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 inoptional object [edit] Example
#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[edit] See also hash function object
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4