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std::nullopt

From cppreference.com
< cpp‎ | utility
Defined in header <optional>
constexpr nullopt_t nullopt{/*unspecified*/};
(since C++14)

std::nullopt is a constant of type std::nullopt_t that is used to indicate optional type with uninitialized state.

[edit] See also

(C++14)
indicator of optional type with uninitialized state
(class) [edit]