std::hash

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Defined in header `<functional>`

template< class Key > struct hash;		(since C++11)

The hash template defines a hash function object such that if two objects K and M of the same type Key compare equal, then std::hash(K)==std::hash(M), although the reverse is not necessarily true.

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.

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*>;

[edit] Standard specializations for library types


std::hash<std::string> std::hash<std::u16string> std::hash<std::u32string> std::hash<std::wstring> (C++11) (C++11) (C++11) (C++11)	hash support for strings (class template specialization)

std::hash<std::error_code> (C++11)	hash support for std::error_code (class template specialization)

std::hash<std::bitset> (C++11)	hash support for std::bitset (class template specialization)

std::hash<std::unique_ptr> (C++11)	hash support for std::unique_ptr (class template specialization)

std::hash<std::shared_ptr> (C++11)	hash support for std::shared_ptr (class template specialization)

std::hash<std::type_index> (C++11)	hash support for std::type_index (class template specialization)

std::hash<std::vector<bool>> (C++11)	hash support for std::vector<bool> (class template specialization)

std::hash<std::thread::id> (C++11)	hash support for std::thread::id (class template specialization)

[edit] Example

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. requires also specialization of std::equal_to.

#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= "Peter";
    std::string s2= "Pan";
    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


`argument_type`	`Key`

`result_type`	std::size_t


(constructor)	constructs a hash function object (public member function)

operator()	calculate the hash of the argument (public member function)

std::hash

From Cppreference

Contents

[edit] Member types

[edit] Member functions

[edit] Standard specializations for basic types

[edit] Standard specializations for library types

[edit] Example