std::hash

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C++
 
Utilities library
 
std::hash
 
Defined in header <functional>
template< class Key >
struct hash; // not defined
 (since C++11)

The hash template defines a function object that implements a hash function. Instances of this function object satisfy Hash. In particular, they define an operator() that:

1. Accepts a single parameter of type Key.

2. Returns a value of type size_t that represents the hash value of the parameter.

3. Does not throw exceptions when called.

4. For two parameters k1 and k2 that are equal, std::hash<Key>()(k1) == std::hash<Key>()(k2).

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().

The hash template is both CopyConstructible and Destructible.

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.

Contents

[edit] Notes

The actual hash functions are implementation-dependent and are not required to fulfill any other quality criteria except those specified above. Notably, some implementations use trivial (identity) hash functions which map an integer to itself. In other words, these hash functions are designed to work with unordered associative containers, but not as cryptographic hashes, for example.

Hash functions are only required to produce the same result for the same input within a single execution of a program; this allows salted hashes that prevent collision DoS attacks. (since C++14)

There is no specialization for C strings. std::hash<const char*> produces a hash of the value of the pointer (the memory address), it does not examine the contents of any character array.

[edit] Member types

argument_type Key
result_type std::size_t

[edit] Member functions

constructs a hash function object
(public member function)
calculate the hash of the argument
(public member function)

[edit] Standard specializations for basic types

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*>;

In addition to the above, standard library provides specializations for all (scoped and unscoped) enumeration types (which are not required, but usually are implemented as std::hash<std::underlying_type<Enum>::type>)

(since C++14)

[edit] Standard specializations for library types

(C++11)(C++11)(C++11)(C++11)
 hash support for strings
(class template specialization)
(C++11)
 hash support for std::error_code
(class template specialization)
(C++11)
 hash support for std::bitset
(class template specialization)
(C++11)
 hash support for std::unique_ptr
(class template specialization)
(C++11)
 hash support for std::shared_ptr
(class template specialization)
(C++11)
 hash support for std::type_index
(class template specialization)
(C++11)
 hash support for std::vector<bool>
(class template specialization)
(C++11)
 hash support for std::thread::id
(class template specialization)

[edit] Examples

Demonstrates the computation of a hash for std::string, a type that already has a hash specialization.

Run this code
#include <iostream>
#include <functional>
#include <string>
 
int main()
{
    std::string str = "Meet the new boss...";
    std::hash<std::string> hash_fn;
    std::size_t str_hash = hash_fn(str);
 
    std::cout << str_hash << '\n';
}

Possible output: 

391070135

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.

Run this code
#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:
    std::size_t operator()(S const& s) const 
    {
        std::size_t h1 = std::hash<std::string>()(s.first_name);
        std::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";
 
}

Possible output: 

hash(s1) = 6119893563398376542
hash(s2) = 14988020022735710972
hash(n1) = 17649170831080298918

Demonstrates how to specialize std::hash for a user defined type.

Run this code
#include <iostream>
#include <functional>
#include <string>
 
struct S
{
    std::string first_name;
    std::string last_name;
};
 
namespace std
{
    template<>
    struct hash<S>
    {
        typedef S argument_type;
        typedef std::size_t result_type;
 
        result_type operator()(argument_type const& s) const
        {
            result_type const h1 ( std::hash<std::string>()(s.first_name) );
            result_type const 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";
}

Possible output: 

hash(s) = 32902390710
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