std::hash <std::experimental::optional>
From cppreference.com
| Defined in header <experimental/optional>
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template< class T >
struct hash<std::experimental::optional<T>>;
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(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) |