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

Technical specifications
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals 2 TS)
Extensions for parallelism (parallelism TS)
Concepts (concepts TS)
Extensions for concurrency (concurrency TS)
Special mathematical functions (special math TR)
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]