std::hash - cppreference.com

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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:

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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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Membri tipi


`argument_type`	`Key`

`result_type`	std::size_t

Membri funzioni


	costruisce un oggetto funzione hash Original: constructs a hash function object The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (metodo pubblico)

	calcolare l'hash dell'argomento Original: calculate the hash of the argument The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (metodo pubblico)

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*>;

Specializzazioni standard per i tipi di libreria


std::hash<std::string> std::hash<std::u16string> std::hash<std::u32string> std::hash<std::wstring> (C++11) (C++11) (C++11) (C++11)	supporto per l'hash di stringhe (classe modello di specializzazione) [modifica]

std::hash<std::error_code> (C++11)	sostegno hash per std::error_code Original: hash support for std::error_code The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe modello di specializzazione) [modifica]

std::hash<std::bitset> (C++11)	sostegno hash per std::bitset Original: hash support for std::bitset The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe modello di specializzazione) [modifica]

std::hash<std::unique_ptr> (C++11)	sostegno hash per std::unique_ptr Original: hash support for std::unique_ptr The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe modello di specializzazione) [modifica]

std::hash<std::shared_ptr> (C++11)	sostegno hash per std::shared_ptr Original: hash support for std::shared_ptr The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe modello di specializzazione) [modifica]

std::hash<std::type_index> (C++11)	sostegno hash per std::type_index Original: hash support for std::type_index The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe modello di specializzazione) [modifica]

std::hash<std::vector<bool>> (C++11)	sostegno hash per `std::vector<bool>` Original: hash support for `std::vector<bool>` The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe modello di specializzazione)

std::hash<std::thread::id> (C++11)	sostegno hash per std::thread::id Original: hash support for std::thread::id The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe modello di specializzazione)

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:

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: