std::hash<std::optional>
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hash<std::optional>
Defined in header
<optional>
template< class T >
struct hash<std::optional <T>>;
(since C++17)
struct hash<std::optional <T>>;
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
T
-
the type of the value contained in
optional object
[edit] Example
Run this code
#include <iostream> #include <optional> #include <string> #include <unordered_set> 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