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std::hash

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< cpp‎ | utility


Defined in header <functional>
template< class Key >
struct hash; // not defined
(seit C++11)
Das Hash-Template definiert ein Funktionsobjekt, das eine Hash-Funktion implementiert. Instanzen dieses Funktionsobjekts definieren einen operator() der:
Original:
The hash template defines a function object that implements a Hash-Funktion. Instances of this function object define an operator() that:
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1. Einen einzelnen Parameter des Typs Key entgegennimmt.
Original:
1. Accepts a single parameter of type Key.
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2. Einen Wert vom Typ size_t zurückgibt, der den Hash-Wert des Parameters darstellt.
Original:
2. Returns a value of type size_t that represents the hash value of the parameter.
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3. Während der Ausführung keine Exception auslösen.
Original:
3. Does not throw exceptions when called.
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4. Für zwei identische Parameter k1 und k2 gilt: 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. Für zwei unterschiedliche Parameter k1 und k2 sollte die Wahrscheinlichkeit, dass std::hash<Key>()(k1) == std::hash<Key>()(k2) minimal sein, näherungsweise 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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Das Hash-Template ist erfüllt die Konzepte CopyConstructible und Destructible.
Original:
The hash template is both CopyConstructible and Destructible.
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Die ungeordnete assoziative Container std::unordered_set, std::unordered_multiset, std::unordered_map, std::unordered_multimap verwenden Spezialisierungen der Vorlage std::hash als Standard-Hash-Funktion .
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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Inhaltsverzeichnis

[Bearbeiten] Mitglied Typen

argument_type Key
result_type std::size_t

[Bearbeiten] Member-Funktionen

konstruiert eine Hash-Funktion Objekt
Original:
constructs a hash function object
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(öffentliche Elementfunktion)
Berechnung der Hash-Wert des Arguments
Original:
calculate the hash of the argument
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(öffentliche Elementfunktion)

[Bearbeiten] Standard Spezialisierungen für Grundtypen

Defined in 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*>;

[Bearbeiten] Standard Spezialisierungen für Bibliothekstypen

Hash-Unterstützung für Strings
Original:
hash support for strings
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(class Template-Spezialisierung) [edit]
Hash-Unterstützung für std::error_code
Original:
hash support for std::error_code
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(class Template-Spezialisierung) [edit]
Hash-Unterstützung für std::bitset
Original:
hash support for std::bitset
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(class Template-Spezialisierung) [edit]
Hash-Unterstützung für std::unique_ptr
Original:
hash support for std::unique_ptr
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(class Template-Spezialisierung) [edit]
Hash-Unterstützung für std::shared_ptr
Original:
hash support for std::shared_ptr
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(class Template-Spezialisierung) [edit]
Hash-Unterstützung für std::type_index
Original:
hash support for std::type_index
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(class Template-Spezialisierung) [edit]
Hash-Unterstützung für std::vector<bool>
Original:
hash support for std::vector<bool>
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(class Template-Spezialisierung)
Hash-Unterstützung für std::thread::id
Original:
hash support for std::thread::id
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(class Template-Spezialisierung)

[Bearbeiten] Beispiele

Demonstriert die Berechnung eines Hash für std::string, ein Typ, der bereits über eine Hash-Spezialisierung .

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

Veranschaulicht Schaffung einer Hash-Funktion für einen Benutzer definierten Typ. Mit diesem als Template-Parameter für std::unordered_map, std::unordered_set, etc. erfordert auch Spezialisierung 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

Veranschaulicht, wie std::hash für einen benutzerdefinierten Typ spezialisieren .

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