std::hash <std::optional>

 

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std::optional

Member functions

optional::optional (C++17)

optional::~optional (C++17)

optional::operator= (C++17)

Observers

optional::operator->optional::operator* (C++17)(C++17)

optional::operator booloptional::has_value (C++17)(C++17)

optional::value (C++17)

optional::value_or (C++17)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace (C++17)

optional::swap (C++17)

optional::reset (C++17)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)

make_optional (C++17)

swap (C++17)

Deduction guides

Helper classes

hash (C++17)

nullopt_t (C++17)

in_place_t (C++17)

bad_optional_access (C++17)

Helper objects

nullopt (C++17)

in_place (C++17)

 

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<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 <optional>
#include <unordered_set>
#include <string>
#include <iostream>
using namespace std::literals;
int main()
{
    using OptStr = std::optional<std::string>;
 
    // hash<optional> makes it possible to use unordered_set
    std::unordered_set<OptStr> s = {
        "ABC"s, "abc"s, std::nullopt, "def"s
    };
 
    for(const auto& 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)