std::hash <std::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.

Template parameters

T - the type of the value contained in optional object


#include <optional>
#include <unordered_set>
#include <string>
#include <iostream>
using namespace std::literals;
int main()
    using OptStr = std::optional<std::string>;
    // hash<optional> makes it possible to use unordered_set
    std::unordered_set<OptStr> s = {
        "ABC"s, "abc"s, std::nullopt, "def"s
    for(const auto& o : s) {
        std::cout << o.value_or("(null)") << '\t' << std::hash<OptStr>{}(o) << '\n';

Possible output:

def        11697390762615875584
(null)        18446744073709548283
abc        3663726644998027833
ABC        11746482041453314842

See also

hash function object
(class template)

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