std:: hash <std::optional>

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

Example

#include <iostream>
#include <optional>
#include <string>
#include <unordered_set>
 
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';
}

def     11697390762615875584
(null)  18446744073709548283
abc     3663726644998027833
ABC     11746482041453314842

See also