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    std::hash

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    C++
     
    Utilities-Bibliothek
     
    std::hash
     
    definiert 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_typeKey
    result_typestd::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

    definiert in 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*>;

    [Bearbeiten]Standard Spezialisierungen für Bibliothekstypen

    (C++11)
    (C++11)
    (C++11)
    (C++11)
    Hash-Unterstützung für Strings
    Original:
    hash support for strings
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    (class Template-Spezialisierung)[edit]
    (C++11)
    Hash-Unterstützung für std::error_code
    Original:
    hash support for std::error_code
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    (class Template-Spezialisierung)[edit]
    (C++11)
    Hash-Unterstützung für std::bitset
    Original:
    hash support for std::bitset
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    (class Template-Spezialisierung)[edit]
    (C++11)
    		 Hash-Unterstützung für std::unique_ptr
    (class Template-Spezialisierung)[edit]
    (C++11)
    		 Hash-Unterstützung für std::shared_ptr
    (class Template-Spezialisierung)[edit]
    (C++11)
    Hash-Unterstützung für std::type_index
    Original:
    hash support for std::type_index
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    (class Template-Spezialisierung)[edit]
    (C++11)
    Hash-Unterstützung für std::vector<bool>
    Original:
    hash support for std::vector<bool>
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    (class Template-Spezialisierung)
    (C++11)
    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
    Von „https://de.cppreference.com/mwiki/index.php?title=cpp/utility/hash&oldid=32116“
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