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# std::hash <std::optional>

From cppreference.com
< 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<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.

### [edit] Template parameters

T | - | the type of the value contained in `optional` object

### [edit] Example

Run this code

    #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)") << ' ';
    }

Possible output:

    def abc (null)

### [edit] See also

hash (C++11) | hash function object (class template) [edit]

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