std::nullopt (3) Linux Manual Page

std::nullopt – std::nullopt

Synopsis

Defined in header <optional>
inline constexpr nullopt_t nullopt{/*unspecified*/}; (since C++17)
std::nullopt is a constant of type std::nullopt_t that is used to indicate optional type with uninitialized state.

See also

nullopt_t indicator of optional type with uninitialized state
          (class)
(C++17)