std::hash <std::optional>

template<class T>
struct hash<std::optional<T>>;

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

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<T>()(*o). For an optional that does not contain a value, the hash is unspecified.

Template parameters

Example

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

def abc (null)

See also