std::hashstd::optional - cppreference.com (original) (raw)

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<std::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.

[edit] Template parameters

[edit] Example

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

Possible output:

def 11697390762615875584 (null) 18446744073709548283 abc 3663726644998027833 ABC 11746482041453314842

[edit] See also

| | hash function object (class template) [edit] | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |