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

Technical specifications
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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.

[edit] 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.

[edit] Example

#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)

[edit] See also

hash function object
(class template) [edit]