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_typeKey
result_typestd::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<signedchar>;
template<>struct hash<unsignedchar>;
template<>struct hash<char16_t>;
template<>struct hash<char32_t>;
template<>struct hash<wchar_t>;
template<>struct hash<short>;
template<>struct hash<unsignedshort>;
template<>struct hash<int>;
template<>struct hash<unsignedint>;
template<>struct hash<long>;
template<>struct hash<longlong>;
template<>struct hash<unsignedlong>;
template<>struct hash<unsignedlonglong>;
template<>struct hash<float>;
template<>struct hash<double>;
template<>struct hash<longdouble>;

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"
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