std::hash<std::optional> (3) - Linux Manuals

std::hash<std::optional>: std::hash<std::optional>

NAME

std::hash<std::optional> - std::hash<std::optional>

Synopsis

Defined in header <optional>
template<class T> (since C++17)
struct hash<std::optional<T>>;

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

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)