|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Member functions | ||||
| Observers | ||||
| Iterators | ||||
(C++26) | ||||
(C++26) | ||||
| Monadic operations | ||||
(C++23) | ||||
(C++23) | ||||
(C++23) | ||||
| Modifiers | ||||
| Non-member functions | ||||
| Deduction guides | ||||
| Helper classes | ||||
hash<std::optional> | ||||
| Helper objects | ||||
Defined in header <optional> | ||
template<class T> struct hash<std::optional<T>>; | (since C++17) | |
The template specialization ofstd::hash for thestd::optional class allows users to obtain hashes of the values contained inoptional objects.
The specializationstd::hash<std::optional<T>> is enabled (seestd::hash) ifstd::hash<std::remove_const_t<T>> is enabled, and is disabled otherwise.
When enabled, for an objecto of typestd::optional<T> that contains a value,std::hash<std::optional<T>>()(o) evaluates to the same value asstd::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.
| T | - | the type of the value contained inoptional object |
#include <iostream>#include <optional>#include <string>#include <unordered_set> usingnamespace std::literals; int main(){using OptStr=std::optional<std::string>; // hash<optional> makes it possible to use unordered_setstd::unordered_set<OptStr> s={"ABC"s,"abc"s,std::nullopt,"def"s}; for(constauto& o: s)std::cout<< o.value_or("(null)")<<'\t'<<std::hash<OptStr>{}(o)<<'\n';}
Possible output:
def 11697390762615875584(null) 18446744073709548283abc 3663726644998027833ABC 11746482041453314842
(C++11) | hash function object (class template)[edit] |
