# std::hash <std::experimental::optional>

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< cpp‎ | experimental‎ | optional

Defined in header `<experimental/optional>` |  |
template<class T> struct hash< std::experimental::optional <T>>; |  | (library fundamentals TS)

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

### Template parameters

T | - | the type of the value contained in `optional` object. The specialization std::hash <T> must meet the requirements of class template `hash`.

### Example

Run this code

    #include <experimental/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::experimental::optional<std::string>> s = {
                "abc"s, std::experimental::nullopt, "def"s
        };

        for(const auto& o : s)
            std::cout << o.value_or("(null)") << ' ';
    }

Possible output:

    def abc (null)

### See also

hash (C++11) | hash function object (class template)

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