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

From cppreference.com
 
 
Technical specifications
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals TS v2)
Library fundamentals 3 (library fundamentals TS v3)
Extensions for parallelism (parallelism TS)
Extensions for parallelism 2 (parallelism TS v2)
Extensions for concurrency (concurrency TS)
Extensions for concurrency 2 (concurrency TS v2)
Concepts (concepts TS)
Ranges (ranges TS)
Reflection (reflection TS)
Mathematical special functions (special functions 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.

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

#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

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