Reference


Header <boost/functional/hash.hpp>

For the full specification, see section 6.3 of the C++ Standard Library Technical Report and issue 6.18 of the Library Extension Technical Report Issues List (page 63).

Header <boost/functional/hash.hpp>

Defines boost::hash, and helper functions.

namespace boost {
  template<typename T> struct hash;

  template<> struct hash<bool>;
  template<> struct hash<char>;
  template<> struct hash<signed char>;
  template<> struct hash<unsigned char>;
  template<> struct hash<wchar_t>;
  template<> struct hash<char16_t>;
  template<> struct hash<char32_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<unsigned long>;
  template<> struct hash<long long>;
  template<> struct hash<unsigned long long>;
  template<> struct hash<float>;
  template<> struct hash<double>;
  template<> struct hash<long double>;
  template<> struct hash<std::string>;
  template<> struct hash<std::wstring>;
  template<> struct hash<std::u16string>;
  template<> struct hash<std::u32string>;
  template<typename T> struct hash<T*>;
  template<> struct hash<std::type_index>;

  // Support functions (Boost extension).
  template<typename T> void hash_combine(size_t &, T const&);
  template<typename It> std::size_t hash_range(It, It);
  template<typename It> void hash_range(std::size_t&, It, It);

  // Overloadable hash implementation (Boost extension).
  std::size_t hash_value(bool);
  std::size_t hash_value(char);
  std::size_t hash_value(signed char);
  std::size_t hash_value(unsigned char);
  std::size_t hash_value(wchar_t);
  std::size_t hash_value(char16_t);
  std::size_t hash_value(char32_t);
  std::size_t hash_value(short);
  std::size_t hash_value(unsigned short);
  std::size_t hash_value(int);
  std::size_t hash_value(unsigned int);
  std::size_t hash_value(long);
  std::size_t hash_value(unsigned long);
  std::size_t hash_value(long long);
  std::size_t hash_value(unsigned long long);
  std::size_t hash_value(float);
  std::size_t hash_value(double);
  std::size_t hash_value(long double);
  template<typename T> std::size_t hash_value(T* const&);
  template<typename T, unsigned N> std::size_t hash_value(T (&val)[N]);
  template<typename T, unsigned N> std::size_t hash_value(const T (&val)[N]);
  template<typename Ch, typename A> 
    std::size_t hash_value(std::basic_string<Ch, std::char_traits<Ch>, A> const&);
  template<typename A, typename B> 
    std::size_t hash_value(std::pair<A, B> const&);
  template<typename T, typename A> 
    std::size_t hash_value(std::vector<T, A> const&);
  template<typename T, typename A> 
    std::size_t hash_value(std::list<T, A> const&);
  template<typename T, typename A> 
    std::size_t hash_value(std::deque<T, A> const&);
  template<typename K, typename C, typename A> 
    std::size_t hash_value(std::set<K, C, A> const&);
  template<typename K, typename C, typename A> 
    std::size_t hash_value(std::multiset<K, C, A> const&);
  template<typename K, typename T, typename C, typename A> 
    std::size_t hash_value(std::map<K, T, C, A> const&);
  template<typename K, typename T, typename C, typename A> 
    std::size_t hash_value(std::multimap<K, T, C, A> const&);
  template<typename T> std::size_t hash_value(std::complex<T> const&);
  std::size_t hash_value(std::type_index);
  template<typename T, std::size_t N> 
    std::size_t hash_value(std::array<T, N> const&);
  template<typename... T> std::size_t hash_value(std::tuple<T...>);
}

Support functions (Boost extension).

template<typename T> void hash_combine(size_t & seed, T const& v);

Called repeatedly to incrementally create a hash value from several variables.

Effects:

Updates seed with a new hash value generated by combining it with the result of hash_value(v). Will always produce the same result for the same combination of seed and hash_value(v) during the single run of a program.

Notes:

hash_value is called without qualification, so that overloads can be found via ADL.

This is an extension to TR1

Forward declared in <boost/functional/hash_fwd.hpp>

This hash function is not intended for general use, and isn't guaranteed to be equal during separate runs of a program - so please don't use it for any persistent storage or communication.

Throws:

Only throws if hash_value(T) throws. Strong exception safety, as long as hash_value(T) also has strong exception safety.

template<typename It> std::size_t hash_range(It first, It last);
template<typename It> void hash_range(std::size_t& seed, It first, It last);

Calculate the combined hash value of the elements of an iterator range.

Effects:

For the two argument overload:

size_t seed = 0;

for(; first != last; ++first)
{
    hash_combine(seed, *first);
}

return seed;

For the three arguments overload:

for(; first != last; ++first)
{
    hash_combine(seed, *first);
}

Notes:

hash_range is sensitive to the order of the elements so it wouldn't be appropriate to use this with an unordered container.

This is an extension to TR1

Forward declared in <boost/functional/hash_fwd.hpp>

This hash function is not intended for general use, and isn't guaranteed to be equal during separate runs of a program - so please don't use it for any persistent storage or communication.

Throws:

Only throws if hash_value(std::iterator_traits<It>::value_type) throws. hash_range(std::size_t&, It, It) has basic exception safety as long as hash_value(std::iterator_traits<It>::value_type) has basic exception safety.

Overloadable hash implementation (Boost extension).

std::size_t hash_value(bool val);
std::size_t hash_value(char val);
std::size_t hash_value(signed char val);
std::size_t hash_value(unsigned char val);
std::size_t hash_value(wchar_t val);
std::size_t hash_value(char16_t val);
std::size_t hash_value(char32_t val);
std::size_t hash_value(short val);
std::size_t hash_value(unsigned short val);
std::size_t hash_value(int val);
std::size_t hash_value(unsigned int val);
std::size_t hash_value(long val);
std::size_t hash_value(unsigned long val);
std::size_t hash_value(long long val);
std::size_t hash_value(unsigned long long val);
std::size_t hash_value(float val);
std::size_t hash_value(double val);
std::size_t hash_value(long double val);
template<typename T> std::size_t hash_value(T* const& val);
template<typename T, unsigned N> std::size_t hash_value(T (&val)[N]);
template<typename T, unsigned N> std::size_t hash_value(const T (&val)[N]);
template<typename Ch, typename A> 
  std::size_t hash_value(std::basic_string<Ch, std::char_traits<Ch>, A> const& val);
template<typename A, typename B> 
  std::size_t hash_value(std::pair<A, B> const& val);
template<typename T, typename A> 
  std::size_t hash_value(std::vector<T, A> const& val);
template<typename T, typename A> 
  std::size_t hash_value(std::list<T, A> const& val);
template<typename T, typename A> 
  std::size_t hash_value(std::deque<T, A> const& val);
template<typename K, typename C, typename A> 
  std::size_t hash_value(std::set<K, C, A> const& val);
template<typename K, typename C, typename A> 
  std::size_t hash_value(std::multiset<K, C, A> const& val);
template<typename K, typename T, typename C, typename A> 
  std::size_t hash_value(std::map<K, T, C, A> const& val);
template<typename K, typename T, typename C, typename A> 
  std::size_t hash_value(std::multimap<K, T, C, A> const& val);
template<typename T> std::size_t hash_value(std::complex<T> const& val);
std::size_t hash_value(std::type_index val);
template<typename T, std::size_t N> 
  std::size_t hash_value(std::array<T, N> const& val);
template<typename... T> std::size_t hash_value(std::tuple<T...> val);

Implementation of the hash function.

Generally shouldn't be called directly by users, instead they should use boost::hash, boost::hash_range or boost::hash_combine which call hash_value without namespace qualification so that overloads for custom types are found via ADL.

Notes:

This is an extension to TR1

This hash function is not intended for general use, and isn't guaranteed to be equal during separate runs of a program - so please don't use it for any persistent storage or communication.

Throws:

Only throws if a user supplied version of hash_value throws for an element of a container, or one of the types stored in a pair.

Returns:

Types	Returns
`bool`, `char`, `signed char`, `unsigned char`, `wchar_t`, `char16_t`, `char32_t`, `short`, `unsigned short`, `int`, `unsigned int`, `long`, `unsigned long`	`val`
`long long`, `unsigned long long`	`val` when `abs(val) <= std::numeric_limits<std::size_t>::max()`.
`float`, `double`, `long double`	An unspecified value, except that equal arguments shall yield the same result.
`T*`	An unspecified value, except that equal arguments shall yield the same result.
`T val[N]`, `const T val[N]`	`hash_range(val, val+N)`
`std:basic_string<Ch, std::char_traits<Ch>, A>`, `std::vector<T, A>`, `std::list<T, A>`, `std::deque<T, A>`, `std::set<K, C, A>`, `std::multiset<K, C, A>`, `std::map<K, T, C, A>`, `std::multimap<K, T, C, A>`, `std::array<T, N>`	`hash_range(val.begin(), val.end())`
`std::pair<A, B>`	size_t seed = 0; hash_combine(seed, val.first); hash_combine(seed, val.second); return seed;
`std::tuple<T...>`	size_t seed = 0; hash_combine(seed, get<0>(val)); hash_combine(seed, get<1>(val)); // .... return seed;
`std::complex<T>`	When `T` is a built in type and `val.imag() == 0`, the result is equal to `hash_value(val.real())`. Otherwise an unspecified value, except that equal arguments shall yield the same result.
`std::type_index`	`val.hash_code()`