std::hash <std::optional>
From cppreference.com
| Defined in header <optional>
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| 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
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Example
Run this code
#include <optional> #include <unordered_set> #include <string> #include <iostream> using namespace std::literals; int main() { // hash<optional> makes it possible to use unordered_set std::unordered_set<std::optional<std::string>> s = { "abc"s, std::nullopt, "def"s }; for(const auto& o : s) std::cout << o.value_or("(null)") << ' '; }
Possible output:
def abc (null)
See also
| (C++11) |
hash function object (class template) |