std::nullopt (3) - Linux Manuals
std::nullopt: std::nullopt
NAME
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
(C++17)