# std::hash<std::optional>

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< cpp‎ | utility‎ | optional

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>

    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)

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