametro di template per std::unordered_map, std::unordered_set, ecc richiede anche la specializzazione di std::equal_to .

Original:

Demonstrates creation of a hash function for a user defined type. Using this as a template parameter for std::unordered_map, std::unordered_set, etc. also requires specialization of std::equal_to.

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#include <iostream>
#include <functional>
#include <string>
 
struct S {
    std::string first_name;
    std::string last_name;
};
 
template<class T> class MyHash;
 
template<>
class MyHash<S> {
public:
    size_t operator()(const S &s) const 
    {
        size_t h1 = std::hash<std::string>()(s.first_name);
        size_t h2 = std::hash<std::string>()(s.last_name);
        return h1 ^ (h2 << 1);
    }
};
 
int main()
{
    std::string s1 = "Hubert";
    std::string s2 = "Farnsworth";
    std::hash<std::string> h1;
 
    S n1;
    n1.first_name = s1;
    n1.last_name =  s2;
 
    std::cout << "hash(s1) = " << h1(s1) << "\n"
              << "hash(s2) = " << std::hash<std::string>()(s2) << "\n"
	      << "hash(n1) = " << MyHash<S>()(n1) << "\n";
 
}

Output: 

hash(s1) = 6119893563398376542
hash(s2) = 14988020022735710972
hash(n1) = 17649170831080298918

Viene illustrato come specializzarsi std::hash di un tipo definito dall'utente .

Original:

Demonstrates how to specialize std::hash for a user defined type.

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#include <iostream>
#include <functional>
#include <string>
 
struct S {
    std::string first_name;
    std::string last_name;
};
 
namespace std {
template<>
class hash<S> {
public:
    size_t operator()(const S &s) const 
    {
        size_t h1 = std::hash<std::string>()(s.first_name);
        size_t h2 = std::hash<std::string>()(s.last_name);
        return h1 ^ ( h2 << 1 );
    }
};
}
 
int main()
{
    S s;
    s.first_name = "Bender";
    s.last_name =  "Rodriguez";
    std::hash<S> hash_fn;
 
    std::cout << "hash(s) = " << hash_fn(s) << "\n";
}

Output: 

hash(s) = 32902390710
Estratto da "https://it.cppreference.com/mwiki/index.php?title=cpp/utility/hash&oldid=27301"