std::hash <std::experimental::optional>
From cppreference.com
< cpp | experimental | optional
| Defined in header <experimental/optional>
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| template< class T > struct hash<std::experimental::optional<T>>; |
(library fundamentals TS) | |
The template specialization of std::hash for the std::experimental::optional class allows users to obtain hashes of the values contained in optional objects.
Template parameters
| T | - | the type of the value contained in optional object. The specialization std::hash<T> must meet the requirements of class template hash.
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Example
Run this code
#include <experimental/optional> #include <iostream> #include <string> #include <unordered_set> using namespace std::literals; int main() { // hash<optional> makes it possible to use unordered_set std::unordered_set<std::experimental::optional<std::string>> s = { "abc"s, std::experimental::nullopt, "def"s }; for (const auto& o : s) std::cout << o.value_or("(null)") << ' '; std::cout << '\n'; }
Possible output:
def abc (null)
See also
| (C++11) |
hash function object (class template) |