std::hash<std::optional>

 

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std::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator booloptional::has_value

optional::value

optional::value_or

Iterators

optional::begin (C++26)

optional::end (C++26)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace

optional::swap

optional::reset

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

make_optional

swap(std::optional)

Deduction guides

Helper classes

hash<std::optional>

nullopt_t

in_place_t

bad_optional_access

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nullopt

in_place

 

Defined in header <optional>
template<class T > struct hash<std::optional<T>>;		(since C++17)

The template specialization of std::hash for the std::optional class allows users to obtain hashes of the values contained in optional objects.

The specialization std::hash<std::optional<T>> is enabled (see std::hash) if std::hash<std::remove_const_t<T>> is enabled, and is disabled otherwise.

When enabled, for an object o of type std::optional<T> that contains a value, std::hash<std::optional<T>>()(o) evaluates to the same value as std::hash<std::remove_const_t<T>>()(*o). For an optional that does not contain a value, the hash is unspecified.

The member functions of this specialization are not guaranteed to be noexcept because the hash of the underlying type might throw.

Template parameters

T

-

the type of the value contained in optional object

Example

Run this code

#include <iostream>#include <optional>#include <string>#include <unordered_set>   usingnamespace std::literals;   int main(){using OptStr =std::optional<std::string>;   // hash<optional> makes it possible to use unordered_setstd::unordered_set<OptStr> s ={"ABC"s, "abc"s, std::nullopt, "def"s };   for(constauto& o : s)std::cout<< o.value_or("(null)")<<'\t'<<std::hash<OptStr>{}(o)<<'\n';}

Possible output:

def 11697390762615875584 (null) 18446744073709548283 abc 3663726644998027833 ABC 11746482041453314842

See also

hash (C++11)

hash function object (class template)